Add per-channel hybrid audio spread spectrum and env feature toggles

Spread spectrum v2: independent per-channel embedding with round-robin
bit distribution, preserving spatial stereo/surround mix. Adaptive chip
tiers (256/512/1024) trade capacity for lossy codec robustness. LFE
channel skipped for 5.1+ layouts. v2 header (20B) with backward-
compatible v0 decode fallback.

Environment toggles (STEGASOO_AUDIO, STEGASOO_VIDEO) gate audio/video
features for minimal builds (e.g. Raspberry Pi image-only). Values:
auto (default, detect deps), 1/true (force on), 0/false (force off).

Web UI fixes: accordion defaults to step 1 on load, chevron arrow
styling, required attribute toggling for audio carrier type switch,
"Images & Mode" renamed to "Reference, Carrier, Mode".

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -5,6 +5,25 @@ All notable changes to Stegasoo will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org).
 
+## [4.3.0] - 2026-02-27
+
+### Added
+- **Audio Steganography** — Hide messages in audio files (WAV, FLAC, MP3, OGG, AAC, M4A)
+  - LSB mode: Direct least-significant-bit embedding in audio samples
+  - Spread Spectrum mode: Noise-resistant encoding using pseudo-random spreading
+  - Automatic format transcoding to WAV for embedding
+  - Full CLI support: `stegasoo audio-encode`, `audio-decode`, `audio-info`
+  - REST API endpoints: `/audio/encode`, `/audio/decode`, `/audio/info`
+  - Web UI: Unified encode/decode pages with carrier type selector (Image/Audio)
+- New `AudioCapacityInfo`, `AudioEmbedStats`, `AudioInfo` model classes
+- Audio-specific exceptions: `AudioError`, `AudioValidationError`, `AudioCapacityError`, `AudioExtractionError`, `AudioTranscodeError`, `UnsupportedAudioFormatError`
+- Subprocess isolation for audio operations (crash protection)
+- `debug.py` module for structured logging across all steganography operations
+
+### Changed
+- Encode/Decode web pages now have a "Carrier Type" step to switch between Image and Audio
+- Version bumped to 4.3.0
+
 ## [4.1.5] - 2026-01-07
 
 ### Added
@@ -201,6 +220,7 @@ and this project adheres to [Semantic Versioning](https://semver.org).
 - CLI interface
 - Basic PIN authentication
 
+[4.3.0]: https://github.com/adlee-was-taken/stegasoo/compare/v4.2.1...v4.3.0
 [4.1.5]: https://github.com/adlee-was-taken/stegasoo/compare/v4.1.3...v4.1.5
 [4.1.3]: https://github.com/adlee-was-taken/stegasoo/compare/v4.1.0...v4.1.3
 [4.1.0]: https://github.com/adlee-was-taken/stegasoo/compare/v4.0.2...v4.1.0

CLAUDE.md

@@ -1,7 +1,7 @@
 # Stegasoo — Claude Code Project Guide
 
 Stegasoo is a secure steganography toolkit with hybrid photo + passphrase + PIN authentication.
-Version 4.2.1 · Python >=3.11 · MIT License
+Version 4.3.0 · Python >=3.11 · MIT License
 
 ## Quick commands
 
@@ -27,6 +27,10 @@ src/stegasoo/          Core library
   models.py              Dataclasses (EncodeResult, DecodeResult, etc.)
   encode.py / decode.py  High-level encode/decode orchestration
   channel.py             Channel key management (v4.0+)
+  audio_steganography.py  LSB audio embedding/extraction (v4.3.0)
+  spread_steganography.py Spread spectrum audio embedding (v4.3.0)
+  audio_utils.py          Audio format detection, validation, transcoding (v4.3.0)
+  debug.py                Structured logging for operations (v4.3.0)
   compression.py         Zstandard / zlib / lz4 payload compression
   cli.py                 Click CLI entry point
   generate.py            Credential generation (passphrase, PIN, RSA keys)

README.md

@@ -1,6 +1,6 @@
 # Stegasoo
 
-A secure steganography system for hiding encrypted messages in images using hybrid authentication.
+A secure steganography system for hiding encrypted messages in images and audio using hybrid authentication.
 
 [![Tests](https://github.com/adlee-was-taken/stegasoo/actions/workflows/test.yml/badge.svg)](https://github.com/adlee-was-taken/stegasoo/actions/workflows/test.yml)
 [![Lint](https://github.com/adlee-was-taken/stegasoo/actions/workflows/lint.yml/badge.svg)](https://github.com/adlee-was-taken/stegasoo/actions/workflows/lint.yml)
@@ -17,15 +17,25 @@ A secure steganography system for hiding encrypted messages in images using hybr
 - **Multiple interfaces**: CLI, Web UI, REST API
 - **File embedding**: Hide any file type (PDF, ZIP, documents)
 - **DCT steganography**: JPEG-resilient embedding for social media
+- **Audio steganography**: Hide messages in WAV, FLAC, MP3, OGG, AAC, M4A files (LSB and Spread Spectrum modes)
 - **Channel keys**: Private group communication channels
 
 ## Embedding Modes
 
+### Image Modes
+
 | Mode | Capacity (1080p) | JPEG Resilient | Best For |
 |------|------------------|----------------|----------|
 | **DCT** (default) | ~150 KB | Yes | Social media, messaging apps |
 | **LSB** | ~750 KB | No | Email, direct file transfer |
 
+### Audio Modes
+
+| Mode | Capacity (5 min WAV) | Noise Resistant | Best For |
+|------|---------------------|-----------------|----------|
+| **LSB** | ~1.3 MB | No | Direct file transfer |
+| **Spread Spectrum** | ~160 KB | Yes | Shared files, light processing |
+
 ## Web UI
 
 | Home | Encode | Decode | Generate |

RELEASE_NOTES.md

@@ -1,3 +1,45 @@
+# v4.3.0 — Audio Steganography
+
+**Release Date:** 2026-02-27
+
+## Highlights
+
+Stegasoo can now hide messages in audio files! This release adds full audio steganography support with two embedding modes:
+
+- **LSB (Least Significant Bit)**: Embeds data directly in audio sample LSBs. High capacity, best for direct file transfers.
+- **Spread Spectrum**: Spreads data across audio frequencies using pseudo-random sequences. Lower capacity but more resistant to noise and light processing.
+
+## What's New
+
+### Audio Steganography
+- Support for WAV, FLAC, MP3, OGG, AAC, and M4A input formats
+- Automatic transcoding to WAV (16-bit PCM) for embedding
+- Same security model: reference photo + passphrase + PIN/RSA + channel key
+- Full CLI, REST API, and Web UI support
+
+### Unified Web UI
+- Encode and Decode pages now feature a "Carrier Type" selector
+- Switch between Image and Audio modes without leaving the page
+- Audio capacity display shows LSB and Spread Spectrum capacities
+- Audio preview player on encode result page
+
+### New Modules
+- `audio_steganography.py` — LSB audio embedding/extraction
+- `spread_steganography.py` — Spread spectrum embedding/extraction
+- `audio_utils.py` — Audio format detection, validation, transcoding
+- `debug.py` — Structured logging for all operations
+
+## Upgrade Notes
+
+Audio steganography requires `numpy` and `soundfile` packages. Install with:
+```bash
+pip install stegasoo[audio]
+```
+
+For full audio format support (MP3, AAC, etc.), install FFmpeg on your system.
+
+---
+
 ## Stegasoo v4.2.1
 
 ### API Security

aur-api/PKGBUILD

@@ -1,6 +1,6 @@
 # Maintainer: Aaron D. Lee <your-email@example.com>
 pkgname=stegasoo-api-git
-pkgver=4.2.1
+pkgver=4.3.0
 pkgrel=1
 pkgdesc="Stegasoo REST API with TLS and API key authentication"
 arch=('x86_64')
@@ -30,7 +30,7 @@ sha256sums=('SKIP')
 pkgver() {
     cd "$pkgname"
     git describe --long --tags 2>/dev/null | sed 's/^v//;s/\([^-]*-g\)/r\1/;s/-/./g' || \
-    printf "%s.r%s.g%s" "4.2.1" "$(git rev-list --count HEAD)" "$(git rev-parse --short HEAD)"
+    printf "%s.r%s.g%s" "4.3.0" "$(git rev-list --count HEAD)" "$(git rev-parse --short HEAD)"
 }
 
 build() {

aur-cli/PKGBUILD

@@ -1,6 +1,6 @@
 # Maintainer: Aaron D. Lee <your-email@example.com>
 pkgname=stegasoo-cli-git
-pkgver=4.2.1
+pkgver=4.3.0
 pkgrel=1
 pkgdesc="Secure steganography CLI with hybrid photo + passphrase + PIN authentication"
 arch=('x86_64')
@@ -29,7 +29,7 @@ sha256sums=('SKIP')
 pkgver() {
     cd "$pkgname"
     git describe --long --tags 2>/dev/null | sed 's/^v//;s/\([^-]*-g\)/r\1/;s/-/./g' || \
-    printf "%s.r%s.g%s" "4.2.1" "$(git rev-list --count HEAD)" "$(git rev-parse --short HEAD)"
+    printf "%s.r%s.g%s" "4.3.0" "$(git rev-list --count HEAD)" "$(git rev-parse --short HEAD)"
 }
 
 build() {

aur/PKGBUILD

@@ -1,6 +1,6 @@
 # Maintainer: Aaron D. Lee <your-email@example.com>
 pkgname=stegasoo-git
-pkgver=4.2.1
+pkgver=4.3.0
 pkgrel=1
 pkgdesc="Secure steganography with hybrid photo + passphrase + PIN authentication"
 arch=('x86_64')
@@ -27,7 +27,7 @@ sha256sums=('SKIP')
 pkgver() {
     cd "$pkgname"
     git describe --long --tags 2>/dev/null | sed 's/^v//;s/\([^-]*-g\)/r\1/;s/-/./g' || \
-    printf "%s.r%s.g%s" "4.2.1" "$(git rev-list --count HEAD)" "$(git rev-parse --short HEAD)"
+    printf "%s.r%s.g%s" "4.3.0" "$(git rev-list --count HEAD)" "$(git rev-parse --short HEAD)"
 }
 
 build() {

docs/stegasoo.1

@@ -1,6 +1,6 @@
 .\" Stegasoo man page
 .\" Generate with: groff -man -Tascii stegasoo.1
-.TH STEGASOO 1 "January 2026" "Stegasoo 4.1.7" "User Commands"
+.TH STEGASOO 1 "February 2026" "Stegasoo 4.3.0" "User Commands"
 .SH NAME
 stegasoo \- steganography with hybrid authentication
 .SH SYNOPSIS
@@ -12,9 +12,10 @@ stegasoo \- steganography with hybrid authentication
 [\fIargs\fR]
 .SH DESCRIPTION
 .B stegasoo
-hides messages and files in images using PIN + passphrase security.
+hides messages and files in images and audio using PIN + passphrase security.
 It uses LSB (Least Significant Bit) steganography with optional DCT
-(Discrete Cosine Transform) encoding for JPEG resilience.
+(Discrete Cosine Transform) encoding for JPEG resilience, and supports
+audio steganography with LSB and Spread Spectrum modes.
 .PP
 Messages are encrypted using a hybrid authentication scheme that combines
 a reference photo (shared secret), passphrase, and PIN code.
@@ -221,6 +222,83 @@ Reset admin password using recovery key.
 .PP
 Options: \fB\-\-db\fR \fIPATH\fR (path to stegasoo.db), \fB\-\-password\fR \fITEXT\fR.
 .RE
+.SS audio\-encode
+Encode a message or file into an audio file.
+.PP
+.B stegasoo audio\-encode
+.I audio
+.B \-r
+.I reference
+[\fB\-m\fR \fImessage\fR | \fB\-f\fR \fIfile\fR]
+[\fIoptions\fR]
+.TP
+.BR \-r ", " \-\-reference " " \fIPATH\fR
+Reference photo (shared secret). Required.
+.TP
+.BR \-m ", " \-\-message " " \fITEXT\fR
+Message to encode.
+.TP
+.BR \-f ", " \-\-file " " \fIPATH\fR
+File to embed instead of message.
+.TP
+.BR \-o ", " \-\-output " " \fIPATH\fR
+Output audio path.
+.TP
+.B \-\-passphrase " " \fITEXT\fR
+Passphrase (recommend 4+ words). Prompts if not provided.
+.TP
+.B \-\-pin " " \fITEXT\fR
+PIN code. Prompts if not provided.
+.TP
+.B \-\-mode " " [\fIlsb\fR|\fIspread\fR]
+Embedding mode: lsb (default) or spread (spread spectrum).
+.PP
+.B Examples:
+.nf
+    stegasoo audio-encode song.wav -r ref.jpg -m "Secret" --passphrase --pin
+    stegasoo audio-encode podcast.mp3 -r ref.jpg -f doc.pdf --mode spread
+.fi
+.SS audio\-decode
+Decode a message or file from a stego audio file.
+.PP
+.B stegasoo audio\-decode
+.I audio
+.B \-r
+.I reference
+[\fIoptions\fR]
+.TP
+.BR \-r ", " \-\-reference " " \fIPATH\fR
+Reference photo (shared secret). Required.
+.TP
+.B \-\-passphrase " " \fITEXT\fR
+Passphrase. Prompts if not provided.
+.TP
+.B \-\-pin " " \fITEXT\fR
+PIN code. Prompts if not provided.
+.TP
+.BR \-o ", " \-\-output " " \fIPATH\fR
+Output path for file payloads.
+.PP
+.B Examples:
+.nf
+    stegasoo audio-decode stego.wav -r ref.jpg --passphrase --pin
+    stegasoo audio-decode stego.wav -r ref.jpg -o ./extracted/
+.fi
+.SS audio\-info
+Display audio file information and steganographic capacity.
+.PP
+.B stegasoo audio\-info
+.I audio
+[\fB\-\-json\fR]
+.PP
+Shows format, sample rate, channels, bit depth, duration, and embedding
+capacity for both LSB and Spread Spectrum modes.
+.PP
+.B Examples:
+.nf
+    stegasoo audio-info song.wav
+    stegasoo audio-info podcast.mp3 --json
+.fi
 .SS tools
 Image security tools.
 .PP

frontends/api/auth.py

@@ -17,9 +17,8 @@ import json
 import os
 import secrets
 from pathlib import Path
-from typing import Optional
 
-from fastapi import Depends, HTTPException, Security
+from fastapi import HTTPException, Security
 from fastapi.security import APIKeyHeader
 
 # API key header name
@@ -55,7 +54,7 @@ def _load_keys(location: str = "user") -> dict:
         try:
             with open(keys_file) as f:
                 return json.load(f)
-        except (json.JSONDecodeError, IOError):
+        except (OSError, json.JSONDecodeError):
             return {"keys": [], "enabled": True}
     return {"keys": [], "enabled": True}
 
@@ -101,11 +100,13 @@ def add_api_key(name: str, location: str = "user") -> str:
         if existing["name"] == name:
             raise ValueError(f"Key with name '{name}' already exists")
 
-    data["keys"].append({
+    data["keys"].append(
+        {
             "name": name,
             "hash": key_hash,
             "created": __import__("datetime").datetime.now().isoformat(),
-    })
+        }
+    )
 
     _save_keys(data, location)
 
@@ -204,12 +205,12 @@ def get_api_key_status() -> dict:
         "keys": {
             "user": user_keys,
             "project": project_keys,
-        }
+        },
     }
 
 
 # FastAPI dependency for API key authentication
-async def require_api_key(api_key: Optional[str] = Security(API_KEY_HEADER)) -> str:
+async def require_api_key(api_key: str | None = Security(API_KEY_HEADER)) -> str:
     """
     FastAPI dependency that requires a valid API key.
 
@@ -243,7 +244,7 @@ async def require_api_key(api_key: Optional[str] = Security(API_KEY_HEADER)) ->
     return api_key
 
 
-async def optional_api_key(api_key: Optional[str] = Security(API_KEY_HEADER)) -> Optional[str]:
+async def optional_api_key(api_key: str | None = Security(API_KEY_HEADER)) -> str | None:
     """
     FastAPI dependency that optionally validates API key.
 

frontends/api/main.py

@@ -1,10 +1,15 @@
 #!/usr/bin/env python3
 """
-Stegasoo REST API (v4.2.1)
+Stegasoo REST API (v4.3.0)
 
 FastAPI-based REST API for steganography operations.
 Supports both text messages and file embedding.
 
+CHANGES in v4.3.0:
+- Audio steganography endpoints (/audio/*)
+- LSB and spread spectrum (DSSS) audio embedding modes
+- Audio info and capacity checking
+
 CHANGES in v4.2.1:
 - API key authentication (X-API-Key header)
 - TLS support with self-signed certificates
@@ -32,11 +37,31 @@ NEW in v3.0.1: DCT color mode and JPEG output format.
 
 import asyncio
 import base64
+import logging
+import os
 import sys
 from functools import partial
 from pathlib import Path
 from typing import Literal
 
+# Configure logging for API frontend
+_log_level = os.environ.get("STEGASOO_LOG_LEVEL", "").strip().upper()
+if _log_level and hasattr(logging, _log_level):
+    logging.basicConfig(
+        level=getattr(logging, _log_level),
+        format="[%(asctime)s.%(msecs)03d] [%(levelname)s] [%(name)s] %(message)s",
+        datefmt="%H:%M:%S",
+        stream=sys.stderr,
+    )
+elif os.environ.get("STEGASOO_DEBUG", "").strip() in ("1", "true", "yes"):
+    logging.basicConfig(
+        level=logging.DEBUG,
+        format="[%(asctime)s.%(msecs)03d] [%(levelname)s] [%(name)s] %(message)s",
+        datefmt="%H:%M:%S",
+        stream=sys.stderr,
+    )
+api_logger = logging.getLogger("stegasoo.api")
+
 from fastapi import Depends, FastAPI, File, Form, HTTPException, Query, UploadFile
 from fastapi.responses import JSONResponse, Response
 from pydantic import BaseModel, Field
@@ -44,28 +69,28 @@ from pydantic import BaseModel, Field
 # API Key Authentication
 try:
     from .auth import (
-        require_api_key,
-        get_api_key_status,
         add_api_key,
-        remove_api_key,
-        list_api_keys,
+        get_api_key_status,
         is_auth_enabled,
+        list_api_keys,
+        remove_api_key,
+        require_api_key,
     )
 except ImportError:
     # When running directly (not as package)
     from auth import (
-        require_api_key,
-        get_api_key_status,
         add_api_key,
-        remove_api_key,
+        get_api_key_status,
         list_api_keys,
-        is_auth_enabled,
+        remove_api_key,
+        require_api_key,
     )
 
 # Add parent to path for development
 sys.path.insert(0, str(Path(__file__).parent.parent.parent / "src"))
 
 from stegasoo import (
+    HAS_AUDIO_SUPPORT,
     MAX_FILE_PAYLOAD_SIZE,
     CapacityError,
     DecryptionError,
@@ -87,6 +112,12 @@ from stegasoo import (
     validate_image,
     will_fit_by_mode,
 )
+
+# Audio steganography (v4.3.0) - conditionally imported
+if HAS_AUDIO_SUPPORT:
+    from stegasoo import decode_audio, encode_audio, get_audio_info
+    from stegasoo.audio_steganography import calculate_audio_lsb_capacity
+    from stegasoo.spread_steganography import calculate_audio_spread_capacity
 from stegasoo.constants import (
     DEFAULT_PASSPHRASE_WORDS,
     MAX_PASSPHRASE_WORDS,
@@ -163,6 +194,8 @@ EmbedModeType = Literal["lsb", "dct"]
 ExtractModeType = Literal["auto", "lsb", "dct"]
 DctColorModeType = Literal["grayscale", "color"]
 DctOutputFormatType = Literal["png", "jpeg"]
+AudioEmbedModeType = Literal["audio_lsb", "audio_spread"]
+AudioExtractModeType = Literal["audio_auto", "audio_lsb", "audio_spread"]
 
 
 # ============================================================================
@@ -405,6 +438,7 @@ class ModesResponse(BaseModel):
 
     lsb: dict
     dct: DctModeInfo
+    audio: dict | None = Field(default=None, description="Audio steganography modes (v4.3.0)")
     # Channel key status (v4.0.0)
     channel: dict | None = Field(default=None, description="Channel key status (v4.0.0)")
 
@@ -415,6 +449,7 @@ class StatusResponse(BaseModel):
     has_qrcode_read: bool
     has_qrcode_write: bool  # v4.2.0: QR generation capability
     has_dct: bool
+    has_audio: bool = Field(default=False, description="Audio steganography support (v4.3.0)")
     max_payload_kb: int
     available_modes: list[str]
     dct_features: dict | None = Field(default=None, description="DCT mode features (v3.0.1+)")
@@ -479,6 +514,124 @@ class ErrorResponse(BaseModel):
     detail: str | None = None
 
 
+# --- Audio models (v4.3.0) ---
+
+
+class AudioEncodeRequest(BaseModel):
+    """Request to encode a text message into audio."""
+
+    message: str
+    reference_photo_base64: str
+    carrier_audio_base64: str
+    passphrase: str = Field(description="Passphrase for key derivation")
+    pin: str = ""
+    rsa_key_base64: str | None = None
+    rsa_password: str | None = None
+    channel_key: str | None = Field(
+        default=None,
+        description="Channel key for deployment isolation. null=auto, ''=public, 'XXXX-...'=explicit",
+    )
+    embed_mode: AudioEmbedModeType = Field(
+        default="audio_lsb",
+        description="Embedding mode: 'audio_lsb' (default) or 'audio_spread' (DSSS)",
+    )
+    chip_tier: int | None = Field(
+        default=None,
+        description="Spread spectrum chip tier: 0=lossless(256), 1=high_lossy(512), 2=low_lossy(1024). Only for audio_spread.",
+    )
+
+
+class AudioEncodeFileRequest(BaseModel):
+    """Request to encode a file into audio."""
+
+    file_data_base64: str
+    filename: str
+    mime_type: str | None = None
+    reference_photo_base64: str
+    carrier_audio_base64: str
+    passphrase: str = Field(description="Passphrase for key derivation")
+    pin: str = ""
+    rsa_key_base64: str | None = None
+    rsa_password: str | None = None
+    channel_key: str | None = Field(
+        default=None,
+        description="Channel key for deployment isolation. null=auto, ''=public, 'XXXX-...'=explicit",
+    )
+    embed_mode: AudioEmbedModeType = Field(
+        default="audio_lsb",
+        description="Embedding mode: 'audio_lsb' (default) or 'audio_spread' (DSSS)",
+    )
+    chip_tier: int | None = Field(
+        default=None,
+        description="Spread spectrum chip tier: 0=lossless(256), 1=high_lossy(512), 2=low_lossy(1024). Only for audio_spread.",
+    )
+
+
+class AudioEncodeResponse(BaseModel):
+    """Response from audio encode operations."""
+
+    stego_audio_base64: str
+    embed_mode: str = Field(description="Embedding mode used: 'audio_lsb' or 'audio_spread'")
+    stats: dict = Field(description="Embedding statistics (samples_modified, capacity_used, etc.)")
+    channel_mode: str = Field(default="public", description="Channel mode: 'public' or 'private'")
+    channel_fingerprint: str | None = Field(
+        default=None, description="Channel key fingerprint (if private mode)"
+    )
+
+
+class AudioDecodeRequest(BaseModel):
+    """Request to decode a message or file from stego audio."""
+
+    stego_audio_base64: str
+    reference_photo_base64: str
+    passphrase: str = Field(description="Passphrase for key derivation")
+    pin: str = ""
+    rsa_key_base64: str | None = None
+    rsa_password: str | None = None
+    channel_key: str | None = Field(
+        default=None,
+        description="Channel key for decryption. null=auto, ''=public, 'XXXX-...'=explicit",
+    )
+    embed_mode: AudioExtractModeType = Field(
+        default="audio_auto",
+        description="Extraction mode: 'audio_auto' (default), 'audio_lsb', or 'audio_spread'",
+    )
+
+
+class AudioInfoResponse(BaseModel):
+    """Response with audio file metadata and capacity info."""
+
+    sample_rate: int
+    channels: int
+    duration_seconds: float
+    num_samples: int
+    format: str
+    bit_depth: int | None = None
+    bitrate: int | None = None
+    capacity_lsb: int = Field(description="LSB mode capacity in bytes")
+    capacity_spread: int = Field(description="Spread spectrum mode capacity in bytes")
+
+
+class AudioCapacityRequest(BaseModel):
+    """Request to check if a payload fits in audio carrier."""
+
+    carrier_audio_base64: str
+    payload_size: int = Field(ge=1, description="Payload size in bytes")
+    embed_mode: AudioEmbedModeType = Field(
+        default="audio_lsb", description="Embedding mode to check capacity for"
+    )
+
+
+class AudioCapacityResponse(BaseModel):
+    """Response for audio capacity check."""
+
+    fits: bool
+    payload_size: int
+    capacity_bytes: int
+    usage_percent: float
+    embed_mode: str
+
+
 # ============================================================================
 # HELPER: RESOLVE CHANNEL KEY
 # ============================================================================
@@ -569,12 +722,18 @@ async def root():
         "source": channel_status.get("source"),
     }
 
+    # Audio modes (v4.3.0)
+    if HAS_AUDIO_SUPPORT:
+        available_modes.append("audio_lsb")
+        available_modes.append("audio_spread")
+
     return StatusResponse(
         version=__version__,
         has_argon2=has_argon2(),
         has_qrcode_read=HAS_QR_READ,
         has_qrcode_write=HAS_QR_WRITE,
         has_dct=has_dct_support(),
+        has_audio=HAS_AUDIO_SUPPORT,
         max_payload_kb=MAX_FILE_PAYLOAD_SIZE // 1024,
         available_modes=available_modes,
         dct_features=dct_features,
@@ -606,6 +765,28 @@ async def api_modes():
         "fingerprint": channel_status.get("fingerprint"),
     }
 
+    # Audio modes (v4.3.0)
+    audio_info = None
+    if HAS_AUDIO_SUPPORT:
+        audio_info = {
+            "available": True,
+            "modes": {
+                "audio_lsb": {
+                    "name": "Audio LSB",
+                    "description": "Embed in audio sample LSBs, high capacity",
+                    "output_format": "WAV",
+                },
+                "audio_spread": {
+                    "name": "Spread Spectrum (DSSS)",
+                    "description": "Direct-sequence spread spectrum with Reed-Solomon ECC, better stealth",
+                    "output_format": "WAV",
+                },
+            },
+            "supported_formats": ["WAV", "FLAC", "MP3", "OGG", "AAC", "M4A"],
+            "output_format": "WAV",
+            "requires": "soundfile",
+        }
+
     return ModesResponse(
         lsb={
             "available": True,
@@ -623,6 +804,7 @@ async def api_modes():
             capacity_ratio="~20% of LSB",
             requires="scipy",
         ),
+        audio=audio_info,
         channel=channel_info,
     )
 
@@ -723,7 +905,7 @@ async def api_channel_set(request: ChannelSetRequest, _: str = Depends(require_a
 @app.delete("/channel")
 async def api_channel_clear(
     _: str = Depends(require_api_key),
-    location: str = Query("user", description="'user', 'project', or 'all'")
+    location: str = Query("user", description="'user', 'project', or 'all'"),
 ):
     """
     Clear/remove channel key from config.
@@ -935,7 +1117,7 @@ async def api_will_fit(request: WillFitRequest, _: str = Depends(require_api_key
 @app.post("/extract-key-from-qr", response_model=QrExtractResponse)
 async def api_extract_key_from_qr(
     _: str = Depends(require_api_key),
-    qr_image: UploadFile = File(..., description="QR code image containing RSA key")
+    qr_image: UploadFile = File(..., description="QR code image containing RSA key"),
 ):
     """
     Extract RSA key from a QR code image.
@@ -1607,6 +1789,454 @@ async def api_image_info(
         raise HTTPException(500, str(e))
 
 
+# ============================================================================
+# ROUTES - AUDIO STEGANOGRAPHY (v4.3.0)
+# ============================================================================
+
+
+def _require_audio():
+    """Check that audio support is available, raise 501 if not."""
+    if not HAS_AUDIO_SUPPORT:
+        raise HTTPException(
+            501, "Audio steganography not available. Install with: pip install stegasoo[audio]"
+        )
+
+
+@app.post("/audio/encode", response_model=AudioEncodeResponse)
+async def api_audio_encode(request: AudioEncodeRequest, _: str = Depends(require_api_key)):
+    """
+    Encode a text message into audio.
+
+    Audio must be base64-encoded. Returns base64-encoded stego WAV.
+
+    v4.3.0: New endpoint for audio steganography.
+    """
+    _require_audio()
+
+    resolved_channel_key = _resolve_channel_key(request.channel_key)
+
+    try:
+        ref_photo = base64.b64decode(request.reference_photo_base64)
+        carrier = base64.b64decode(request.carrier_audio_base64)
+        rsa_key = base64.b64decode(request.rsa_key_base64) if request.rsa_key_base64 else None
+
+        stego_audio, stats = await run_in_thread(
+            encode_audio,
+            message=request.message,
+            reference_photo=ref_photo,
+            carrier_audio=carrier,
+            passphrase=request.passphrase,
+            pin=request.pin,
+            rsa_key_data=rsa_key,
+            rsa_password=request.rsa_password,
+            embed_mode=request.embed_mode,
+            channel_key=resolved_channel_key,
+            chip_tier=request.chip_tier,
+        )
+
+        stego_b64 = base64.b64encode(stego_audio).decode("utf-8")
+        channel_mode, channel_fingerprint = _get_channel_info(resolved_channel_key)
+
+        return AudioEncodeResponse(
+            stego_audio_base64=stego_b64,
+            embed_mode=stats.embed_mode,
+            stats={
+                "samples_modified": stats.samples_modified,
+                "total_samples": stats.total_samples,
+                "capacity_used": round(stats.capacity_used * 100, 1),
+                "bytes_embedded": stats.bytes_embedded,
+                "sample_rate": stats.sample_rate,
+                "channels": stats.channels,
+                "duration_seconds": round(stats.duration_seconds, 2),
+            },
+            channel_mode=channel_mode,
+            channel_fingerprint=channel_fingerprint,
+        )
+
+    except CapacityError as e:
+        raise HTTPException(400, str(e))
+    except StegasooError as e:
+        raise HTTPException(400, str(e))
+    except Exception as e:
+        raise HTTPException(500, str(e))
+
+
+@app.post("/audio/encode/file", response_model=AudioEncodeResponse)
+async def api_audio_encode_file(request: AudioEncodeFileRequest, _: str = Depends(require_api_key)):
+    """
+    Encode a file into audio (JSON with base64).
+
+    v4.3.0: New endpoint for audio steganography.
+    """
+    _require_audio()
+
+    resolved_channel_key = _resolve_channel_key(request.channel_key)
+
+    try:
+        file_data = base64.b64decode(request.file_data_base64)
+        ref_photo = base64.b64decode(request.reference_photo_base64)
+        carrier = base64.b64decode(request.carrier_audio_base64)
+        rsa_key = base64.b64decode(request.rsa_key_base64) if request.rsa_key_base64 else None
+
+        payload = FilePayload(
+            data=file_data, filename=request.filename, mime_type=request.mime_type
+        )
+
+        stego_audio, stats = await run_in_thread(
+            encode_audio,
+            message=payload,
+            reference_photo=ref_photo,
+            carrier_audio=carrier,
+            passphrase=request.passphrase,
+            pin=request.pin,
+            rsa_key_data=rsa_key,
+            rsa_password=request.rsa_password,
+            embed_mode=request.embed_mode,
+            channel_key=resolved_channel_key,
+            chip_tier=request.chip_tier,
+        )
+
+        stego_b64 = base64.b64encode(stego_audio).decode("utf-8")
+        channel_mode, channel_fingerprint = _get_channel_info(resolved_channel_key)
+
+        return AudioEncodeResponse(
+            stego_audio_base64=stego_b64,
+            embed_mode=stats.embed_mode,
+            stats={
+                "samples_modified": stats.samples_modified,
+                "total_samples": stats.total_samples,
+                "capacity_used": round(stats.capacity_used * 100, 1),
+                "bytes_embedded": stats.bytes_embedded,
+                "sample_rate": stats.sample_rate,
+                "channels": stats.channels,
+                "duration_seconds": round(stats.duration_seconds, 2),
+            },
+            channel_mode=channel_mode,
+            channel_fingerprint=channel_fingerprint,
+        )
+
+    except CapacityError as e:
+        raise HTTPException(400, str(e))
+    except StegasooError as e:
+        raise HTTPException(400, str(e))
+    except Exception as e:
+        raise HTTPException(500, str(e))
+
+
+@app.post("/audio/encode/multipart")
+async def api_audio_encode_multipart(
+    _: str = Depends(require_api_key),
+    passphrase: str = Form(..., description="Passphrase for key derivation"),
+    reference_photo: UploadFile = File(...),
+    carrier: UploadFile = File(...),
+    message: str = Form(""),
+    payload_file: UploadFile | None = File(None),
+    pin: str = Form(""),
+    rsa_key: UploadFile | None = File(None),
+    rsa_password: str = Form(""),
+    channel_key: str = Form(
+        "auto", description="Channel key: 'auto'=server config, 'none'=public, 'XXXX-...'=explicit"
+    ),
+    embed_mode: str = Form("audio_lsb"),
+    chip_tier: int | None = Form(
+        None,
+        description="Spread spectrum chip tier: 0=lossless, 1=high_lossy, 2=low_lossy. Only for audio_spread.",
+    ),
+):
+    """
+    Encode audio using multipart form data (file uploads).
+
+    Provide either 'message' (text) or 'payload_file' (binary file).
+    Returns the stego WAV directly with metadata headers.
+
+    v4.3.0: New endpoint for audio steganography.
+    """
+    _require_audio()
+
+    if embed_mode not in ("audio_lsb", "audio_spread"):
+        raise HTTPException(400, "embed_mode must be 'audio_lsb' or 'audio_spread'")
+
+    # Resolve channel key
+    if channel_key.lower() == "auto":
+        resolved_channel_key = None
+    elif channel_key.lower() == "none":
+        resolved_channel_key = ""
+    else:
+        resolved_channel_key = _resolve_channel_key(channel_key)
+
+    try:
+        ref_data = await reference_photo.read()
+        carrier_data = await carrier.read()
+
+        rsa_key_data = None
+        if rsa_key and rsa_key.filename:
+            rsa_key_data = await rsa_key.read()
+
+        effective_password = rsa_password if rsa_password else None
+
+        # Determine payload
+        if payload_file and payload_file.filename:
+            file_data = await payload_file.read()
+            payload = FilePayload(
+                data=file_data, filename=payload_file.filename, mime_type=payload_file.content_type
+            )
+        elif message:
+            payload = message
+        else:
+            raise HTTPException(400, "Must provide either 'message' or 'payload_file'")
+
+        stego_audio, stats = await run_in_thread(
+            encode_audio,
+            message=payload,
+            reference_photo=ref_data,
+            carrier_audio=carrier_data,
+            passphrase=passphrase,
+            pin=pin,
+            rsa_key_data=rsa_key_data,
+            rsa_password=effective_password,
+            embed_mode=embed_mode,
+            channel_key=resolved_channel_key,
+            chip_tier=chip_tier,
+        )
+
+        channel_mode, channel_fingerprint = _get_channel_info(resolved_channel_key)
+
+        headers = {
+            "Content-Disposition": "attachment; filename=stego_audio.wav",
+            "X-Stegasoo-Embed-Mode": stats.embed_mode,
+            "X-Stegasoo-Capacity-Percent": f"{stats.capacity_used * 100:.1f}",
+            "X-Stegasoo-Samples-Modified": str(stats.samples_modified),
+            "X-Stegasoo-Duration": f"{stats.duration_seconds:.2f}",
+            "X-Stegasoo-Channel-Mode": channel_mode,
+            "X-Stegasoo-Version": __version__,
+        }
+        if channel_fingerprint:
+            headers["X-Stegasoo-Channel-Fingerprint"] = channel_fingerprint
+
+        return Response(
+            content=stego_audio,
+            media_type="audio/wav",
+            headers=headers,
+        )
+
+    except CapacityError as e:
+        raise HTTPException(400, str(e))
+    except StegasooError as e:
+        raise HTTPException(400, str(e))
+    except HTTPException:
+        raise
+    except Exception as e:
+        raise HTTPException(500, str(e))
+
+
+@app.post("/audio/decode", response_model=DecodeResponse)
+async def api_audio_decode(request: AudioDecodeRequest, _: str = Depends(require_api_key)):
+    """
+    Decode a message or file from stego audio.
+
+    Returns payload_type to indicate if result is text or file.
+
+    v4.3.0: New endpoint for audio steganography.
+    """
+    _require_audio()
+
+    resolved_channel_key = _resolve_channel_key(request.channel_key)
+
+    try:
+        stego = base64.b64decode(request.stego_audio_base64)
+        ref_photo = base64.b64decode(request.reference_photo_base64)
+        rsa_key = base64.b64decode(request.rsa_key_base64) if request.rsa_key_base64 else None
+
+        result = await run_in_thread(
+            decode_audio,
+            stego_audio=stego,
+            reference_photo=ref_photo,
+            passphrase=request.passphrase,
+            pin=request.pin,
+            rsa_key_data=rsa_key,
+            rsa_password=request.rsa_password,
+            embed_mode=request.embed_mode,
+            channel_key=resolved_channel_key,
+        )
+
+        if result.is_file:
+            return DecodeResponse(
+                payload_type="file",
+                file_data_base64=base64.b64encode(result.file_data).decode("utf-8"),
+                filename=result.filename,
+                mime_type=result.mime_type,
+            )
+        else:
+            return DecodeResponse(payload_type="text", message=result.message)
+
+    except DecryptionError as e:
+        error_msg = str(e)
+        if "channel key" in error_msg.lower():
+            raise HTTPException(401, error_msg)
+        raise HTTPException(401, "Decryption failed. Check credentials.")
+    except StegasooError as e:
+        raise HTTPException(400, str(e))
+    except Exception as e:
+        raise HTTPException(500, str(e))
+
+
+@app.post("/audio/decode/multipart", response_model=DecodeResponse)
+async def api_audio_decode_multipart(
+    _: str = Depends(require_api_key),
+    passphrase: str = Form(..., description="Passphrase for key derivation"),
+    reference_photo: UploadFile = File(...),
+    stego_audio: UploadFile = File(...),
+    pin: str = Form(""),
+    rsa_key: UploadFile | None = File(None),
+    rsa_password: str = Form(""),
+    channel_key: str = Form(
+        "auto", description="Channel key: 'auto'=server config, 'none'=public, 'XXXX-...'=explicit"
+    ),
+    embed_mode: str = Form("audio_auto"),
+):
+    """
+    Decode audio using multipart form data (file uploads).
+
+    Returns JSON with payload_type indicating text or file.
+
+    v4.3.0: New endpoint for audio steganography.
+    """
+    _require_audio()
+
+    if embed_mode not in ("audio_auto", "audio_lsb", "audio_spread"):
+        raise HTTPException(400, "embed_mode must be 'audio_auto', 'audio_lsb', or 'audio_spread'")
+
+    # Resolve channel key
+    if channel_key.lower() == "auto":
+        resolved_channel_key = None
+    elif channel_key.lower() == "none":
+        resolved_channel_key = ""
+    else:
+        resolved_channel_key = _resolve_channel_key(channel_key)
+
+    try:
+        ref_data = await reference_photo.read()
+        stego_data = await stego_audio.read()
+
+        rsa_key_data = None
+        if rsa_key and rsa_key.filename:
+            rsa_key_data = await rsa_key.read()
+
+        effective_password = rsa_password if rsa_password else None
+
+        result = await run_in_thread(
+            decode_audio,
+            stego_audio=stego_data,
+            reference_photo=ref_data,
+            passphrase=passphrase,
+            pin=pin,
+            rsa_key_data=rsa_key_data,
+            rsa_password=effective_password,
+            embed_mode=embed_mode,
+            channel_key=resolved_channel_key,
+        )
+
+        if result.is_file:
+            return DecodeResponse(
+                payload_type="file",
+                file_data_base64=base64.b64encode(result.file_data).decode("utf-8"),
+                filename=result.filename,
+                mime_type=result.mime_type,
+            )
+        else:
+            return DecodeResponse(payload_type="text", message=result.message)
+
+    except DecryptionError as e:
+        error_msg = str(e)
+        if "channel key" in error_msg.lower():
+            raise HTTPException(401, error_msg)
+        raise HTTPException(401, "Decryption failed. Check credentials.")
+    except StegasooError as e:
+        raise HTTPException(400, str(e))
+    except HTTPException:
+        raise
+    except Exception as e:
+        raise HTTPException(500, str(e))
+
+
+@app.post("/audio/info", response_model=AudioInfoResponse)
+async def api_audio_info(
+    _: str = Depends(require_api_key),
+    audio: UploadFile = File(...),
+):
+    """
+    Get audio file metadata and embedding capacity.
+
+    v4.3.0: New endpoint for audio steganography.
+    """
+    _require_audio()
+
+    try:
+        audio_data = await audio.read()
+
+        info = await run_in_thread(get_audio_info, audio_data)
+
+        # Calculate capacities for both modes
+        lsb_capacity = await run_in_thread(calculate_audio_lsb_capacity, audio_data)
+        try:
+            spread_info = await run_in_thread(calculate_audio_spread_capacity, audio_data)
+            spread_capacity = spread_info.usable_capacity_bytes
+        except Exception:
+            spread_capacity = 0
+
+        return AudioInfoResponse(
+            sample_rate=info.sample_rate,
+            channels=info.channels,
+            duration_seconds=round(info.duration_seconds, 2),
+            num_samples=info.num_samples,
+            format=info.format,
+            bit_depth=info.bit_depth,
+            bitrate=info.bitrate,
+            capacity_lsb=lsb_capacity,
+            capacity_spread=spread_capacity,
+        )
+
+    except StegasooError as e:
+        raise HTTPException(400, str(e))
+    except Exception as e:
+        raise HTTPException(500, str(e))
+
+
+@app.post("/audio/capacity", response_model=AudioCapacityResponse)
+async def api_audio_capacity(request: AudioCapacityRequest, _: str = Depends(require_api_key)):
+    """
+    Check if a payload of a given size will fit in an audio carrier.
+
+    v4.3.0: New endpoint for audio steganography.
+    """
+    _require_audio()
+
+    try:
+        carrier = base64.b64decode(request.carrier_audio_base64)
+
+        if request.embed_mode == "audio_lsb":
+            capacity = await run_in_thread(calculate_audio_lsb_capacity, carrier)
+        else:
+            spread_info = await run_in_thread(calculate_audio_spread_capacity, carrier)
+            capacity = spread_info.usable_capacity_bytes
+
+        fits = request.payload_size <= capacity
+        usage = (request.payload_size / capacity * 100) if capacity > 0 else 100.0
+
+        return AudioCapacityResponse(
+            fits=fits,
+            payload_size=request.payload_size,
+            capacity_bytes=capacity,
+            usage_percent=round(usage, 1),
+            embed_mode=request.embed_mode,
+        )
+
+    except StegasooError as e:
+        raise HTTPException(400, str(e))
+    except Exception as e:
+        raise HTTPException(500, str(e))
+
+
 # ============================================================================
 # ERROR HANDLERS
 # ============================================================================

frontends/cli/main.py

@@ -361,7 +361,7 @@ def generate(pin, rsa, pin_length, rsa_bits, words, output, password, as_json, q
 
                     qr_bytes = generate_qr_code(creds.rsa_key_pem, compress=True, output_format=fmt)
                     qr_path.write_bytes(qr_bytes)
-                    click.secho(f"─── RSA KEY QR CODE ───", fg="green")
+                    click.secho("─── RSA KEY QR CODE ───", fg="green")
                     click.secho(f"    Saved to: {qr}", fg="bright_white")
                     click.secho("    ⚠️  Contains unencrypted private key!", fg="yellow")
                     click.echo()

frontends/web/app.py

@@ -146,6 +146,7 @@ sys.path.insert(0, str(Path(__file__).parent.parent.parent / "src"))
 
 import stegasoo
 from stegasoo import (
+    HAS_AUDIO_SUPPORT,
     CapacityError,
     DecryptionError,
     FilePayload,
@@ -463,6 +464,9 @@ def inject_globals():
         "is_admin": is_admin(),
         # NEW in v4.2.0 - Saved channel keys
         "saved_channel_keys": saved_channel_keys,
+        # NEW in v4.3.0 - Audio support
+        "has_audio": HAS_AUDIO_SUPPORT,
+        "supported_audio_formats": "WAV, FLAC, MP3, OGG, AAC, M4A" if HAS_AUDIO_SUPPORT else "",
     }
 
 
@@ -564,6 +568,14 @@ def allowed_image(filename: str) -> bool:
     return ext in {"png", "jpg", "jpeg", "bmp", "gif"}
 
 
+def allowed_audio(filename: str) -> bool:
+    """Check if file has allowed audio extension."""
+    if not filename or "." not in filename:
+        return False
+    ext = filename.rsplit(".", 1)[1].lower()
+    return ext in {"wav", "flac", "mp3", "ogg", "aac", "m4a", "aiff", "aif"}
+
+
 def format_size(size_bytes: int) -> str:
     """Format file size for display."""
     if size_bytes < 1024:
@@ -710,11 +722,15 @@ def generate():
                 if not qr_too_large:
                     qr_token = secrets.token_urlsafe(16)
                     cleanup_temp_files()
-                    temp_storage.save_temp_file(qr_token, creds.rsa_key_pem.encode(), {
+                    temp_storage.save_temp_file(
+                        qr_token,
+                        creds.rsa_key_pem.encode(),
+                        {
                             "filename": "rsa_key.pem",
                             "type": "rsa_key",
                             "compress": qr_needs_compression,
-                    })
+                        },
+                    )
 
             # v3.2.0: Single passphrase instead of daily phrases
             return render_template(
@@ -1001,6 +1017,37 @@ def api_check_fit():
         return jsonify({"error": str(e)}), 500
 
 
+@app.route("/api/audio-capacity", methods=["POST"])
+@login_required
+def api_audio_capacity():
+    """Get audio file capacity for steganography (v4.3.0)."""
+    audio_file = request.files.get("carrier")
+    if not audio_file:
+        return jsonify({"error": "No audio file provided"}), 400
+
+    try:
+        audio_data = audio_file.read()
+        result = subprocess_stego.audio_info(audio_data)
+
+        if not result.success:
+            return jsonify({"error": result.error or "Audio analysis failed"}), 500
+
+        return jsonify(
+            {
+                "success": True,
+                "sample_rate": result.sample_rate,
+                "channels": result.channels,
+                "duration": round(result.duration_seconds, 2),
+                "format": result.format,
+                "bit_depth": result.bit_depth,
+                "lsb_capacity": result.capacity_lsb,
+                "spread_capacity": result.capacity_spread,
+            }
+        )
+    except Exception as e:
+        return jsonify({"error": str(e)}), 500
+
+
 # ============================================================================
 # ENCODE
 # ============================================================================
@@ -1078,7 +1125,10 @@ def _run_encode_job(job_id: str, encode_params: dict) -> None:
 
         # Store result
         file_id = secrets.token_urlsafe(16)
-        temp_storage.save_temp_file(file_id, encode_result.stego_data, {
+        temp_storage.save_temp_file(
+            file_id,
+            encode_result.stego_data,
+            {
                 "filename": filename,
                 "embed_mode": embed_mode,
                 "output_format": dct_output_format if embed_mode == "dct" else "png",
@@ -1086,7 +1136,94 @@ def _run_encode_job(job_id: str, encode_params: dict) -> None:
                 "mime_type": output_mime,
                 "channel_mode": encode_result.channel_mode,
                 "channel_fingerprint": encode_result.channel_fingerprint,
-        })
+            },
+        )
+
+        _store_job(
+            job_id,
+            {
+                "status": "complete",
+                "file_id": file_id,
+                "created": time.time(),
+            },
+        )
+
+    except Exception as e:
+        _store_job(
+            job_id,
+            {
+                "status": "error",
+                "error": str(e),
+                "created": time.time(),
+            },
+        )
+    finally:
+        cleanup_progress_file(job_id)
+
+
+def _run_encode_audio_job(job_id: str, encode_params: dict) -> None:
+    """Background thread function for async audio encode (v4.3.0)."""
+    progress_file = get_progress_file_path(job_id)
+
+    try:
+        _store_job(job_id, {"status": "running", "created": time.time()})
+
+        if encode_params.get("file_data"):
+            encode_result = subprocess_stego.encode_audio(
+                carrier_data=encode_params["carrier_data"],
+                reference_data=encode_params["ref_data"],
+                file_data=encode_params["file_data"],
+                file_name=encode_params["file_name"],
+                file_mime=encode_params["file_mime"],
+                passphrase=encode_params["passphrase"],
+                pin=encode_params.get("pin"),
+                rsa_key_data=encode_params.get("rsa_key_data"),
+                rsa_password=encode_params.get("key_password"),
+                embed_mode=encode_params["embed_mode"],
+                channel_key=encode_params.get("channel_key"),
+                progress_file=progress_file,
+                chip_tier=encode_params.get("chip_tier"),
+            )
+        else:
+            encode_result = subprocess_stego.encode_audio(
+                carrier_data=encode_params["carrier_data"],
+                reference_data=encode_params["ref_data"],
+                message=encode_params.get("message"),
+                passphrase=encode_params["passphrase"],
+                pin=encode_params.get("pin"),
+                rsa_key_data=encode_params.get("rsa_key_data"),
+                rsa_password=encode_params.get("key_password"),
+                embed_mode=encode_params["embed_mode"],
+                channel_key=encode_params.get("channel_key"),
+                progress_file=progress_file,
+                chip_tier=encode_params.get("chip_tier"),
+            )
+
+        if not encode_result.success:
+            _store_job(
+                job_id,
+                {
+                    "status": "error",
+                    "error": encode_result.error or "Audio encoding failed",
+                    "created": time.time(),
+                },
+            )
+            return
+
+        filename = generate_filename("stego_audio", ".wav")
+        file_id = secrets.token_urlsafe(16)
+        temp_storage.save_temp_file(
+            file_id,
+            encode_result.stego_data,
+            {
+                "filename": filename,
+                "embed_mode": encode_params["embed_mode"],
+                "carrier_type": "audio",
+                "mime_type": "audio/wav",
+                "channel_mode": encode_result.channel_mode,
+                "channel_fingerprint": encode_result.channel_fingerprint,
+            },
+        )
 
         _store_job(
             job_id,
@@ -1131,6 +1268,196 @@ def encode_page():
             rsa_key_file = request.files.get("rsa_key")
             payload_file = request.files.get("payload_file")
 
+            # Determine carrier type (v4.3.0)
+            carrier_type = request.form.get("carrier_type", "image")
+
+            if carrier_type == "audio":
+                # ========== AUDIO ENCODE PATH (v4.3.0) ==========
+                if not HAS_AUDIO_SUPPORT:
+                    return _error_response(
+                        "Audio steganography is not available. Install audio dependencies."
+                    )
+
+                if not ref_photo or not carrier:
+                    return _error_response("Both reference photo and audio carrier are required")
+
+                if not allowed_image(ref_photo.filename):
+                    return _error_response("Reference must be an image (PNG, JPG, BMP)")
+
+                if not allowed_audio(carrier.filename):
+                    return _error_response(
+                        "Invalid audio format. Use WAV, FLAC, MP3, OGG, AAC, or M4A"
+                    )
+
+                # Get form data
+                message = request.form.get("message", "")
+                passphrase = request.form.get("passphrase", "")
+                pin = request.form.get("pin", "").strip()
+                rsa_password = request.form.get("rsa_password", "")
+                payload_type = request.form.get("payload_type", "text")
+
+                embed_mode = request.form.get("embed_mode", "audio_lsb")
+                if embed_mode not in ("audio_lsb", "audio_spread"):
+                    embed_mode = "audio_lsb"
+
+                # Chip tier for spread spectrum (None = default)
+                chip_tier_str = request.form.get("chip_tier")
+                chip_tier = None
+                if chip_tier_str and chip_tier_str.isdigit():
+                    chip_tier = int(chip_tier_str)
+                    if chip_tier not in (0, 1, 2):
+                        chip_tier = None
+
+                channel_key = resolve_channel_key_form(request.form.get("channel_key", "auto"))
+
+                # Determine payload
+                if payload_type == "file" and payload_file and payload_file.filename:
+                    file_data = payload_file.read()
+                    result = validate_file_payload(file_data, payload_file.filename)
+                    if not result.is_valid:
+                        return _error_response(result.error_message)
+                    mime_type, _ = mimetypes.guess_type(payload_file.filename)
+                    payload = FilePayload(
+                        data=file_data,
+                        filename=payload_file.filename,
+                        mime_type=mime_type,
+                    )
+                else:
+                    result = validate_message(message)
+                    if not result.is_valid:
+                        return _error_response(result.error_message)
+                    payload = message
+
+                if not passphrase:
+                    return _error_response("Passphrase is required")
+
+                result = validate_passphrase(passphrase)
+                if not result.is_valid:
+                    return _error_response(result.error_message)
+                if result.warning:
+                    flash(result.warning, "warning")
+
+                ref_data = ref_photo.read()
+                carrier_data = carrier.read()
+
+                # Handle RSA key (same as image path)
+                rsa_key_data = None
+                rsa_key_pem = request.form.get("rsa_key_pem", "").strip()
+                rsa_key_qr = request.files.get("rsa_key_qr")
+                rsa_key_from_qr = False
+
+                if rsa_key_pem:
+                    if is_compressed(rsa_key_pem):
+                        rsa_key_pem = decompress_data(rsa_key_pem)
+                    rsa_key_data = rsa_key_pem.encode("utf-8")
+                    rsa_key_from_qr = True
+                elif rsa_key_file and rsa_key_file.filename:
+                    rsa_key_data = rsa_key_file.read()
+                elif rsa_key_qr and rsa_key_qr.filename and HAS_QRCODE_READ:
+                    qr_image_data = rsa_key_qr.read()
+                    key_pem = extract_key_from_qr(qr_image_data)
+                    if key_pem:
+                        rsa_key_data = key_pem.encode("utf-8")
+                        rsa_key_from_qr = True
+                    else:
+                        return _error_response("Could not extract RSA key from QR code image.")
+
+                result = validate_security_factors(pin, rsa_key_data)
+                if not result.is_valid:
+                    return _error_response(result.error_message)
+
+                if pin:
+                    result = validate_pin(pin)
+                    if not result.is_valid:
+                        return _error_response(result.error_message)
+
+                key_password = None if rsa_key_from_qr else (rsa_password if rsa_password else None)
+
+                if rsa_key_data:
+                    result = validate_rsa_key(rsa_key_data, key_password)
+                    if not result.is_valid:
+                        return _error_response(result.error_message)
+
+                # Build audio encode params
+                encode_params = {
+                    "carrier_data": carrier_data,
+                    "ref_data": ref_data,
+                    "passphrase": passphrase,
+                    "pin": pin if pin else None,
+                    "rsa_key_data": rsa_key_data,
+                    "key_password": key_password,
+                    "embed_mode": embed_mode,
+                    "channel_key": channel_key,
+                    "carrier_type": "audio",
+                    "chip_tier": chip_tier,
+                }
+
+                if payload_type == "file" and payload_file and payload_file.filename:
+                    encode_params["file_data"] = payload.data
+                    encode_params["file_name"] = payload.filename
+                    encode_params["file_mime"] = payload.mime_type
+                else:
+                    encode_params["message"] = payload
+
+                if is_async:
+                    job_id = generate_job_id()
+                    _store_job(job_id, {"status": "pending", "created": time.time()})
+                    _executor.submit(_run_encode_audio_job, job_id, encode_params)
+                    return jsonify({"job_id": job_id, "status": "pending"})
+
+                # Sync audio encode
+                if encode_params.get("file_data"):
+                    encode_result = subprocess_stego.encode_audio(
+                        carrier_data=carrier_data,
+                        reference_data=ref_data,
+                        file_data=encode_params["file_data"],
+                        file_name=encode_params["file_name"],
+                        file_mime=encode_params["file_mime"],
+                        passphrase=passphrase,
+                        pin=pin if pin else None,
+                        rsa_key_data=rsa_key_data,
+                        rsa_password=key_password,
+                        embed_mode=embed_mode,
+                        channel_key=channel_key,
+                        chip_tier=chip_tier,
+                    )
+                else:
+                    encode_result = subprocess_stego.encode_audio(
+                        carrier_data=carrier_data,
+                        reference_data=ref_data,
+                        message=payload,
+                        passphrase=passphrase,
+                        pin=pin if pin else None,
+                        rsa_key_data=rsa_key_data,
+                        rsa_password=key_password,
+                        embed_mode=embed_mode,
+                        channel_key=channel_key,
+                        chip_tier=chip_tier,
+                    )
+
+                if not encode_result.success:
+                    error_msg = encode_result.error or "Audio encoding failed"
+                    return _error_response(error_msg)
+
+                filename = generate_filename("stego_audio", ".wav")
+                file_id = secrets.token_urlsafe(16)
+                cleanup_temp_files()
+                temp_storage.save_temp_file(
+                    file_id,
+                    encode_result.stego_data,
+                    {
+                        "filename": filename,
+                        "embed_mode": embed_mode,
+                        "carrier_type": "audio",
+                        "mime_type": "audio/wav",
+                        "channel_mode": encode_result.channel_mode,
+                        "channel_fingerprint": encode_result.channel_fingerprint,
+                    },
+                )
+
+                return redirect(url_for("encode_result", file_id=file_id))
+
+            # ========== IMAGE ENCODE PATH (original) ==========
             if not ref_photo or not carrier:
                 return _error_response("Both reference photo and carrier image are required")
 
@@ -1356,7 +1683,10 @@ def encode_page():
             # Store temporarily
             file_id = secrets.token_urlsafe(16)
             cleanup_temp_files()
-            temp_storage.save_temp_file(file_id, encode_result.stego_data, {
+            temp_storage.save_temp_file(
+                file_id,
+                encode_result.stego_data,
+                {
                     "filename": filename,
                     "embed_mode": embed_mode,
                     "output_format": dct_output_format if embed_mode == "dct" else "png",
@@ -1365,7 +1695,8 @@ def encode_page():
                     # Channel info (v4.0.0)
                     "channel_mode": encode_result.channel_mode,
                     "channel_fingerprint": encode_result.channel_fingerprint,
-            })
+                },
+            )
 
             return redirect(url_for("encode_result", file_id=file_id))
 
@@ -1434,10 +1765,13 @@ def encode_result(file_id):
         flash("File expired or not found. Please encode again.", "error")
         return redirect(url_for("encode_page"))
 
-    # Generate thumbnail
-    thumbnail_data = generate_thumbnail(file_info["data"])
-    thumbnail_id = None
+    carrier_type = file_info.get("carrier_type", "image")
 
+    # Generate thumbnail only for images
+    thumbnail_data = None
+    thumbnail_id = None
+    if carrier_type != "audio":
+        thumbnail_data = generate_thumbnail(file_info["data"])
         if thumbnail_data:
             thumbnail_id = f"{file_id}_thumb"
             temp_storage.save_thumbnail(thumbnail_id, thumbnail_data)
@@ -1450,6 +1784,7 @@ def encode_result(file_id):
         embed_mode=file_info.get("embed_mode", "lsb"),
         output_format=file_info.get("output_format", "png"),
         color_mode=file_info.get("color_mode"),
+        carrier_type=carrier_type,
         # Channel info (v4.0.0)
         channel_mode=file_info.get("channel_mode", "public"),
         channel_fingerprint=file_info.get("channel_fingerprint"),
@@ -1464,9 +1799,7 @@ def encode_thumbnail(thumb_id):
     if not thumb_data:
         return "Thumbnail not found", 404
 
-    return send_file(
-        io.BytesIO(thumb_data), mimetype="image/jpeg", as_attachment=False
-    )
+    return send_file(io.BytesIO(thumb_data), mimetype="image/jpeg", as_attachment=False)
 
 
 @app.route("/encode/download/<file_id>")
@@ -1559,10 +1892,92 @@ def _run_decode_job(job_id: str, decode_params: dict) -> None:
         if decode_result.is_file:
             file_id = secrets.token_urlsafe(16)
             filename = decode_result.filename or "decoded_file"
-            temp_storage.save_temp_file(file_id, decode_result.file_data, {
+            temp_storage.save_temp_file(
+                file_id,
+                decode_result.file_data,
+                {
                     "filename": filename,
                     "mime_type": decode_result.mime_type,
-            })
+                },
+            )
+            _store_job(
+                job_id,
+                {
+                    "status": "complete",
+                    "file_id": file_id,
+                    "is_file": True,
+                    "filename": filename,
+                    "file_size": len(decode_result.file_data),
+                    "mime_type": decode_result.mime_type,
+                    "created": time.time(),
+                },
+            )
+        else:
+            _store_job(
+                job_id,
+                {
+                    "status": "complete",
+                    "is_file": False,
+                    "message": decode_result.message,
+                    "created": time.time(),
+                },
+            )
+
+    except Exception as e:
+        _store_job(
+            job_id,
+            {
+                "status": "error",
+                "error": str(e),
+                "created": time.time(),
+            },
+        )
+    finally:
+        cleanup_progress_file(job_id)
+
+
+def _run_decode_audio_job(job_id: str, decode_params: dict) -> None:
+    """Background thread function for async audio decode (v4.3.0)."""
+    progress_file = get_progress_file_path(job_id)
+
+    try:
+        _store_job(job_id, {"status": "running", "created": time.time()})
+
+        decode_result = subprocess_stego.decode_audio(
+            stego_data=decode_params["stego_data"],
+            reference_data=decode_params["ref_data"],
+            passphrase=decode_params["passphrase"],
+            pin=decode_params.get("pin"),
+            rsa_key_data=decode_params.get("rsa_key_data"),
+            rsa_password=decode_params.get("rsa_password"),
+            embed_mode=decode_params.get("embed_mode", "audio_auto"),
+            channel_key=decode_params.get("channel_key"),
+            progress_file=progress_file,
+        )
+
+        if not decode_result.success:
+            _store_job(
+                job_id,
+                {
+                    "status": "error",
+                    "error": decode_result.error or "Audio decoding failed",
+                    "error_type": decode_result.error_type,
+                    "created": time.time(),
+                },
+            )
+            return
+
+        if decode_result.is_file:
+            file_id = secrets.token_urlsafe(16)
+            filename = decode_result.filename or "decoded_file"
+            temp_storage.save_temp_file(
+                file_id,
+                decode_result.file_data,
+                {
+                    "filename": filename,
+                    "mime_type": decode_result.mime_type,
+                },
+            )
             _store_job(
                 job_id,
                 {
@@ -1609,6 +2024,163 @@ def decode_page():
             stego_image = request.files.get("stego_image")
             rsa_key_file = request.files.get("rsa_key")
 
+            # Determine carrier type (v4.3.0)
+            carrier_type = request.form.get("carrier_type", "image")
+
+            if carrier_type == "audio":
+                # ========== AUDIO DECODE PATH (v4.3.0) ==========
+                if not HAS_AUDIO_SUPPORT:
+                    flash("Audio steganography is not available.", "error")
+                    return render_template("decode.html", has_qrcode_read=HAS_QRCODE_READ)
+
+                if not ref_photo or not stego_image:
+                    flash("Both reference photo and stego audio are required", "error")
+                    return render_template("decode.html", has_qrcode_read=HAS_QRCODE_READ)
+
+                if not allowed_image(ref_photo.filename):
+                    flash("Reference must be an image", "error")
+                    return render_template("decode.html", has_qrcode_read=HAS_QRCODE_READ)
+
+                if not allowed_audio(stego_image.filename):
+                    flash("Invalid audio format", "error")
+                    return render_template("decode.html", has_qrcode_read=HAS_QRCODE_READ)
+
+                passphrase = request.form.get("passphrase", "")
+                pin = request.form.get("pin", "").strip()
+                rsa_password = request.form.get("rsa_password", "")
+
+                embed_mode = request.form.get("embed_mode", "audio_auto")
+                if embed_mode not in ("audio_auto", "audio_lsb", "audio_spread"):
+                    embed_mode = "audio_auto"
+
+                channel_key = resolve_channel_key_form(request.form.get("channel_key", "auto"))
+
+                if not passphrase:
+                    flash("Passphrase is required", "error")
+                    return render_template("decode.html", has_qrcode_read=HAS_QRCODE_READ)
+
+                ref_data = ref_photo.read()
+                stego_data = stego_image.read()
+
+                # Handle RSA key (same as image path)
+                rsa_key_data = None
+                rsa_key_pem = request.form.get("rsa_key_pem", "").strip()
+                rsa_key_qr = request.files.get("rsa_key_qr")
+                rsa_key_from_qr = False
+
+                if rsa_key_pem:
+                    if is_compressed(rsa_key_pem):
+                        rsa_key_pem = decompress_data(rsa_key_pem)
+                    rsa_key_data = rsa_key_pem.encode("utf-8")
+                    rsa_key_from_qr = True
+                elif rsa_key_file and rsa_key_file.filename:
+                    rsa_key_data = rsa_key_file.read()
+                elif rsa_key_qr and rsa_key_qr.filename and HAS_QRCODE_READ:
+                    qr_image_data = rsa_key_qr.read()
+                    key_pem = extract_key_from_qr(qr_image_data)
+                    if key_pem:
+                        rsa_key_data = key_pem.encode("utf-8")
+                        rsa_key_from_qr = True
+                    else:
+                        flash("Could not extract RSA key from QR code image.", "error")
+                        return render_template("decode.html", has_qrcode_read=HAS_QRCODE_READ)
+
+                result = validate_security_factors(pin, rsa_key_data)
+                if not result.is_valid:
+                    flash(result.error_message, "error")
+                    return render_template("decode.html", has_qrcode_read=HAS_QRCODE_READ)
+
+                if pin:
+                    result = validate_pin(pin)
+                    if not result.is_valid:
+                        flash(result.error_message, "error")
+                        return render_template("decode.html", has_qrcode_read=HAS_QRCODE_READ)
+
+                key_password = None if rsa_key_from_qr else (rsa_password if rsa_password else None)
+
+                if rsa_key_data:
+                    result = validate_rsa_key(rsa_key_data, key_password)
+                    if not result.is_valid:
+                        flash(result.error_message, "error")
+                        return render_template("decode.html", has_qrcode_read=HAS_QRCODE_READ)
+
+                is_async = (
+                    request.form.get("async") == "true" or request.headers.get("X-Async") == "true"
+                )
+
+                decode_params = {
+                    "stego_data": stego_data,
+                    "ref_data": ref_data,
+                    "passphrase": passphrase,
+                    "pin": pin if pin else None,
+                    "rsa_key_data": rsa_key_data,
+                    "rsa_password": key_password,
+                    "embed_mode": embed_mode,
+                    "channel_key": channel_key,
+                }
+
+                if is_async:
+                    job_id = generate_job_id()
+                    _store_job(job_id, {"status": "pending", "created": time.time()})
+                    _executor.submit(_run_decode_audio_job, job_id, decode_params)
+                    return jsonify({"job_id": job_id, "status": "pending"})
+
+                # Sync audio decode
+                decode_result = subprocess_stego.decode_audio(
+                    stego_data=stego_data,
+                    reference_data=ref_data,
+                    passphrase=passphrase,
+                    pin=pin if pin else None,
+                    rsa_key_data=rsa_key_data,
+                    rsa_password=key_password,
+                    embed_mode=embed_mode,
+                    channel_key=channel_key,
+                )
+
+                if not decode_result.success:
+                    error_msg = decode_result.error or "Audio decoding failed"
+                    if (
+                        "decrypt" in error_msg.lower()
+                        or decode_result.error_type == "DecryptionError"
+                    ):
+                        flash(
+                            "Wrong credentials. Double-check your reference photo, "
+                            "passphrase, PIN, and channel key.",
+                            "warning",
+                        )
+                    else:
+                        flash(error_msg, "error")
+                    return render_template("decode.html", has_qrcode_read=HAS_QRCODE_READ)
+
+                if decode_result.is_file:
+                    file_id = secrets.token_urlsafe(16)
+                    cleanup_temp_files()
+                    filename = decode_result.filename or "decoded_file"
+                    temp_storage.save_temp_file(
+                        file_id,
+                        decode_result.file_data,
+                        {
+                            "filename": filename,
+                            "mime_type": decode_result.mime_type,
+                        },
+                    )
+                    return render_template(
+                        "decode.html",
+                        decoded_file=True,
+                        file_id=file_id,
+                        filename=filename,
+                        file_size=format_size(len(decode_result.file_data)),
+                        mime_type=decode_result.mime_type,
+                        has_qrcode_read=HAS_QRCODE_READ,
+                    )
+                else:
+                    return render_template(
+                        "decode.html",
+                        decoded_message=decode_result.message,
+                        has_qrcode_read=HAS_QRCODE_READ,
+                    )
+
+            # ========== IMAGE DECODE PATH (original) ==========
             if not ref_photo or not stego_image:
                 flash("Both reference photo and stego image are required", "error")
                 return render_template("decode.html", has_qrcode_read=HAS_QRCODE_READ)
@@ -1690,7 +2262,9 @@ def decode_page():
                     return render_template("decode.html", has_qrcode_read=HAS_QRCODE_READ)
 
             # Check for async mode (v4.1.5)
-            is_async = request.form.get("async") == "true" or request.headers.get("X-Async") == "true"
+            is_async = (
+                request.form.get("async") == "true" or request.headers.get("X-Async") == "true"
+            )
 
             # Build decode params
             decode_params = {
@@ -1742,10 +2316,14 @@ def decode_page():
                 cleanup_temp_files()
 
                 filename = decode_result.filename or "decoded_file"
-                temp_storage.save_temp_file(file_id, decode_result.file_data, {
+                temp_storage.save_temp_file(
+                    file_id,
+                    decode_result.file_data,
+                    {
                         "filename": filename,
                         "mime_type": decode_result.mime_type,
-                })
+                    },
+                )
 
                 return render_template(
                     "decode.html",
@@ -2101,11 +2679,12 @@ def api_tools_exif_clear():
 @login_required
 def api_tools_rotate():
     """Rotate and/or flip an image, using lossless jpegtran for JPEGs."""
-    from PIL import Image
     import shutil
     import subprocess
     import tempfile
 
+    from PIL import Image
+
     image_file = request.files.get("image")
     if not image_file:
         return jsonify({"success": False, "error": "No image provided"}), 400
@@ -2136,9 +2715,18 @@ def api_tools_rotate():
                 output_path = tempfile.mktemp(suffix=".jpg")
                 try:
                     result = subprocess.run(
-                        ["jpegtran", "-rotate", str(rotation), "-copy", "all",
-                         "-outfile", output_path, input_path],
-                        capture_output=True, timeout=30
+                        [
+                            "jpegtran",
+                            "-rotate",
+                            str(rotation),
+                            "-copy",
+                            "all",
+                            "-outfile",
+                            output_path,
+                            input_path,
+                        ],
+                        capture_output=True,
+                        timeout=30,
                     )
                     if result.returncode == 0:
                         with open(output_path, "rb") as f:
@@ -2158,9 +2746,18 @@ def api_tools_rotate():
                 output_path = tempfile.mktemp(suffix=".jpg")
                 try:
                     result = subprocess.run(
-                        ["jpegtran", "-flip", "horizontal", "-copy", "all",
-                         "-outfile", output_path, input_path],
-                        capture_output=True, timeout=30
+                        [
+                            "jpegtran",
+                            "-flip",
+                            "horizontal",
+                            "-copy",
+                            "all",
+                            "-outfile",
+                            output_path,
+                            input_path,
+                        ],
+                        capture_output=True,
+                        timeout=30,
                     )
                     if result.returncode == 0:
                         with open(output_path, "rb") as f:
@@ -2180,9 +2777,18 @@ def api_tools_rotate():
                 output_path = tempfile.mktemp(suffix=".jpg")
                 try:
                     result = subprocess.run(
-                        ["jpegtran", "-flip", "vertical", "-copy", "all",
-                         "-outfile", output_path, input_path],
-                        capture_output=True, timeout=30
+                        [
+                            "jpegtran",
+                            "-flip",
+                            "vertical",
+                            "-copy",
+                            "all",
+                            "-outfile",
+                            output_path,
+                            input_path,
+                        ],
+                        capture_output=True,
+                        timeout=30,
                     )
                     if result.returncode == 0:
                         with open(output_path, "rb") as f:
@@ -2839,10 +3445,7 @@ def admin_settings_unlock():
     channel_status = get_channel_status()
     channel_key = channel_status.get("key") if channel_status["configured"] else ""
 
-    return jsonify({
-        "success": True,
-        "channel_key": channel_key
-    })
+    return jsonify({"success": True, "channel_key": channel_key})
 
 
 @app.route("/admin/users")
@@ -2976,6 +3579,7 @@ if __name__ == "__main__":
     ssl_context = None
     if app.config.get("HTTPS_ENABLED", False):
         import socket
+
         hostname = os.environ.get("STEGASOO_HOSTNAME") or socket.gethostname()
         try:
             cert_path, key_path = ensure_certs(base_dir, hostname)

frontends/web/auth.py

@@ -77,14 +77,10 @@ def init_db():
     db = get_db()
 
     # Check if we need to migrate from old single-user schema
-    cursor = db.execute(
-        "SELECT name FROM sqlite_master WHERE type='table' AND name='admin_user'"
-    )
+    cursor = db.execute("SELECT name FROM sqlite_master WHERE type='table' AND name='admin_user'")
     has_old_table = cursor.fetchone() is not None
 
-    cursor = db.execute(
-        "SELECT name FROM sqlite_master WHERE type='table' AND name='users'"
-    )
+    cursor = db.execute("SELECT name FROM sqlite_master WHERE type='table' AND name='users'")
     has_new_table = cursor.fetchone() is not None
 
     if has_old_table and not has_new_table:
@@ -189,9 +185,7 @@ def _ensure_channel_keys_table(db: sqlite3.Connection):
 
 def _ensure_app_settings_table(db: sqlite3.Connection):
     """Ensure app_settings table exists (v4.1.0 migration)."""
-    cursor = db.execute(
-        "SELECT name FROM sqlite_master WHERE type='table' AND name='app_settings'"
-    )
+    cursor = db.execute("SELECT name FROM sqlite_master WHERE type='table' AND name='app_settings'")
     if cursor.fetchone() is None:
         db.executescript("""
             CREATE TABLE IF NOT EXISTS app_settings (
@@ -212,9 +206,7 @@ def _ensure_app_settings_table(db: sqlite3.Connection):
 def get_app_setting(key: str) -> str | None:
     """Get an app-level setting value."""
     db = get_db()
-    row = db.execute(
-        "SELECT value FROM app_settings WHERE key = ?", (key,)
-    ).fetchone()
+    row = db.execute("SELECT value FROM app_settings WHERE key = ?", (key,)).fetchone()
     return row["value"] if row else None
 
 
@@ -384,12 +376,10 @@ def get_user_by_username(username: str) -> User | None:
 def get_all_users() -> list[User]:
     """Get all users, admins first, then by creation date."""
     db = get_db()
-    rows = db.execute(
-        """
+    rows = db.execute("""
         SELECT id, username, role, created_at FROM users
         ORDER BY role = 'admin' DESC, created_at ASC
-        """
-    ).fetchall()
+        """).fetchall()
     return [
         User(
             id=row["id"],
@@ -596,9 +586,7 @@ def create_admin_user(username: str, password: str) -> tuple[bool, str]:
     return success, msg
 
 
-def change_password(
-    user_id: int, current_password: str, new_password: str
-) -> tuple[bool, str]:
+def change_password(user_id: int, current_password: str, new_password: str) -> tuple[bool, str]:
     """Change a user's password (requires current password)."""
     user = get_user_by_id(user_id)
     if not user:
@@ -667,9 +655,7 @@ def delete_user(user_id: int, current_user_id: int) -> tuple[bool, str]:
     # Check if this is the last admin
     if user.role == ROLE_ADMIN:
         db = get_db()
-        admin_count = db.execute(
-            "SELECT COUNT(*) FROM users WHERE role = 'admin'"
-        ).fetchone()[0]
+        admin_count = db.execute("SELECT COUNT(*) FROM users WHERE role = 'admin'").fetchone()[0]
         if admin_count <= 1:
             return False, "Cannot delete the last admin"
 
@@ -848,9 +834,7 @@ def save_channel_key(
         return False, "This channel key is already saved", None
 
 
-def update_channel_key_name(
-    key_id: int, user_id: int, new_name: str
-) -> tuple[bool, str]:
+def update_channel_key_name(key_id: int, user_id: int, new_name: str) -> tuple[bool, str]:
     """Update the name of a saved channel key."""
     new_name = new_name.strip()
     if not new_name:

frontends/web/ssl_utils.py

@@ -81,10 +81,12 @@ def generate_self_signed_cert(
     )
 
     # Create certificate
-    subject = issuer = x509.Name([
+    subject = issuer = x509.Name(
+        [
             x509.NameAttribute(NameOID.ORGANIZATION_NAME, "Stegasoo"),
             x509.NameAttribute(NameOID.COMMON_NAME, hostname),
-    ])
+        ]
+    )
 
     # Subject Alternative Names
     san_list = [
@@ -112,7 +114,7 @@ def generate_self_signed_cert(
         except (ipaddress.AddressValueError, ValueError):
             pass
 
-    now = datetime.datetime.now(datetime.timezone.utc)
+    now = datetime.datetime.now(datetime.UTC)
     cert = (
         x509.CertificateBuilder()
         .subject_name(subject)

frontends/web/static/js/stegasoo.js

@@ -95,7 +95,16 @@ const Stegasoo = {
             if (!isPayloadZone && !isQrZone) {
                 input.addEventListener('change', function() {
                     if (this.files && this.files[0]) {
-                        Stegasoo.showImagePreview(this.files[0], preview, label, zone);
+                        const file = this.files[0];
+                        if (file.type.startsWith('image/') && preview) {
+                            Stegasoo.showImagePreview(file, preview, label, zone);
+                        } else if (file.type.startsWith('audio/') || !file.type.startsWith('image/')) {
+                            // Audio or non-image files: show file info instead of image preview
+                            Stegasoo.showAudioFileInfo(file, zone);
+                            if (label) {
+                                label.classList.add('d-none');
+                            }
+                        }
                     }
                 });
             }
@@ -154,6 +163,20 @@ const Stegasoo = {
         reader.readAsDataURL(file);
     },
 
+    /**
+     * Format audio file info for display in drop zones (v4.3.0)
+     */
+    showAudioFileInfo(file, zone) {
+        const filenameEl = zone.querySelector('.pixel-data-filename span, .scan-data-filename span');
+        const sizeEl = zone.querySelector('.pixel-data-value, .scan-data-value');
+        if (filenameEl) filenameEl.textContent = file.name;
+        if (sizeEl) {
+            const kb = file.size / 1024;
+            sizeEl.textContent = kb >= 1024 ? (kb / 1024).toFixed(1) + ' MB' : kb.toFixed(1) + ' KB';
+        }
+        zone.classList.add('has-file');
+    },
+
     // ========================================================================
     // REFERENCE PHOTO SCAN ANIMATION
     // ========================================================================
@@ -1036,6 +1059,10 @@ const Stegasoo = {
             'saving': 'Saving image...',
             'finalizing': 'Finalizing...',
             'complete': 'Complete!',
+            // Audio encode phases (v4.3.0)
+            'audio_transcoding': 'Transcoding audio...',
+            'audio_embedding': 'Embedding in audio...',
+            'spread_embedding': 'Spread spectrum embedding...',
         };
         return phases[phase] || phase;
     },
@@ -1252,6 +1279,10 @@ const Stegasoo = {
             'verifying': 'Verifying...',
             'finalizing': 'Finalizing...',
             'complete': 'Complete!',
+            // Audio decode phases (v4.3.0)
+            'audio_transcoding': 'Transcoding audio...',
+            'audio_extracting': 'Extracting from audio...',
+            'spread_extracting': 'Spread spectrum extracting...',
         };
         return phases[phase] || phase;
     },

frontends/web/stego_worker.py

@@ -19,6 +19,8 @@ Usage:
 
 import base64
 import json
+import logging
+import os
 import sys
 import traceback
 from pathlib import Path
@@ -27,6 +29,24 @@ from pathlib import Path
 sys.path.insert(0, str(Path(__file__).parent.parent.parent / "src"))
 sys.path.insert(0, str(Path(__file__).parent))
 
+# Configure logging for worker subprocess
+_log_level = os.environ.get("STEGASOO_LOG_LEVEL", "").strip().upper()
+if _log_level and hasattr(logging, _log_level):
+    logging.basicConfig(
+        level=getattr(logging, _log_level),
+        format="[%(asctime)s.%(msecs)03d] [%(levelname)s] [%(name)s] %(message)s",
+        datefmt="%H:%M:%S",
+        stream=sys.stderr,
+    )
+elif os.environ.get("STEGASOO_DEBUG", "").strip() in ("1", "true", "yes"):
+    logging.basicConfig(
+        level=logging.DEBUG,
+        format="[%(asctime)s.%(msecs)03d] [%(levelname)s] [%(name)s] %(message)s",
+        datefmt="%H:%M:%S",
+        stream=sys.stderr,
+    )
+logger = logging.getLogger("stegasoo.worker")
+
 
 def _resolve_channel_key(channel_key_param):
     """
@@ -73,6 +93,7 @@ def _get_channel_info(resolved_key):
 
 def encode_operation(params: dict) -> dict:
     """Handle encode operation."""
+    logger.debug("encode_operation: mode=%s", params.get("embed_mode", "lsb"))
     from stegasoo import FilePayload, encode
 
     # Decode base64 inputs
@@ -142,6 +163,7 @@ def _write_decode_progress(progress_file: str | None, percent: int, phase: str)
         return
     try:
         import json
+
         with open(progress_file, "w") as f:
             json.dump({"percent": percent, "phase": phase}, f)
     except Exception:
@@ -150,6 +172,7 @@ def _write_decode_progress(progress_file: str | None, percent: int, phase: str)
 
 def decode_operation(params: dict) -> dict:
     """Handle decode operation."""
+    logger.debug("decode_operation: mode=%s", params.get("embed_mode", "auto"))
     from stegasoo import decode
 
     progress_file = params.get("progress_file")
@@ -233,6 +256,145 @@ def capacity_check_operation(params: dict) -> dict:
     }
 
 
+def encode_audio_operation(params: dict) -> dict:
+    """Handle audio encode operation (v4.3.0)."""
+    logger.debug("encode_audio_operation: mode=%s", params.get("embed_mode", "audio_lsb"))
+    from stegasoo import FilePayload, encode_audio
+
+    carrier_data = base64.b64decode(params["carrier_b64"])
+    reference_data = base64.b64decode(params["reference_b64"])
+
+    # Optional RSA key
+    rsa_key_data = None
+    if params.get("rsa_key_b64"):
+        rsa_key_data = base64.b64decode(params["rsa_key_b64"])
+
+    # Determine payload type
+    if params.get("file_b64"):
+        file_data = base64.b64decode(params["file_b64"])
+        payload = FilePayload(
+            data=file_data,
+            filename=params.get("file_name", "file"),
+            mime_type=params.get("file_mime", "application/octet-stream"),
+        )
+    else:
+        payload = params.get("message", "")
+
+    resolved_channel_key = _resolve_channel_key(params.get("channel_key", "auto"))
+
+    # Resolve chip_tier from params (None means use default)
+    chip_tier_val = params.get("chip_tier")
+    if chip_tier_val is not None:
+        chip_tier_val = int(chip_tier_val)
+
+    stego_audio, stats = encode_audio(
+        message=payload,
+        reference_photo=reference_data,
+        carrier_audio=carrier_data,
+        passphrase=params.get("passphrase", ""),
+        pin=params.get("pin"),
+        rsa_key_data=rsa_key_data,
+        rsa_password=params.get("rsa_password"),
+        embed_mode=params.get("embed_mode", "audio_lsb"),
+        channel_key=resolved_channel_key,
+        progress_file=params.get("progress_file"),
+        chip_tier=chip_tier_val,
+    )
+
+    channel_mode, channel_fingerprint = _get_channel_info(resolved_channel_key)
+
+    return {
+        "success": True,
+        "stego_b64": base64.b64encode(stego_audio).decode("ascii"),
+        "stats": {
+            "samples_modified": stats.samples_modified,
+            "total_samples": stats.total_samples,
+            "capacity_used": stats.capacity_used,
+            "bytes_embedded": stats.bytes_embedded,
+            "sample_rate": stats.sample_rate,
+            "channels": stats.channels,
+            "duration_seconds": stats.duration_seconds,
+            "embed_mode": stats.embed_mode,
+        },
+        "channel_mode": channel_mode,
+        "channel_fingerprint": channel_fingerprint,
+    }
+
+
+def decode_audio_operation(params: dict) -> dict:
+    """Handle audio decode operation (v4.3.0)."""
+    logger.debug("decode_audio_operation: mode=%s", params.get("embed_mode", "audio_auto"))
+    from stegasoo import decode_audio
+
+    progress_file = params.get("progress_file")
+    _write_decode_progress(progress_file, 5, "reading")
+
+    stego_data = base64.b64decode(params["stego_b64"])
+    reference_data = base64.b64decode(params["reference_b64"])
+
+    _write_decode_progress(progress_file, 15, "reading")
+
+    rsa_key_data = None
+    if params.get("rsa_key_b64"):
+        rsa_key_data = base64.b64decode(params["rsa_key_b64"])
+
+    resolved_channel_key = _resolve_channel_key(params.get("channel_key", "auto"))
+
+    result = decode_audio(
+        stego_audio=stego_data,
+        reference_photo=reference_data,
+        passphrase=params.get("passphrase", ""),
+        pin=params.get("pin"),
+        rsa_key_data=rsa_key_data,
+        rsa_password=params.get("rsa_password"),
+        embed_mode=params.get("embed_mode", "audio_auto"),
+        channel_key=resolved_channel_key,
+        progress_file=progress_file,
+    )
+
+    if result.is_file:
+        return {
+            "success": True,
+            "is_file": True,
+            "file_b64": base64.b64encode(result.file_data).decode("ascii"),
+            "filename": result.filename,
+            "mime_type": result.mime_type,
+        }
+    else:
+        return {
+            "success": True,
+            "is_file": False,
+            "message": result.message,
+        }
+
+
+def audio_info_operation(params: dict) -> dict:
+    """Handle audio info operation (v4.3.0)."""
+    from stegasoo import get_audio_info
+    from stegasoo.audio_steganography import calculate_audio_lsb_capacity
+    from stegasoo.spread_steganography import calculate_audio_spread_capacity
+
+    audio_data = base64.b64decode(params["audio_b64"])
+
+    info = get_audio_info(audio_data)
+    lsb_capacity = calculate_audio_lsb_capacity(audio_data)
+    spread_capacity = calculate_audio_spread_capacity(audio_data)
+
+    return {
+        "success": True,
+        "info": {
+            "sample_rate": info.sample_rate,
+            "channels": info.channels,
+            "duration_seconds": round(info.duration_seconds, 2),
+            "num_samples": info.num_samples,
+            "format": info.format,
+            "bit_depth": info.bit_depth,
+            "capacity_lsb": lsb_capacity,
+            "capacity_spread": spread_capacity.usable_capacity_bytes,
+        },
+    }
+
+
 def channel_status_operation(params: dict) -> dict:
     """Handle channel status check (v4.0.0)."""
     from stegasoo import get_channel_status
@@ -263,6 +425,7 @@ def main():
         else:
             params = json.loads(input_text)
             operation = params.get("operation")
+            logger.info("Worker handling operation: %s", operation)
 
             if operation == "encode":
                 output = encode_operation(params)
@@ -274,6 +437,13 @@ def main():
                 output = capacity_check_operation(params)
             elif operation == "channel_status":
                 output = channel_status_operation(params)
+            # Audio operations (v4.3.0)
+            elif operation == "encode_audio":
+                output = encode_audio_operation(params)
+            elif operation == "decode_audio":
+                output = decode_audio_operation(params)
+            elif operation == "audio_info":
+                output = audio_info_operation(params)
             else:
                 output = {"success": False, "error": f"Unknown operation: {operation}"}
 

frontends/web/subprocess_stego.py

@@ -115,6 +115,35 @@ class CapacityResult:
     error: str | None = None
 
 
+@dataclass
+class AudioEncodeResult:
+    """Result from audio encode operation (v4.3.0)."""
+
+    success: bool
+    stego_data: bytes | None = None
+    stats: dict[str, Any] | None = None
+    channel_mode: str | None = None
+    channel_fingerprint: str | None = None
+    error: str | None = None
+    error_type: str | None = None
+
+
+@dataclass
+class AudioInfoResult:
+    """Result from audio info operation (v4.3.0)."""
+
+    success: bool
+    sample_rate: int = 0
+    channels: int = 0
+    duration_seconds: float = 0.0
+    num_samples: int = 0
+    format: str = ""
+    bit_depth: int | None = None
+    capacity_lsb: int = 0
+    capacity_spread: int = 0
+    error: str | None = None
+
+
 @dataclass
 class ChannelStatusResult:
     """Result from channel status check (v4.0.0)."""
@@ -456,6 +485,201 @@ class SubprocessStego:
                 error=result.get("error", "Unknown error"),
             )
 
+    # =========================================================================
+    # Audio Steganography (v4.3.0)
+    # =========================================================================
+
+    def encode_audio(
+        self,
+        carrier_data: bytes,
+        reference_data: bytes,
+        message: str | None = None,
+        file_data: bytes | None = None,
+        file_name: str | None = None,
+        file_mime: str | None = None,
+        passphrase: str = "",
+        pin: str | None = None,
+        rsa_key_data: bytes | None = None,
+        rsa_password: str | None = None,
+        embed_mode: str = "audio_lsb",
+        channel_key: str | None = "auto",
+        timeout: int | None = None,
+        progress_file: str | None = None,
+        chip_tier: int | None = None,
+    ) -> AudioEncodeResult:
+        """
+        Encode a message or file into an audio carrier.
+
+        Args:
+            carrier_data: Carrier audio bytes (WAV, FLAC, MP3, etc.)
+            reference_data: Reference photo bytes
+            message: Text message to encode (if not file)
+            file_data: File bytes to encode (if not message)
+            file_name: Original filename (for file payload)
+            file_mime: MIME type (for file payload)
+            passphrase: Encryption passphrase
+            pin: Optional PIN
+            rsa_key_data: Optional RSA key PEM bytes
+            rsa_password: RSA key password if encrypted
+            embed_mode: 'audio_lsb' or 'audio_spread'
+            channel_key: 'auto', 'none', or explicit key
+            timeout: Operation timeout (default 300s for audio)
+            progress_file: Path to write progress updates
+
+        Returns:
+            AudioEncodeResult with stego audio data on success
+        """
+        params = {
+            "operation": "encode_audio",
+            "carrier_b64": base64.b64encode(carrier_data).decode("ascii"),
+            "reference_b64": base64.b64encode(reference_data).decode("ascii"),
+            "message": message,
+            "passphrase": passphrase,
+            "pin": pin,
+            "embed_mode": embed_mode,
+            "channel_key": channel_key,
+            "progress_file": progress_file,
+            "chip_tier": chip_tier,
+        }
+
+        if file_data:
+            params["file_b64"] = base64.b64encode(file_data).decode("ascii")
+            params["file_name"] = file_name
+            params["file_mime"] = file_mime
+
+        if rsa_key_data:
+            params["rsa_key_b64"] = base64.b64encode(rsa_key_data).decode("ascii")
+            params["rsa_password"] = rsa_password
+
+        # Audio operations can be slower (especially spread spectrum)
+        result = self._run_worker(params, timeout or 300)
+
+        if result.get("success"):
+            return AudioEncodeResult(
+                success=True,
+                stego_data=base64.b64decode(result["stego_b64"]),
+                stats=result.get("stats"),
+                channel_mode=result.get("channel_mode"),
+                channel_fingerprint=result.get("channel_fingerprint"),
+            )
+        else:
+            return AudioEncodeResult(
+                success=False,
+                error=result.get("error", "Unknown error"),
+                error_type=result.get("error_type"),
+            )
+
+    def decode_audio(
+        self,
+        stego_data: bytes,
+        reference_data: bytes,
+        passphrase: str = "",
+        pin: str | None = None,
+        rsa_key_data: bytes | None = None,
+        rsa_password: str | None = None,
+        embed_mode: str = "audio_auto",
+        channel_key: str | None = "auto",
+        timeout: int | None = None,
+        progress_file: str | None = None,
+    ) -> DecodeResult:
+        """
+        Decode a message or file from stego audio.
+
+        Args:
+            stego_data: Stego audio bytes
+            reference_data: Reference photo bytes
+            passphrase: Decryption passphrase
+            pin: Optional PIN
+            rsa_key_data: Optional RSA key PEM bytes
+            rsa_password: RSA key password if encrypted
+            embed_mode: 'audio_auto', 'audio_lsb', or 'audio_spread'
+            channel_key: 'auto', 'none', or explicit key
+            timeout: Operation timeout (default 300s for audio)
+            progress_file: Path to write progress updates
+
+        Returns:
+            DecodeResult with message or file_data on success
+        """
+        params = {
+            "operation": "decode_audio",
+            "stego_b64": base64.b64encode(stego_data).decode("ascii"),
+            "reference_b64": base64.b64encode(reference_data).decode("ascii"),
+            "passphrase": passphrase,
+            "pin": pin,
+            "embed_mode": embed_mode,
+            "channel_key": channel_key,
+            "progress_file": progress_file,
+        }
+
+        if rsa_key_data:
+            params["rsa_key_b64"] = base64.b64encode(rsa_key_data).decode("ascii")
+            params["rsa_password"] = rsa_password
+
+        result = self._run_worker(params, timeout or 300)
+
+        if result.get("success"):
+            if result.get("is_file"):
+                return DecodeResult(
+                    success=True,
+                    is_file=True,
+                    file_data=base64.b64decode(result["file_b64"]),
+                    filename=result.get("filename"),
+                    mime_type=result.get("mime_type"),
+                )
+            else:
+                return DecodeResult(
+                    success=True,
+                    is_file=False,
+                    message=result.get("message"),
+                )
+        else:
+            return DecodeResult(
+                success=False,
+                error=result.get("error", "Unknown error"),
+                error_type=result.get("error_type"),
+            )
+
+    def audio_info(
+        self,
+        audio_data: bytes,
+        timeout: int | None = None,
+    ) -> AudioInfoResult:
+        """
+        Get audio file information and steganographic capacity.
+
+        Args:
+            audio_data: Audio file bytes
+            timeout: Operation timeout in seconds
+
+        Returns:
+            AudioInfoResult with metadata and capacity info
+        """
+        params = {
+            "operation": "audio_info",
+            "audio_b64": base64.b64encode(audio_data).decode("ascii"),
+        }
+
+        result = self._run_worker(params, timeout)
+
+        if result.get("success"):
+            info = result.get("info", {})
+            return AudioInfoResult(
+                success=True,
+                sample_rate=info.get("sample_rate", 0),
+                channels=info.get("channels", 0),
+                duration_seconds=info.get("duration_seconds", 0.0),
+                num_samples=info.get("num_samples", 0),
+                format=info.get("format", ""),
+                bit_depth=info.get("bit_depth"),
+                capacity_lsb=info.get("capacity_lsb", 0),
+                capacity_spread=info.get("capacity_spread", 0),
+            )
+        else:
+            return AudioInfoResult(
+                success=False,
+                error=result.get("error", "Unknown error"),
+            )
+
     def get_channel_status(
         self,
         reveal: bool = False,

frontends/web/templates/decode.html

@@ -24,7 +24,11 @@
     border-left: 3px solid #ffe699;
 }
 .step-accordion .accordion-button::after {
-    filter: invert(1) sepia(1) saturate(2) hue-rotate(5deg) brightness(1.2);
+    filter: brightness(0) invert(1);
+    opacity: 0.5;
+}
+.step-accordion .accordion-button:not(.collapsed)::after {
+    opacity: 0.9;
 }
 .step-accordion .accordion-body {
     background: rgba(30, 40, 50, 0.4);
@@ -172,19 +176,51 @@
                     <div class="accordion step-accordion" id="decodeAccordion">
 
                         <!-- ================================================================
-                             STEP 1: IMAGES & MODE
+                             STEP 1: CARRIER TYPE (v4.3.0)
+                             ================================================================ -->
+                        <div class="accordion-item" id="carrierTypeStep">
+                            <h2 class="accordion-header">
+                                <button class="accordion-button" type="button" data-bs-toggle="collapse" data-bs-target="#stepCarrierType">
+                                    <span class="step-title">
+                                        <span class="step-number" id="stepCarrierTypeNumber">1</span>
+                                        <i class="bi bi-collection me-1"></i> Carrier Type
+                                    </span>
+                                    <span class="step-summary" id="stepCarrierTypeSummary"></span>
+                                </button>
+                            </h2>
+                            <div id="stepCarrierType" class="accordion-collapse collapse show" data-bs-parent="#decodeAccordion">
+                                <div class="accordion-body">
+                                    <input type="hidden" name="carrier_type" id="carrierTypeInput" value="image">
+                                    <div class="btn-group w-100" role="group">
+                                        <input type="radio" class="btn-check" name="carrier_type_select" id="typeImage" value="image" checked>
+                                        <label class="btn btn-outline-secondary" for="typeImage">
+                                            <i class="bi bi-image me-1"></i> Image
+                                        </label>
+                                        <input type="radio" class="btn-check" name="carrier_type_select" id="typeAudio" value="audio"
+                                               {% if not has_audio %}disabled{% endif %}>
+                                        <label class="btn btn-outline-secondary {% if not has_audio %}disabled text-muted{% endif %}" for="typeAudio">
+                                            <i class="bi bi-music-note-beamed me-1"></i> Audio
+                                            {% if not has_audio %}<small class="d-block" style="font-size: 0.65rem;">(not available)</small>{% endif %}
+                                        </label>
+                                    </div>
+                                </div>
+                            </div>
+                        </div>
+
+                        <!-- ================================================================
+                             STEP 2: IMAGES & MODE
                              ================================================================ -->
                         <div class="accordion-item">
                             <h2 class="accordion-header">
-                                <button class="accordion-button" type="button" data-bs-toggle="collapse" data-bs-target="#stepImages">
+                                <button class="accordion-button collapsed" type="button" data-bs-toggle="collapse" data-bs-target="#stepImages">
                                     <span class="step-title">
-                                        <span class="step-number" id="stepImagesNumber">1</span>
-                                        <i class="bi bi-images me-1"></i> Images & Mode
+                                        <span class="step-number" id="stepImagesNumber">2</span>
+                                        <i class="bi bi-images me-1"></i> Reference, Carrier, Mode
                                     </span>
                                     <span class="step-summary" id="stepImagesSummary">Select reference & stego</span>
                                 </button>
                             </h2>
-                            <div id="stepImages" class="accordion-collapse collapse show" data-bs-parent="#decodeAccordion">
+                            <div id="stepImages" class="accordion-collapse collapse" data-bs-parent="#decodeAccordion">
                                 <div class="accordion-body">
 
                                     <div class="row">
@@ -213,6 +249,7 @@
                                         </div>
 
                                         <div class="col-md-6 mb-3">
+                                            <div id="imageStegoSection">
                                                 <label class="form-label">
                                                     <i class="bi bi-file-earmark-image me-1"></i> Stego Image
                                                 </label>
@@ -237,10 +274,30 @@
                                                 </div>
                                                 <div class="form-text">Image containing the hidden message</div>
                                             </div>
+                                            <!-- Audio Stego (hidden by default) -->
+                                            <div class="d-none" id="audioStegoSection">
+                                                <label class="form-label">
+                                                    <i class="bi bi-file-earmark-music me-1"></i> Stego Audio
+                                                </label>
+                                                <div class="drop-zone pixel-container" id="audioStegoDropZone">
+                                                    <input type="file" name="stego_image" accept="audio/*" id="audioStegoInput">
+                                                    <div class="drop-zone-label">
+                                                        <i class="bi bi-music-note-beamed fs-3 d-block mb-2 text-muted"></i>
+                                                        <span class="text-muted">Drop audio or click</span>
+                                                    </div>
+                                                    <div class="pixel-data-panel">
+                                                        <div class="pixel-data-filename"><i class="bi bi-check-circle-fill"></i><span id="audioStegoFileName">audio.wav</span></div>
+                                                        <div class="pixel-data-row"><span class="pixel-status-badge">Audio Loaded</span><span class="pixel-data-value" id="audioStegoFileSize">--</span></div>
+                                                    </div>
+                                                </div>
+                                                <div class="form-text">Audio file containing the hidden message</div>
+                                            </div>
+                                        </div>
                                     </div>
 
                                     <!-- Extraction Mode -->
                                     <div class="d-flex gap-2 align-items-center flex-wrap mb-2">
+                                        <div id="imageModeGroup">
                                             <div class="btn-group" role="group">
                                                 <input type="radio" class="btn-check" name="embed_mode" id="modeAuto" value="auto" checked>
                                                 <label class="btn btn-outline-secondary text-nowrap" for="modeAuto"><i class="bi bi-magic me-1"></i>Auto</label>
@@ -250,6 +307,18 @@
                                                 <label class="btn btn-outline-secondary text-nowrap" for="modeDct" id="dctModeLabel"><i class="bi bi-soundwave me-1"></i>DCT</label>
                                             </div>
                                         </div>
+                                        <!-- Audio Extraction Modes (hidden by default) -->
+                                        <div class="d-none" id="audioModeGroup">
+                                            <div class="btn-group" role="group">
+                                                <input type="radio" class="btn-check" name="embed_mode" id="modeAudioAuto" value="audio_auto">
+                                                <label class="btn btn-outline-secondary text-nowrap" for="modeAudioAuto"><i class="bi bi-magic me-1"></i>Auto</label>
+                                                <input type="radio" class="btn-check" name="embed_mode" id="modeAudioLsb" value="audio_lsb">
+                                                <label class="btn btn-outline-secondary text-nowrap" for="modeAudioLsb"><i class="bi bi-grid-3x3-gap me-1"></i>LSB</label>
+                                                <input type="radio" class="btn-check" name="embed_mode" id="modeAudioSpread" value="audio_spread">
+                                                <label class="btn btn-outline-secondary text-nowrap" for="modeAudioSpread"><i class="bi bi-broadcast me-1"></i>Spread</label>
+                                            </div>
+                                        </div>
+                                    </div>
                                     <div class="form-text" id="modeHint">
                                         <i class="bi bi-lightning me-1"></i>Tries LSB first, then DCT
                                     </div>
@@ -259,13 +328,13 @@
                         </div>
 
                         <!-- ================================================================
-                             STEP 2: SECURITY
+                             STEP 3: SECURITY
                              ================================================================ -->
                         <div class="accordion-item">
                             <h2 class="accordion-header">
                                 <button class="accordion-button collapsed" type="button" data-bs-toggle="collapse" data-bs-target="#stepSecurity">
                                     <span class="step-title">
-                                        <span class="step-number" id="stepSecurityNumber">2</span>
+                                        <span class="step-number" id="stepSecurityNumber">3</span>
                                         <i class="bi bi-shield-lock me-1"></i> Security
                                     </span>
                                     <span class="step-summary" id="stepSecuritySummary">Passphrase & keys</span>
@@ -425,7 +494,10 @@
 const modeHints = {
     auto: { icon: 'lightning', text: 'Tries LSB first, then DCT' },
     lsb: { icon: 'hdd', text: 'For email and direct transfers' },
-    dct: { icon: 'phone', text: 'For social media images' }
+    dct: { icon: 'phone', text: 'For social media images' },
+    audio_auto: { icon: 'lightning', text: 'Tries LSB first, then Spread Spectrum' },
+    audio_lsb: { icon: 'grid-3x3-gap', text: 'Direct bit embedding in audio samples' },
+    audio_spread: { icon: 'broadcast', text: 'Noise-resistant spread spectrum encoding' }
 };
 
 document.querySelectorAll('input[name="embed_mode"]').forEach(radio => {
@@ -442,9 +514,14 @@ document.querySelectorAll('input[name="embed_mode"]').forEach(radio => {
 // ACCORDION SUMMARY UPDATES
 // ============================================================================
 
+const carrierTypeInput = document.getElementById('carrierTypeInput');
+
 function updateImagesSummary() {
     const ref = document.getElementById('refPhotoInput')?.files[0];
-    const stego = document.getElementById('stegoInput')?.files[0];
+    const isAudio = carrierTypeInput?.value === 'audio';
+    const stego = isAudio
+        ? document.getElementById('audioStegoInput')?.files[0]
+        : document.getElementById('stegoInput')?.files[0];
     const mode = document.querySelector('input[name="embed_mode"]:checked')?.value?.toUpperCase() || 'AUTO';
     const summary = document.getElementById('stepImagesSummary');
     const stepNum = document.getElementById('stepImagesNumber');
@@ -460,12 +537,12 @@ function updateImagesSummary() {
         summary.textContent = ref ? ref.name.slice(0, 15) : stego.name.slice(0, 15);
         summary.classList.remove('has-content');
         stepNum.classList.remove('complete');
-        stepNum.textContent = '1';
+        stepNum.textContent = '2';
     } else {
-        summary.textContent = 'Select reference & stego';
+        summary.textContent = isAudio ? 'Select reference & audio' : 'Select reference & stego';
         summary.classList.remove('has-content');
         stepNum.classList.remove('complete');
-        stepNum.textContent = '1';
+        stepNum.textContent = '2';
     }
 }
 
@@ -493,19 +570,99 @@ function updateSecuritySummary() {
         summary.textContent = 'Passphrase & keys';
         summary.classList.remove('has-content');
         stepNum.classList.remove('complete');
-        stepNum.textContent = '2';
+        stepNum.textContent = '3';
     }
 }
 
 // Attach listeners
 document.getElementById('refPhotoInput')?.addEventListener('change', updateImagesSummary);
 document.getElementById('stegoInput')?.addEventListener('change', updateImagesSummary);
+document.getElementById('audioStegoInput')?.addEventListener('change', updateImagesSummary);
 document.querySelectorAll('input[name="embed_mode"]').forEach(r => r.addEventListener('change', updateImagesSummary));
+document.querySelectorAll('#audioModeGroup input[name="embed_mode"]').forEach(r => r.addEventListener('change', updateImagesSummary));
 
 document.getElementById('passphraseInput')?.addEventListener('input', updateSecuritySummary);
 document.getElementById('pinInput')?.addEventListener('input', updateSecuritySummary);
 document.querySelector('input[name="rsa_key"]')?.addEventListener('change', updateSecuritySummary);
 
+// ============================================================================
+// CARRIER TYPE TOGGLE (v4.3.0)
+// ============================================================================
+
+const carrierTypeRadios = document.querySelectorAll('input[name="carrier_type_select"]');
+const imageStegoSection = document.getElementById('imageStegoSection');
+const audioStegoSection = document.getElementById('audioStegoSection');
+const imageModeGroup = document.getElementById('imageModeGroup');
+const audioModeGroup = document.getElementById('audioModeGroup');
+const stepCarrierTypeSummary = document.getElementById('stepCarrierTypeSummary');
+
+carrierTypeRadios.forEach(radio => {
+    radio.addEventListener('change', function() {
+        const isAudio = this.value === 'audio';
+        carrierTypeInput.value = this.value;
+
+        // Toggle stego sections
+        if (imageStegoSection) imageStegoSection.classList.toggle('d-none', isAudio);
+        if (audioStegoSection) audioStegoSection.classList.toggle('d-none', !isAudio);
+
+        // Toggle required attribute so hidden inputs don't block form submission
+        const imgStego = document.getElementById('stegoInput');
+        const audStego = document.getElementById('audioStegoInput');
+        if (imgStego) { if (isAudio) imgStego.removeAttribute('required'); else imgStego.setAttribute('required', ''); }
+        if (audStego) { if (isAudio) audStego.setAttribute('required', ''); else audStego.removeAttribute('required'); }
+
+        // Toggle mode groups
+        if (imageModeGroup) imageModeGroup.classList.toggle('d-none', isAudio);
+        if (audioModeGroup) audioModeGroup.classList.toggle('d-none', !isAudio);
+
+        // Update summary
+        if (stepCarrierTypeSummary) {
+            stepCarrierTypeSummary.textContent = isAudio ? 'Audio' : 'Image';
+        }
+
+        // Select default mode
+        if (isAudio) {
+            const audioAuto = document.getElementById('modeAudioAuto');
+            if (audioAuto) audioAuto.checked = true;
+        } else {
+            const autoMode = document.getElementById('modeAuto');
+            if (autoMode) autoMode.checked = true;
+        }
+
+        // Clear stego file selections
+        const stegoInput = document.getElementById('stegoInput');
+        const audioStegoInput = document.getElementById('audioStegoInput');
+        if (stegoInput) stegoInput.value = '';
+        if (audioStegoInput) audioStegoInput.value = '';
+
+        // Reset previews
+        document.getElementById('stegoPreview')?.classList.add('d-none');
+
+        // Update mode hint
+        const hint = document.getElementById('modeHint');
+        if (hint) {
+            if (isAudio) {
+                hint.innerHTML = '<i class="bi bi-lightning me-1"></i>Tries LSB first, then Spread Spectrum';
+            } else {
+                hint.innerHTML = '<i class="bi bi-lightning me-1"></i>Tries LSB first, then DCT';
+            }
+        }
+
+        updateImagesSummary();
+    });
+});
+
+// Audio stego file info display
+const audioStegoInput = document.getElementById('audioStegoInput');
+audioStegoInput?.addEventListener('change', function() {
+    if (this.files && this.files[0]) {
+        const file = this.files[0];
+        document.getElementById('audioStegoFileName').textContent = file.name;
+        document.getElementById('audioStegoFileSize').textContent = (file.size / 1024).toFixed(1) + ' KB';
+        updateImagesSummary();
+    }
+});
+
 // ============================================================================
 // MODE SWITCHING
 // ============================================================================

frontends/web/templates/encode.html

@@ -24,7 +24,11 @@
     border-left: 3px solid #ffe699;
 }
 .step-accordion .accordion-button::after {
-    filter: invert(1) sepia(1) saturate(2) hue-rotate(5deg) brightness(1.2);
+    filter: brightness(0) invert(1);
+    opacity: 0.5;
+}
+.step-accordion .accordion-button:not(.collapsed)::after {
+    opacity: 0.9;
 }
 .step-accordion .accordion-body {
     background: rgba(30, 40, 50, 0.4);
@@ -126,19 +130,56 @@
                     <div class="accordion step-accordion" id="encodeAccordion">
 
                         <!-- ================================================================
-                             STEP 1: IMAGES
+                             STEP 1: CARRIER TYPE (v4.3.0)
+                             ================================================================ -->
+                        <div class="accordion-item" id="carrierTypeStep">
+                            <h2 class="accordion-header">
+                                <button class="accordion-button" type="button" data-bs-toggle="collapse" data-bs-target="#stepCarrierType">
+                                    <span class="step-title">
+                                        <span class="step-number" id="stepCarrierTypeNumber">1</span>
+                                        <i class="bi bi-collection me-1"></i> Carrier Type
+                                    </span>
+                                    <span class="step-summary" id="stepCarrierTypeSummary"></span>
+                                </button>
+                            </h2>
+                            <div id="stepCarrierType" class="accordion-collapse collapse show" data-bs-parent="#encodeAccordion">
+                                <div class="accordion-body">
+                                    <input type="hidden" name="carrier_type" id="carrierTypeInput" value="image">
+                                    <div class="btn-group w-100" role="group">
+                                        <input type="radio" class="btn-check" name="carrier_type_select" id="typeImage" value="image" checked>
+                                        <label class="btn btn-outline-secondary" for="typeImage">
+                                            <i class="bi bi-image me-1"></i> Image
+                                        </label>
+                                        <input type="radio" class="btn-check" name="carrier_type_select" id="typeAudio" value="audio"
+                                               {% if not has_audio %}disabled{% endif %}>
+                                        <label class="btn btn-outline-secondary {% if not has_audio %}disabled text-muted{% endif %}" for="typeAudio">
+                                            <i class="bi bi-music-note-beamed me-1"></i> Audio
+                                            {% if not has_audio %}<small class="d-block" style="font-size: 0.65rem;">(not available)</small>{% endif %}
+                                        </label>
+                                    </div>
+                                    {% if not has_audio %}
+                                    <div class="form-text text-warning mt-2">
+                                        <i class="bi bi-exclamation-triangle me-1"></i>Audio requires numpy and soundfile packages
+                                    </div>
+                                    {% endif %}
+                                </div>
+                            </div>
+                        </div>
+
+                        <!-- ================================================================
+                             STEP 2: IMAGES & MODE
                              ================================================================ -->
                         <div class="accordion-item">
                             <h2 class="accordion-header">
-                                <button class="accordion-button" type="button" data-bs-toggle="collapse" data-bs-target="#stepImages">
+                                <button class="accordion-button collapsed" type="button" data-bs-toggle="collapse" data-bs-target="#stepImages">
                                     <span class="step-title">
-                                        <span class="step-number" id="stepImagesNumber">1</span>
-                                        <i class="bi bi-images me-1"></i> Images & Mode
+                                        <span class="step-number" id="stepImagesNumber">2</span>
+                                        <i class="bi bi-images me-1"></i> Reference, Carrier, Mode
                                     </span>
                                     <span class="step-summary" id="stepImagesSummary">Select reference & carrier</span>
                                 </button>
                             </h2>
-                            <div id="stepImages" class="accordion-collapse collapse show" data-bs-parent="#encodeAccordion">
+                            <div id="stepImages" class="accordion-collapse collapse" data-bs-parent="#encodeAccordion">
                                 <div class="accordion-body">
 
                                     <div class="row">
@@ -167,6 +208,7 @@
                                         </div>
 
                                         <div class="col-md-6 mb-3">
+                                            <div id="imageCarrierSection">
                                                 <label class="form-label">
                                                     <i class="bi bi-file-earmark-image me-1"></i> Carrier Image
                                                 </label>
@@ -191,6 +233,27 @@
                                                 </div>
                                                 <div class="form-text">Image to hide your message in</div>
                                             </div>
+
+                                            <!-- Audio Carrier (hidden by default, shown when audio type selected) -->
+                                            <div class="d-none" id="audioCarrierSection">
+                                                <label class="form-label">
+                                                    <i class="bi bi-file-earmark-music me-1"></i> Carrier Audio
+                                                </label>
+                                                <div class="drop-zone pixel-container" id="audioCarrierDropZone">
+                                                    <input type="file" name="carrier" accept="audio/*" id="audioCarrierInput">
+                                                    <div class="drop-zone-label">
+                                                        <i class="bi bi-music-note-beamed fs-3 d-block mb-2 text-muted"></i>
+                                                        <span class="text-muted">Drop audio or click</span>
+                                                    </div>
+                                                    <div class="pixel-data-panel">
+                                                        <div class="pixel-data-filename"><i class="bi bi-check-circle-fill"></i><span id="audioCarrierFileName">audio.wav</span></div>
+                                                        <div class="pixel-data-row"><span class="pixel-status-badge">Audio Loaded</span><span class="pixel-data-value" id="audioCarrierFileSize">--</span></div>
+                                                        <div class="pixel-dimensions" id="audioCarrierDuration">--:-- duration</div>
+                                                    </div>
+                                                </div>
+                                                <div class="form-text">Audio file to hide your message in</div>
+                                            </div>
+                                        </div>
                                     </div>
 
                                     <!-- Capacity Info -->
@@ -204,7 +267,19 @@
                                         </div>
                                     </div>
 
+                                    <!-- Audio Capacity Info (v4.3.0) -->
+                                    <div class="alert alert-info small d-none mb-3" id="audioCapacityPanel">
+                                        <div class="d-flex justify-content-between align-items-center">
+                                            <span><i class="bi bi-music-note-beamed me-1"></i><span id="audioInfo">-</span></span>
+                                            <span>
+                                                <span class="badge bg-primary me-1" id="lsbAudioCapacityBadge">LSB: -</span>
+                                                <span class="badge bg-warning text-dark" id="spreadCapacityBadge">Spread: -</span>
+                                            </span>
+                                        </div>
+                                    </div>
+
                                     <!-- Embedding Mode (compact inline) -->
+                                    <div id="imageModeGroup">
                                         <div class="d-flex gap-2 align-items-center flex-wrap mb-2">
                                             <div class="btn-group btn-group-sm" role="group">
                                                 <input type="radio" class="btn-check" name="embed_mode" id="modeDct" value="dct" {% if has_dct %}checked{% endif %} {% if not has_dct %}disabled{% endif %}>
@@ -228,6 +303,18 @@
                                                 </div>
                                             </span>
                                         </div>
+                                    </div>
+
+                                    <!-- Audio Modes (hidden by default) -->
+                                    <div class="d-none" id="audioModeGroup">
+                                        <div class="btn-group btn-group-sm" role="group">
+                                            <input type="radio" class="btn-check" name="embed_mode" id="modeAudioLsb" value="audio_lsb">
+                                            <label class="btn btn-outline-secondary btn-sm text-nowrap" for="modeAudioLsb"><i class="bi bi-grid-3x3-gap me-1"></i>LSB</label>
+                                            <input type="radio" class="btn-check" name="embed_mode" id="modeAudioSpread" value="audio_spread">
+                                            <label class="btn btn-outline-secondary btn-sm text-nowrap" for="modeAudioSpread"><i class="bi bi-broadcast me-1"></i>Spread</label>
+                                        </div>
+                                    </div>
+
                                     <div class="form-text" id="modeHint">
                                         <i class="bi bi-{% if has_dct %}phone{% else %}hdd{% endif %} me-1"></i>{% if has_dct %}Survives social media compression{% else %}Higher capacity for direct transfers{% endif %}
                                     </div>
@@ -237,13 +324,13 @@
                         </div>
 
                         <!-- ================================================================
-                             STEP 2: PAYLOAD
+                             STEP 3: PAYLOAD
                              ================================================================ -->
                         <div class="accordion-item">
                             <h2 class="accordion-header">
                                 <button class="accordion-button collapsed" type="button" data-bs-toggle="collapse" data-bs-target="#stepPayload">
                                     <span class="step-title">
-                                        <span class="step-number" id="stepPayloadNumber">2</span>
+                                        <span class="step-number" id="stepPayloadNumber">3</span>
                                         <i class="bi bi-box me-1"></i> Payload
                                     </span>
                                     <span class="step-summary" id="stepPayloadSummary">Message or file to hide</span>
@@ -295,13 +382,13 @@
                         </div>
 
                         <!-- ================================================================
-                             STEP 3: SECURITY
+                             STEP 4: SECURITY
                              ================================================================ -->
                         <div class="accordion-item">
                             <h2 class="accordion-header">
                                 <button class="accordion-button collapsed" type="button" data-bs-toggle="collapse" data-bs-target="#stepSecurity">
                                     <span class="step-title">
-                                        <span class="step-number" id="stepSecurityNumber">3</span>
+                                        <span class="step-number" id="stepSecurityNumber">4</span>
                                         <i class="bi bi-shield-lock me-1"></i> Security
                                     </span>
                                     <span class="step-summary" id="stepSecuritySummary">Passphrase & keys</span>
@@ -462,13 +549,131 @@ document.querySelectorAll('input[name="embed_mode"]').forEach(radio => {
     });
 });
 
+// ============================================================================
+// CARRIER TYPE TOGGLE (v4.3.0)
+// ============================================================================
+
+const carrierTypeRadios = document.querySelectorAll('input[name="carrier_type_select"]');
+const carrierTypeInput = document.getElementById('carrierTypeInput');
+const imageCarrierSection = document.getElementById('imageCarrierSection');
+const audioCarrierSection = document.getElementById('audioCarrierSection');
+const imageModeGroup = document.getElementById('imageModeGroup');
+const audioModeGroup = document.getElementById('audioModeGroup');
+const capacityPanel = document.getElementById('capacityPanel');
+const audioCapacityPanel = document.getElementById('audioCapacityPanel');
+const stepCarrierTypeSummary = document.getElementById('stepCarrierTypeSummary');
+
+carrierTypeRadios.forEach(radio => {
+    radio.addEventListener('change', function() {
+        const isAudio = this.value === 'audio';
+        carrierTypeInput.value = this.value;
+
+        // Toggle carrier sections
+        if (imageCarrierSection) imageCarrierSection.classList.toggle('d-none', isAudio);
+        if (audioCarrierSection) audioCarrierSection.classList.toggle('d-none', !isAudio);
+
+        // Toggle required attribute so hidden inputs don't block form submission
+        const imgCarrier = document.getElementById('carrierInput');
+        const audCarrier = document.getElementById('audioCarrierInput');
+        if (imgCarrier) { if (isAudio) imgCarrier.removeAttribute('required'); else imgCarrier.setAttribute('required', ''); }
+        if (audCarrier) { if (isAudio) audCarrier.setAttribute('required', ''); else audCarrier.removeAttribute('required'); }
+
+        // Toggle mode groups
+        if (imageModeGroup) imageModeGroup.classList.toggle('d-none', isAudio);
+        if (audioModeGroup) audioModeGroup.classList.toggle('d-none', !isAudio);
+
+        // Toggle capacity panels
+        if (capacityPanel) capacityPanel.classList.add('d-none');
+        if (audioCapacityPanel) audioCapacityPanel.classList.add('d-none');
+
+        // Update summary
+        if (stepCarrierTypeSummary) {
+            stepCarrierTypeSummary.textContent = isAudio ? 'Audio' : 'Image';
+        }
+
+        // Select default mode for the active type
+        if (isAudio) {
+            const audioLsb = document.getElementById('modeAudioLsb');
+            if (audioLsb) audioLsb.checked = true;
+        } else {
+            // Reset to DCT if available, else LSB
+            const dctRadio = document.getElementById('modeDct');
+            const lsbRadio = document.getElementById('modeLsb');
+            if (dctRadio && !dctRadio.disabled) {
+                dctRadio.checked = true;
+            } else if (lsbRadio) {
+                lsbRadio.checked = true;
+            }
+        }
+
+        // Clear carrier file selections
+        const carrierInput = document.getElementById('carrierInput');
+        const audioCarrierInput = document.getElementById('audioCarrierInput');
+        if (carrierInput) carrierInput.value = '';
+        if (audioCarrierInput) audioCarrierInput.value = '';
+
+        // Reset previews
+        document.getElementById('carrierPreview')?.classList.add('d-none');
+
+        // Update step title
+        const stepImagesTitle = document.querySelector('#stepImages')?.closest('.accordion-item')?.querySelector('.accordion-button .step-title');
+        if (stepImagesTitle) {
+            const icon = stepImagesTitle.querySelector('i:not(.step-number i)');
+            const textNode = stepImagesTitle.childNodes[stepImagesTitle.childNodes.length - 1];
+            if (icon) {
+                icon.className = isAudio ? 'bi bi-music-note-beamed me-1' : 'bi bi-images me-1';
+            }
+        }
+
+        updateImagesSummary();
+    });
+});
+
+// Audio carrier file change handler
+const audioCarrierInput = document.getElementById('audioCarrierInput');
+audioCarrierInput?.addEventListener('change', function() {
+    if (this.files && this.files[0]) {
+        const file = this.files[0];
+        document.getElementById('audioCarrierFileName').textContent = file.name;
+        document.getElementById('audioCarrierFileSize').textContent = (file.size / 1024).toFixed(1) + ' KB';
+
+        // Fetch audio capacity
+        const formData = new FormData();
+        formData.append('carrier', file);
+        fetch('/api/audio-capacity', { method: 'POST', body: formData })
+            .then(r => r.json())
+            .then(data => {
+                if (data.error) return;
+                const info = `${data.format || 'Audio'} · ${data.sample_rate}Hz · ${data.channels}ch · ${data.duration}s`;
+                document.getElementById('audioInfo').textContent = info;
+                document.getElementById('lsbAudioCapacityBadge').textContent = `LSB: ${(data.lsb_capacity / 1024).toFixed(1)} KB`;
+                document.getElementById('spreadCapacityBadge').textContent = `Spread: ${(data.spread_capacity / 1024).toFixed(1)} KB`;
+                document.getElementById('audioCapacityPanel')?.classList.remove('d-none');
+                if (data.duration) {
+                    document.getElementById('audioCarrierDuration').textContent = data.duration + 's duration';
+                }
+            }).catch(() => {});
+
+        // Trigger the drop zone animation
+        const dropZone = document.getElementById('audioCarrierDropZone');
+        if (dropZone) {
+            dropZone.classList.add('has-file');
+        }
+
+        updateImagesSummary();
+    }
+});
+
 // ============================================================================
 // ACCORDION SUMMARY UPDATES
 // ============================================================================
 
 function updateImagesSummary() {
     const ref = document.getElementById('refPhotoInput')?.files[0];
-    const carrier = document.getElementById('carrierInput')?.files[0];
+    const isAudio = carrierTypeInput?.value === 'audio';
+    const carrier = isAudio
+        ? document.getElementById('audioCarrierInput')?.files[0]
+        : document.getElementById('carrierInput')?.files[0];
     const mode = document.querySelector('input[name="embed_mode"]:checked')?.value?.toUpperCase() || 'LSB';
     const summary = document.getElementById('stepImagesSummary');
     const stepNum = document.getElementById('stepImagesNumber');
@@ -484,12 +689,12 @@ function updateImagesSummary() {
         summary.textContent = ref ? ref.name.slice(0, 15) : carrier.name.slice(0, 15);
         summary.classList.remove('has-content');
         stepNum.classList.remove('complete');
-        stepNum.textContent = '1';
+        stepNum.textContent = '2';
     } else {
-        summary.textContent = 'Select reference & carrier';
+        summary.textContent = isAudio ? 'Select reference & audio' : 'Select reference & carrier';
         summary.classList.remove('has-content');
         stepNum.classList.remove('complete');
-        stepNum.textContent = '1';
+        stepNum.textContent = '2';
     }
 }
 
@@ -515,7 +720,7 @@ function updatePayloadSummary() {
         summary.textContent = 'Message or file to hide';
         summary.classList.remove('has-content');
         stepNum.classList.remove('complete');
-        stepNum.textContent = '2';
+        stepNum.textContent = '3';
     }
 }
 
@@ -543,14 +748,16 @@ function updateSecuritySummary() {
         summary.textContent = 'Passphrase & keys';
         summary.classList.remove('has-content');
         stepNum.classList.remove('complete');
-        stepNum.textContent = '3';
+        stepNum.textContent = '4';
     }
 }
 
 // Attach listeners
 document.getElementById('refPhotoInput')?.addEventListener('change', updateImagesSummary);
 document.getElementById('carrierInput')?.addEventListener('change', updateImagesSummary);
+document.getElementById('audioCarrierInput')?.addEventListener('change', updateImagesSummary);
 document.querySelectorAll('input[name="embed_mode"]').forEach(r => r.addEventListener('change', updateImagesSummary));
+document.querySelectorAll('#audioModeGroup input[name="embed_mode"]').forEach(r => r.addEventListener('change', updateImagesSummary));
 
 document.getElementById('messageInput')?.addEventListener('input', updatePayloadSummary);
 document.getElementById('payloadFileInput')?.addEventListener('change', updatePayloadSummary);

frontends/web/templates/encode_result.html

@@ -12,6 +12,20 @@
                 </h5>
             </div>
             <div class="card-body text-center">
+                {% if carrier_type == 'audio' %}
+                <!-- Audio Preview -->
+                <div class="my-4">
+                    <div class="text-center">
+                        <i class="bi bi-music-note-beamed text-success" style="font-size: 4rem;"></i>
+                        <div class="mt-2">
+                            <audio controls src="{{ url_for('encode_file_route', file_id=file_id) }}" class="w-100" style="max-width: 400px;"></audio>
+                        </div>
+                        <div class="mt-2 small text-muted">
+                            <i class="bi bi-music-note-beamed me-1"></i>Encoded Audio Preview
+                        </div>
+                    </div>
+                </div>
+                {% else %}
                 <div class="my-4">
                     {% if thumbnail_url %}
                     <!-- Thumbnail of the actual encoded image -->
@@ -29,8 +43,9 @@
                     <i class="bi bi-file-earmark-image text-success" style="font-size: 4rem;"></i>
                     {% endif %}
                 </div>
+                {% endif %}
                 
-                <p class="lead mb-4">Your secret has been hidden in the image.</p>
+                <p class="lead mb-4">Your secret has been hidden in the {{ 'audio file' if carrier_type == 'audio' else 'image' }}.</p>
                 
                 <div class="mb-3">
                     <code class="fs-5">{{ filename }}</code>
@@ -38,7 +53,28 @@
                 
                 <!-- Mode and format badges -->
                 <div class="mb-4">
-                    {% if embed_mode == 'dct' %}
+                    {% if carrier_type == 'audio' %}
+                    <!-- Audio mode badges -->
+                    {% if embed_mode == 'audio_spread' %}
+                    <span class="badge bg-warning text-dark fs-6">
+                        <i class="bi bi-broadcast me-1"></i>Spread Spectrum
+                    </span>
+                    {% else %}
+                    <span class="badge bg-primary fs-6">
+                        <i class="bi bi-grid-3x3-gap me-1"></i>Audio LSB
+                    </span>
+                    {% endif %}
+                    <span class="badge bg-info fs-6 ms-1">
+                        <i class="bi bi-file-earmark-music me-1"></i>WAV
+                    </span>
+                    <div class="small text-muted mt-2">
+                        {% if embed_mode == 'audio_spread' %}
+                        Spread spectrum embedding in audio samples
+                        {% else %}
+                        LSB embedding in audio samples, WAV output
+                        {% endif %}
+                    </div>
+                    {% elif embed_mode == 'dct' %}
                         <span class="badge bg-info fs-6">
                             <i class="bi bi-soundwave me-1"></i>DCT Mode
                         </span>
@@ -114,7 +150,7 @@
                 <div class="d-grid gap-2">
                     <a href="{{ url_for('encode_download', file_id=file_id) }}" 
                        class="btn btn-primary btn-lg" id="downloadBtn">
-                        <i class="bi bi-download me-2"></i>Download Image
+                        <i class="bi bi-download me-2"></i>Download {{ 'Audio' if carrier_type == 'audio' else 'Image' }}
                     </a>
                     
                     <button type="button" class="btn btn-outline-primary" id="shareBtn" style="display: none;">
@@ -129,6 +165,11 @@
                     <strong>Important:</strong>
                     <ul class="mb-0 mt-2">
                         <li>This file expires in <strong>10 minutes</strong></li>
+                        {% if carrier_type == 'audio' %}
+                        <li>Do <strong>not</strong> re-encode or convert the audio file</li>
+                        <li>WAV format preserves your hidden data losslessly</li>
+                        <li>Sharing via platforms that re-encode audio will destroy the hidden data</li>
+                        {% else %}
                         <li>Do <strong>not</strong> resize or recompress the image</li>
                         {% if embed_mode == 'dct' and output_format == 'jpeg' %}
                         <li>JPEG format is lossy - avoid re-saving or editing</li>
@@ -141,6 +182,7 @@
                         <li>Color preserved - extraction works on both color and grayscale</li>
                         {% endif %}
                         {% endif %}
+                        {% endif %}
                         {% if channel_mode == 'private' %}
                         <li><i class="bi bi-shield-lock text-warning me-1"></i>Recipient needs the <strong>same channel key</strong> to decode</li>
                         {% endif %}
@@ -148,7 +190,7 @@
                 </div>
                 
                 <a href="{{ url_for('encode_page') }}" class="btn btn-outline-secondary">
-                    <i class="bi bi-arrow-repeat me-2"></i>Encode Another Message
+                    <i class="bi bi-arrow-repeat me-2"></i>Encode Another
                 </a>
             </div>
         </div>
@@ -162,7 +204,7 @@
 const shareBtn = document.getElementById('shareBtn');
 const fileUrl = "{{ url_for('encode_file_route', file_id=file_id, _external=True) }}";
 const fileName = "{{ filename }}";
-const mimeType = "{{ 'image/jpeg' if embed_mode == 'dct' and output_format == 'jpeg' else 'image/png' }}";
+const mimeType = "{{ 'audio/wav' if carrier_type == 'audio' else ('image/jpeg' if embed_mode == 'dct' and output_format == 'jpeg' else 'image/png') }}";
 
 if (navigator.share && navigator.canShare) {
     // Check if we can share files

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "stegasoo"
-version = "4.2.1"
+version = "4.3.0"
 description = "Secure steganography with hybrid photo + passphrase + PIN authentication"
 readme = "README.md"
 license = "MIT"

src/stegasoo/__init__.py

@@ -7,7 +7,7 @@ Changes in v4.0.0:
 - encode() and decode() now accept channel_key parameter
 """
 
-__version__ = "4.2.1"
+__version__ = "4.3.0"
 
 # Core functionality
 # Channel key management (v4.0.0)
@@ -24,8 +24,8 @@ from .channel import (
 
 # Crypto functions
 from .crypto import get_active_channel_key, get_channel_fingerprint, has_argon2
-from .decode import decode, decode_audio, decode_file, decode_text
-from .encode import encode, encode_audio
+from .decode import decode, decode_file, decode_text
+from .encode import encode
 
 # Credential generation
 from .generate import (
@@ -54,22 +54,28 @@ from .steganography import (
 # Utilities
 from .utils import generate_filename
 
-# Audio utilities - optional, may not be available (v4.3.0)
-try:
+# Audio support — gated by STEGASOO_AUDIO env var and dependency availability
+from .constants import AUDIO_ENABLED, VIDEO_ENABLED
+
+HAS_AUDIO_SUPPORT = AUDIO_ENABLED
+HAS_VIDEO_SUPPORT = VIDEO_ENABLED
+
+if AUDIO_ENABLED:
     from .audio_utils import (
         detect_audio_format,
         get_audio_info,
         has_ffmpeg_support,
         validate_audio,
     )
-
-    HAS_AUDIO_SUPPORT = True
-except ImportError:
-    HAS_AUDIO_SUPPORT = False
+    from .decode import decode_audio
+    from .encode import encode_audio
+else:
     detect_audio_format = None
     get_audio_info = None
     has_ffmpeg_support = None
     validate_audio = None
+    encode_audio = None
+    decode_audio = None
 
 # QR Code utilities - optional, may not be available
 try:
@@ -203,6 +209,7 @@ __all__ = [
     "has_ffmpeg_support",
     "validate_audio",
     "HAS_AUDIO_SUPPORT",
+    "HAS_VIDEO_SUPPORT",
     "validate_audio_embed_mode",
     "validate_audio_file",
     # Generation

src/stegasoo/audio_steganography.py

@@ -283,7 +283,9 @@ def embed_in_audio_lsb(
         # 2. Prepend magic + length prefix
         header = AUDIO_MAGIC_LSB + struct.pack(">I", len(data))
         payload = header + data
-        debug.print(f"Payload with header: {len(payload)} bytes (magic 4 + len 4 + data {len(data)})")
+        debug.print(
+            f"Payload with header: {len(payload)} bytes (magic 4 + len 4 + data {len(data)})"
+        )
 
         # 3. Check capacity
         max_bytes = (num_samples * bits_per_sample) // 8
@@ -463,9 +465,7 @@ def extract_from_audio_lsb(
         total_samples_needed = (total_bits + bits_per_sample - 1) // bits_per_sample
 
         if total_samples_needed > num_samples:
-            debug.print(
-                f"Need {total_samples_needed} samples but only {num_samples} available"
-            )
+            debug.print(f"Need {total_samples_needed} samples but only {num_samples} available")
             return None
 
         debug.print(f"Need {total_samples_needed} samples to extract {data_length} bytes")
@@ -483,14 +483,10 @@ def extract_from_audio_lsb(
                 binary_data += str((val >> bit_pos) & 1)
 
             if progress_file and progress_idx % PROGRESS_INTERVAL == 0:
-                _write_progress(
-                    progress_file, progress_idx, total_samples_needed, "extracting"
-                )
+                _write_progress(progress_file, progress_idx, total_samples_needed, "extracting")
 
         if progress_file:
-            _write_progress(
-                progress_file, total_samples_needed, total_samples_needed, "extracting"
-            )
+            _write_progress(progress_file, total_samples_needed, total_samples_needed, "extracting")
 
         # Skip the 8-byte header (magic + length) = 64 bits
         data_bits = binary_data[64 : 64 + (data_length * 8)]

src/stegasoo/audio_utils.py

@@ -13,7 +13,6 @@ Both are optional — functions degrade gracefully when unavailable.
 from __future__ import annotations
 
 import io
-import logging
 import shutil
 
 from .constants import (
@@ -24,10 +23,11 @@ from .constants import (
     MIN_AUDIO_SAMPLE_RATE,
     VALID_AUDIO_EMBED_MODES,
 )
+from .debug import get_logger
 from .exceptions import AudioTranscodeError, AudioValidationError, UnsupportedAudioFormatError
 from .models import AudioInfo, ValidationResult
 
-logger = logging.getLogger(__name__)
+logger = get_logger(__name__)
 
 
 # =============================================================================
@@ -69,10 +69,12 @@ def detect_audio_format(audio_data: bytes) -> str:
         Format string: "wav", "flac", "mp3", "ogg", "aac", "m4a", or "unknown".
     """
     if len(audio_data) < 12:
+        logger.debug("detect_audio_format: data too short (%d bytes)", len(audio_data))
         return "unknown"
 
     # WAV: RIFF....WAVE
     if audio_data[:4] == b"RIFF" and audio_data[8:12] == b"WAVE":
+        logger.debug("Detected WAV format (%d bytes)", len(audio_data))
         return "wav"
 
     # FLAC
@@ -124,6 +126,7 @@ def transcode_to_wav(audio_data: bytes) -> bytes:
         UnsupportedAudioFormatError: If the format cannot be detected.
     """
     fmt = detect_audio_format(audio_data)
+    logger.info("transcode_to_wav: input format=%s, size=%d bytes", fmt, len(audio_data))
 
     if fmt == "unknown":
         raise UnsupportedAudioFormatError(
@@ -325,7 +328,9 @@ def _get_info_soundfile(audio_data: bytes, fmt: str) -> AudioInfo:
     try:
         import soundfile as sf
     except ImportError:
-        raise AudioTranscodeError("soundfile package is required. Install with: pip install soundfile")
+        raise AudioTranscodeError(
+            "soundfile package is required. Install with: pip install soundfile"
+        )
 
     try:
         buf = io.BytesIO(audio_data)
@@ -460,8 +465,7 @@ def validate_audio(
     fmt = detect_audio_format(audio_data)
     if fmt == "unknown":
         return ValidationResult.error(
-            f"Could not detect {name} format. "
-            "Supported formats: WAV, FLAC, MP3, OGG, AAC, M4A."
+            f"Could not detect {name} format. " "Supported formats: WAV, FLAC, MP3, OGG, AAC, M4A."
         )
 
     # Extract metadata for further validation

src/stegasoo/channel.py

@@ -69,6 +69,7 @@ def _get_machine_key() -> bytes:
     # Fallback to hostname
     if not machine_id:
         import socket
+
         machine_id = socket.gethostname()
 
     # Hash to get consistent 32 bytes
@@ -87,10 +88,7 @@ def _encrypt_for_storage(plaintext: str) -> str:
     plaintext_bytes = plaintext.encode()
 
     # XOR with key (cycling if needed)
-    encrypted = bytes(
-        pb ^ key[i % len(key)]
-        for i, pb in enumerate(plaintext_bytes)
-    )
+    encrypted = bytes(pb ^ key[i % len(key)] for i, pb in enumerate(plaintext_bytes))
 
     return ENCRYPTED_PREFIX + base64.b64encode(encrypted).decode()
 
@@ -108,14 +106,11 @@ def _decrypt_from_storage(stored: str) -> str | None:
         return stored
 
     try:
-        encrypted = base64.b64decode(stored[len(ENCRYPTED_PREFIX):])
+        encrypted = base64.b64decode(stored[len(ENCRYPTED_PREFIX) :])
         key = _get_machine_key()
 
         # XOR to decrypt
-        decrypted = bytes(
-            eb ^ key[i % len(key)]
-            for i, eb in enumerate(encrypted)
-        )
+        decrypted = bytes(eb ^ key[i % len(key)] for i, eb in enumerate(encrypted))
 
         return decrypted.decode()
     except Exception:
@@ -413,7 +408,11 @@ def get_channel_status() -> dict:
                     try:
                         stored = config_path.read_text().strip()
                         file_key = _decrypt_from_storage(stored)
-                        if file_key and validate_channel_key(file_key) and format_channel_key(file_key) == key:
+                        if (
+                            file_key
+                            and validate_channel_key(file_key)
+                            and format_channel_key(file_key) == key
+                        ):
                             source = str(config_path)
                             break
                     except (OSError, PermissionError, ValueError):
@@ -485,7 +484,9 @@ def resolve_channel_key(
         >>> resolve_channel_key("ABCD-1234-...")  # -> "ABCD-1234-..."
         >>> resolve_channel_key(file_path="key.txt")  # reads from file
     """
-    debug.print(f"resolve_channel_key: value={value}, file_path={file_path}, no_channel={no_channel}")
+    debug.print(
+        f"resolve_channel_key: value={value}, file_path={file_path}, no_channel={no_channel}"
+    )
 
     # no_channel flag takes precedence
     if no_channel:

src/stegasoo/cli.py

@@ -108,8 +108,9 @@ CONTEXT_SETTINGS = dict(help_option_names=["-h", "--help"])
 @click.group(context_settings=CONTEXT_SETTINGS)
 @click.version_option(__version__, "-v", "--version")
 @click.option("--json", "json_output", is_flag=True, help="Output results as JSON")
+@click.option("--debug", "debug_mode", is_flag=True, help="Enable debug logging to stderr")
 @click.pass_context
-def cli(ctx, json_output):
+def cli(ctx, json_output, debug_mode):
     """
     Stegasoo - Steganography with hybrid authentication.
 
@@ -120,6 +121,11 @@ def cli(ctx, json_output):
     ctx.ensure_object(dict)
     ctx.obj["json"] = json_output
 
+    if debug_mode:
+        from .debug import debug
+
+        debug.enable(True)
+
 
 # =============================================================================
 # ENCODE COMMANDS
@@ -179,9 +185,7 @@ def cli(ctx, json_output):
 @click.option("--pin", prompt=True, hide_input=True, confirmation_prompt=True, help="PIN code")
 @click.option("--dry-run", is_flag=True, help="Show capacity usage without encoding")
 @click.pass_context
-def encode(
-    ctx, carrier, reference, message, file_payload, output, passphrase, pin, dry_run
-):
+def encode(ctx, carrier, reference, message, file_payload, output, passphrase, pin, dry_run):
     """
     Encode a message or file into an image.
 
@@ -245,14 +249,14 @@ def encode(
     # Default to JPEG for JPEG carriers (preserves DCT mode benefits)
     carrier_ext = Path(carrier).suffix.lower()
     if not output:
-        if carrier_ext in ('.jpg', '.jpeg'):
+        if carrier_ext in (".jpg", ".jpeg"):
             output = f"{Path(carrier).stem}_encoded.jpg"
         else:
             output = f"{Path(carrier).stem}_encoded.png"
 
     # Detect output format from extension
     output_ext = Path(output).suffix.lower()
-    use_dct = output_ext in ('.jpg', '.jpeg')
+    use_dct = output_ext in (".jpg", ".jpeg")
 
     from .steganography import EMBED_MODE_DCT, EMBED_MODE_LSB
 
@@ -442,8 +446,38 @@ def decode(ctx, image, reference, passphrase, pin, output):
     help="Passphrase (recommend 4+ words)",
 )
 @click.option("--pin", prompt=True, hide_input=True, confirmation_prompt=True, help="PIN code")
+@click.option(
+    "--rsa-key",
+    type=click.Path(exists=True),
+    help="RSA private key PEM file",
+)
+@click.option("--rsa-password", default=None, help="Password for encrypted RSA key")
+@click.option("--channel-key", default=None, help="Channel key for deployment isolation")
+@click.option(
+    "--chip-tier",
+    "chip_tier",
+    default=None,
+    type=click.Choice(["lossless", "high", "low"]),
+    help="Spread spectrum chip tier (lossless=256, high=512, low=1024). Only for audio_spread.",
+)
+@click.option("--dry-run", is_flag=True, help="Show capacity usage without encoding")
 @click.pass_context
-def audio_encode(ctx, carrier, reference, message, file_payload, output, embed_mode, passphrase, pin):
+def audio_encode(
+    ctx,
+    carrier,
+    reference,
+    message,
+    file_payload,
+    output,
+    embed_mode,
+    passphrase,
+    pin,
+    rsa_key,
+    rsa_password,
+    channel_key,
+    chip_tier,
+    dry_run,
+):
     """
     Encode a message or file into an audio carrier.
 
@@ -452,26 +486,100 @@ def audio_encode(ctx, carrier, reference, message, file_payload, output, embed_m
         stegasoo audio-encode carrier.wav -r ref.jpg -m "Secret" --mode audio_lsb
 
         stegasoo audio-encode carrier.wav -r ref.jpg -f secret.pdf --mode audio_spread
+
+        stegasoo audio-encode carrier.wav -r ref.jpg -m "Secret" --dry-run
     """
+    from .constants import AUDIO_ENABLED
+
+    if not AUDIO_ENABLED:
+        raise click.UsageError(
+            "Audio support is disabled. Install audio extras (pip install stegasoo[audio]) "
+            "or set STEGASOO_AUDIO=1 to force enable."
+        )
+
+    from .audio_steganography import calculate_audio_lsb_capacity
     from .encode import encode_audio
     from .models import FilePayload
+    from .spread_steganography import calculate_audio_spread_capacity
 
     if not message and not file_payload:
         raise click.UsageError("Either --message or --file is required")
 
+    # Read RSA key if provided
+    rsa_key_data = None
+    if rsa_key:
+        with open(rsa_key, "rb") as f:
+            rsa_key_data = f.read()
+
+    # Calculate payload size
+    if file_payload:
+        payload_size = Path(file_payload).stat().st_size
+        payload_type = "file"
+    else:
+        payload_size = len(message.encode("utf-8"))
+        payload_type = "text"
+
     # Read input files
     with open(reference, "rb") as f:
         reference_data = f.read()
     with open(carrier, "rb") as f:
         carrier_data = f.read()
 
+    if dry_run:
+        try:
+            from .audio_utils import get_audio_info
+
+            info = get_audio_info(carrier_data)
+            lsb_capacity = calculate_audio_lsb_capacity(carrier_data)
+            spread_capacity = calculate_audio_spread_capacity(carrier_data)
+
+            if embed_mode == "audio_lsb":
+                capacity = lsb_capacity
+            else:
+                capacity = spread_capacity.usable_capacity_bytes
+
+            result = {
+                "carrier": carrier,
+                "reference": reference,
+                "format": info.format,
+                "sample_rate": info.sample_rate,
+                "channels": info.channels,
+                "duration_seconds": round(info.duration_seconds, 2),
+                "embed_mode": embed_mode,
+                "capacity_bytes": capacity,
+                "lsb_capacity_bytes": lsb_capacity,
+                "spread_capacity_bytes": spread_capacity.usable_capacity_bytes,
+                "payload_type": payload_type,
+                "payload_size": payload_size,
+                "usage_percent": round(payload_size / capacity * 100, 1) if capacity > 0 else 0,
+                "fits": payload_size < capacity,
+            }
+
+            if ctx.obj.get("json"):
+                click.echo(json.dumps(result, indent=2))
+            else:
+                click.echo(
+                    f"Carrier: {carrier} ({info.format}, {info.sample_rate}Hz, {info.channels}ch)"
+                )
+                click.echo(f"Duration: {info.duration_seconds:.1f}s")
+                click.echo(f"Reference: {reference}")
+                click.echo(f"Mode: {embed_mode}")
+                click.echo(f"LSB capacity: {lsb_capacity:,} bytes ({lsb_capacity // 1024} KB)")
+                click.echo(f"Spread capacity: {spread_capacity.usable_capacity_bytes:,} bytes")
+                click.echo(f"Payload: {payload_size:,} bytes ({payload_type})")
+                click.echo(f"Usage: {result['usage_percent']}%")
+                click.echo(f"Status: {'✓ Fits' if result['fits'] else '✗ Too large'}")
+        except Exception as e:
+            if ctx.obj.get("json"):
+                click.echo(json.dumps({"status": "error", "error": str(e)}, indent=2))
+            else:
+                click.echo(f"✗ Capacity check failed: {e}", err=True)
+            raise SystemExit(1)
+        return
+
     # Determine output path
     if not output:
-        carrier_path = Path(carrier)
-        if embed_mode == "audio_lsb":
-            output = f"{carrier_path.stem}_encoded.wav"
-        else:
-            output = f"{carrier_path.stem}_encoded.wav"
+        output = f"{Path(carrier).stem}_encoded.wav"
 
     try:
         if file_payload:
@@ -479,13 +587,24 @@ def audio_encode(ctx, carrier, reference, message, file_payload, output, embed_m
         else:
             payload = message
 
+        # Resolve chip tier name to integer
+        resolved_chip_tier = None
+        if chip_tier is not None:
+            from .constants import AUDIO_SS_CHIP_TIER_NAMES
+
+            resolved_chip_tier = AUDIO_SS_CHIP_TIER_NAMES.get(chip_tier)
+
         stego_audio, stats = encode_audio(
             message=payload,
             reference_photo=reference_data,
             carrier_audio=carrier_data,
             passphrase=passphrase,
             pin=pin,
+            rsa_key_data=rsa_key_data,
+            rsa_password=rsa_password,
             embed_mode=embed_mode,
+            channel_key=channel_key,
+            chip_tier=resolved_chip_tier,
         )
 
         with open(output, "wb") as f:
@@ -539,9 +658,18 @@ def audio_encode(ctx, carrier, reference, message, file_payload, output, embed_m
 )
 @click.option("--passphrase", prompt=True, hide_input=True, help="Passphrase")
 @click.option("--pin", prompt=True, hide_input=True, help="PIN code")
+@click.option(
+    "--rsa-key",
+    type=click.Path(exists=True),
+    help="RSA private key PEM file",
+)
+@click.option("--rsa-password", default=None, help="Password for encrypted RSA key")
+@click.option("--channel-key", default=None, help="Channel key for deployment isolation")
 @click.option("-o", "--output", type=click.Path(), help="Output path for file payloads")
 @click.pass_context
-def audio_decode(ctx, audio, reference, embed_mode, passphrase, pin, output):
+def audio_decode(
+    ctx, audio, reference, embed_mode, passphrase, pin, rsa_key, rsa_password, channel_key, output
+):
     """
     Decode a message or file from stego audio.
 
@@ -551,8 +679,22 @@ def audio_decode(ctx, audio, reference, embed_mode, passphrase, pin, output):
 
         stegasoo audio-decode stego.wav -r ref.jpg --mode audio_lsb -o ./extracted/
     """
+    from .constants import AUDIO_ENABLED
+
+    if not AUDIO_ENABLED:
+        raise click.UsageError(
+            "Audio support is disabled. Install audio extras (pip install stegasoo[audio]) "
+            "or set STEGASOO_AUDIO=1 to force enable."
+        )
+
     from .decode import decode_audio
 
+    # Read RSA key if provided
+    rsa_key_data = None
+    if rsa_key:
+        with open(rsa_key, "rb") as f:
+            rsa_key_data = f.read()
+
     with open(audio, "rb") as f:
         audio_data = f.read()
     with open(reference, "rb") as f:
@@ -564,7 +706,10 @@ def audio_decode(ctx, audio, reference, embed_mode, passphrase, pin, output):
             reference_photo=reference_data,
             passphrase=passphrase,
             pin=pin,
+            rsa_key_data=rsa_key_data,
+            rsa_password=rsa_password,
             embed_mode=embed_mode,
+            channel_key=channel_key,
         )
 
         if result.is_file:
@@ -617,6 +762,97 @@ def audio_decode(ctx, audio, reference, embed_mode, passphrase, pin, output):
         raise SystemExit(1)
 
 
+@cli.command("audio-info")
+@click.argument("audio", type=click.Path(exists=True))
+@click.pass_context
+def audio_info(ctx, audio):
+    """
+    Show audio file information and steganographic capacity.
+
+    Examples:
+
+        stegasoo audio-info carrier.wav
+
+        stegasoo --json audio-info carrier.wav
+    """
+    from .constants import AUDIO_ENABLED
+
+    if not AUDIO_ENABLED:
+        raise click.UsageError(
+            "Audio support is disabled. Install audio extras (pip install stegasoo[audio]) "
+            "or set STEGASOO_AUDIO=1 to force enable."
+        )
+
+    from .audio_steganography import calculate_audio_lsb_capacity
+    from .audio_utils import get_audio_info
+    from .spread_steganography import calculate_audio_spread_capacity
+
+    with open(audio, "rb") as f:
+        audio_data = f.read()
+
+    try:
+        info = get_audio_info(audio_data)
+        lsb_capacity = calculate_audio_lsb_capacity(audio_data)
+
+        # Calculate spread capacity at each chip tier
+        spread_tiers = {}
+        for tier_name, tier_val in [("lossless", 0), ("high", 1), ("low", 2)]:
+            cap = calculate_audio_spread_capacity(audio_data, chip_tier=tier_val)
+            spread_tiers[tier_name] = {
+                "bytes": cap.usable_capacity_bytes,
+                "kb": round(cap.usable_capacity_bytes / 1024, 1),
+                "chip_length": cap.chip_length,
+                "embeddable_channels": cap.embeddable_channels,
+            }
+
+        result = {
+            "file": audio,
+            "format": info.format,
+            "sample_rate": info.sample_rate,
+            "channels": info.channels,
+            "duration_seconds": round(info.duration_seconds, 2),
+            "num_samples": info.num_samples,
+            "bit_depth": info.bit_depth,
+            "file_size": len(audio_data),
+            "capacity": {
+                "audio_lsb": {
+                    "bytes": lsb_capacity,
+                    "kb": round(lsb_capacity / 1024, 1),
+                },
+                "audio_spread": spread_tiers,
+            },
+        }
+
+        if ctx.obj.get("json"):
+            click.echo(json.dumps(result, indent=2))
+        else:
+            click.echo(f"File: {audio}")
+            click.echo(f"Format: {info.format}")
+            click.echo(f"Sample rate: {info.sample_rate} Hz")
+            click.echo(f"Channels: {info.channels}")
+            click.echo(f"Duration: {info.duration_seconds:.1f}s")
+            click.echo(f"Samples: {info.num_samples:,}")
+            if info.bit_depth:
+                click.echo(f"Bit depth: {info.bit_depth}-bit")
+            click.echo(f"File size: {len(audio_data):,} bytes")
+            click.echo()
+            click.echo("Steganographic capacity:")
+            click.echo(f"  LSB:              {lsb_capacity:,} bytes ({lsb_capacity // 1024} KB)")
+            for tier_name in ("lossless", "high", "low"):
+                t = spread_tiers[tier_name]
+                click.echo(
+                    f"  Spread ({tier_name:>8}, chip={t['chip_length']}): "
+                    f"{t['bytes']:,} bytes ({t['kb']} KB)"
+                )
+
+    except Exception as e:
+        if ctx.obj.get("json"):
+            click.echo(json.dumps({"status": "error", "error": str(e)}, indent=2))
+        else:
+            click.echo(f"✗ Audio info failed: {e}", err=True)
+        raise SystemExit(1)
+
+
 # =============================================================================
 # BATCH COMMANDS
 # =============================================================================
@@ -828,9 +1064,7 @@ def batch_check(ctx, images, recursive):
 @click.option(
     "--pin-length", default=DEFAULT_PIN_LENGTH, help=f"PIN length (default: {DEFAULT_PIN_LENGTH})"
 )
-@click.option(
-    "--channel-key", is_flag=True, help="Also generate a 256-bit channel key"
-)
+@click.option("--channel-key", is_flag=True, help="Also generate a 256-bit channel key")
 @click.pass_context
 def generate(ctx, words, pin_length, channel_key):
     """
@@ -889,6 +1123,7 @@ def generate(ctx, words, pin_length, channel_key):
     # Generate channel key if requested
     if channel_key:
         from .channel import generate_channel_key
+
         result["channel_key"] = generate_channel_key()
 
     if ctx.obj.get("json"):
@@ -912,6 +1147,7 @@ def info(ctx, full):
     # Check for DCT support
     try:
         from .dct_steganography import HAS_JPEGIO, HAS_SCIPY
+
         has_dct = HAS_SCIPY and HAS_JPEGIO
     except ImportError:
         has_dct = False
@@ -954,6 +1190,7 @@ def info(ctx, full):
     channel_source = None
     try:
         from .channel import get_channel_fingerprint, get_channel_key, get_channel_status
+
         key = get_channel_key()
         if key:
             channel_fingerprint = get_channel_fingerprint(key)
@@ -986,7 +1223,7 @@ def info(ctx, full):
         try:
             # Disk free
             st = os.statvfs("/")
-            disk_free = (st.f_bavail * st.f_frsize) / (1024 ** 3)  # GB
+            disk_free = (st.f_bavail * st.f_frsize) / (1024**3)  # GB
         except OSError:
             pass
 
@@ -1005,20 +1242,28 @@ def info(ctx, full):
         "service": service_status,
         "url": service_url,
         "dct_support": has_dct,
-        "channel": {
+        "channel": (
+            {
                 "fingerprint": channel_fingerprint,
                 "source": channel_source,
-        } if channel_fingerprint else None,
+            }
+            if channel_fingerprint
+            else None
+        ),
         "limits": {
             "max_message_bytes": MAX_MESSAGE_SIZE,
             "max_file_payload_bytes": MAX_FILE_PAYLOAD_SIZE,
         },
-        "system": {
+        "system": (
+            {
                 "cpu_mhz": cpu_freq,
                 "temp_c": cpu_temp,
                 "disk_free_gb": round(disk_free, 1) if disk_free else None,
                 "uptime": uptime,
-        } if full else None,
+            }
+            if full
+            else None
+        ),
     }
 
     if ctx.obj.get("json"):
@@ -1055,7 +1300,9 @@ def info(ctx, full):
             if cpu_freq:
                 click.echo(f"  CPU:         {cpu_freq} MHz")
             if cpu_temp:
-                temp_color = "\033[32m" if cpu_temp < 60 else "\033[33m" if cpu_temp < 75 else "\033[31m"
+                temp_color = (
+                    "\033[32m" if cpu_temp < 60 else "\033[33m" if cpu_temp < 75 else "\033[31m"
+                )
                 click.echo(f"  Temp:        {temp_color}{cpu_temp:.1f}°C\033[0m")
             if uptime:
                 click.echo(f"  Uptime:      {uptime}")
@@ -1384,7 +1631,7 @@ def tools_capacity(image, as_json):
         click.echo(f"  Megapixels:   {result['megapixels']} MP")
         click.echo(f"  {'─' * 40}")
         click.echo(f"  LSB Capacity: {result['lsb']['capacity_kb']:.1f} KB")
-        if result['dct']['available']:
+        if result["dct"]["available"]:
             click.echo(f"  DCT Capacity: {result['dct']['capacity_kb']:.1f} KB")
         else:
             click.echo("  DCT Capacity: N/A (scipy required)")
@@ -1394,7 +1641,9 @@ def tools_capacity(image, as_json):
 @tools.command("strip")
 @click.argument("image", type=click.Path(exists=True))
 @click.option("-o", "--output", type=click.Path(), help="Output file (default: <name>_clean.png)")
-@click.option("--format", "fmt", type=click.Choice(["png", "bmp"]), default="png", help="Output format")
+@click.option(
+    "--format", "fmt", type=click.Choice(["png", "bmp"]), default="png", help="Output format"
+)
 def tools_strip(image, output, fmt):
     """Strip EXIF/metadata from an image.
 
@@ -1529,7 +1778,9 @@ def tools_exif(image, clear, set_fields, output, as_json):
 @tools.command("compress")
 @click.argument("image", type=click.Path(exists=True))
 @click.option("-q", "--quality", type=int, default=75, help="JPEG quality (1-100, default: 75)")
-@click.option("-o", "--output", type=click.Path(), help="Output file (default: <name>_q<quality>.jpg)")
+@click.option(
+    "-o", "--output", type=click.Path(), help="Output file (default: <name>_q<quality>.jpg)"
+)
 def tools_compress(image, quality, output):
     """Compress a JPEG image.
 
@@ -1541,9 +1792,10 @@ def tools_compress(image, quality, output):
         stegasoo tools compress photo.jpg -q 60
         stegasoo tools compress photo.jpg -q 80 -o smaller.jpg
     """
-    from PIL import Image
     import io
 
+    from PIL import Image
+
     if not 1 <= quality <= 100:
         raise click.UsageError("Quality must be between 1 and 100")
 
@@ -1578,7 +1830,9 @@ def tools_compress(image, quality, output):
 
 @tools.command("rotate")
 @click.argument("image", type=click.Path(exists=True))
-@click.option("-r", "--rotation", type=click.Choice(["90", "180", "270"]), help="Rotation degrees clockwise")
+@click.option(
+    "-r", "--rotation", type=click.Choice(["90", "180", "270"]), help="Rotation degrees clockwise"
+)
 @click.option("--flip-h", is_flag=True, help="Flip horizontally")
 @click.option("--flip-v", is_flag=True, help="Flip vertically")
 @click.option("-o", "--output", type=click.Path(), help="Output file")
@@ -1593,10 +1847,11 @@ def tools_rotate(image, rotation, flip_h, flip_v, output):
         stegasoo tools rotate photo.jpg -r 90
         stegasoo tools rotate photo.jpg -r 180 --flip-h -o rotated.jpg
     """
-    from PIL import Image
     import io
     import shutil
 
+    from PIL import Image
+
     with open(image, "rb") as f:
         image_data = f.read()
 
@@ -1622,9 +1877,9 @@ def tools_rotate(image, rotation, flip_h, flip_v, output):
 
         # Apply flips using jpegtran
         if flip_h or flip_v:
+            import os
             import subprocess
             import tempfile
-            import os
 
             for flip_type in (["horizontal"] if flip_h else []) + (["vertical"] if flip_v else []):
                 with tempfile.NamedTemporaryFile(suffix=".jpg", delete=False) as f:
@@ -1633,9 +1888,19 @@ def tools_rotate(image, rotation, flip_h, flip_v, output):
                 output_path = tempfile.mktemp(suffix=".jpg")
                 try:
                     subprocess.run(
-                        ["jpegtran", "-flip", flip_type, "-copy", "all",
-                         "-outfile", output_path, input_path],
-                        capture_output=True, timeout=30, check=True
+                        [
+                            "jpegtran",
+                            "-flip",
+                            flip_type,
+                            "-copy",
+                            "all",
+                            "-outfile",
+                            output_path,
+                            input_path,
+                        ],
+                        capture_output=True,
+                        timeout=30,
+                        check=True,
                     )
                     with open(output_path, "rb") as f:
                         result_data = f.read()
@@ -1680,8 +1945,17 @@ def tools_rotate(image, rotation, flip_h, flip_v, output):
 
 @tools.command("convert")
 @click.argument("image", type=click.Path(exists=True))
-@click.option("-f", "--format", "fmt", type=click.Choice(["png", "jpg", "bmp", "webp"]), required=True, help="Output format")
-@click.option("-q", "--quality", type=int, default=95, help="Quality for lossy formats (default: 95)")
+@click.option(
+    "-f",
+    "--format",
+    "fmt",
+    type=click.Choice(["png", "jpg", "bmp", "webp"]),
+    required=True,
+    help="Output format",
+)
+@click.option(
+    "-q", "--quality", type=int, default=95, help="Quality for lossy formats (default: 95)"
+)
 @click.option("-o", "--output", type=click.Path(), help="Output file")
 def tools_convert(image, fmt, quality, output):
     """Convert image to a different format.
@@ -1691,9 +1965,10 @@ def tools_convert(image, fmt, quality, output):
         stegasoo tools convert photo.png -f jpg
         stegasoo tools convert photo.jpg -f png -o lossless.png
     """
-    from PIL import Image
     import io
 
+    from PIL import Image
+
     with open(image, "rb") as f:
         image_data = f.read()
 
@@ -1737,12 +2012,14 @@ def admin(ctx):
 
 @admin.command("recover")
 @click.option(
-    "--db", "db_path",
+    "--db",
+    "db_path",
     type=click.Path(exists=True),
-    help="Path to stegasoo.db (default: frontends/web/instance/stegasoo.db)"
+    help="Path to stegasoo.db (default: frontends/web/instance/stegasoo.db)",
+)
+@click.option(
+    "--password", prompt=True, hide_input=True, confirmation_prompt=True, help="New admin password"
 )
-@click.option("--password", prompt=True, hide_input=True, confirmation_prompt=True,
-              help="New admin password")
 def admin_recover(db_path, password):
     """Reset admin password using recovery key.
 
@@ -1772,9 +2049,7 @@ def admin_recover(db_path, password):
                 break
 
     if not db_path or not Path(db_path).exists():
-        raise click.UsageError(
-            "Database not found. Use --db to specify path to stegasoo.db"
-        )
+        raise click.UsageError("Database not found. Use --db to specify path to stegasoo.db")
 
     click.echo(f"Database: {db_path}")
 
@@ -1783,16 +2058,13 @@ def admin_recover(db_path, password):
     db.row_factory = sqlite3.Row
 
     # Get recovery key hash from app_settings
-    cursor = db.execute(
-        "SELECT value FROM app_settings WHERE key = 'recovery_key_hash'"
-    )
+    cursor = db.execute("SELECT value FROM app_settings WHERE key = 'recovery_key_hash'")
     row = cursor.fetchone()
 
     if not row:
         db.close()
         raise click.ClickException(
-            "No recovery key configured for this instance. "
-            "Password reset is not possible."
+            "No recovery key configured for this instance. " "Password reset is not possible."
         )
 
     stored_hash = row["value"]
@@ -1869,6 +2141,7 @@ def admin_generate_key(show_qr):
     if show_qr:
         try:
             import qrcode
+
             qr = qrcode.QRCode(box_size=1, border=1)
             qr.add_data(key)
             qr.make()
@@ -1920,8 +2193,12 @@ def api_keys():
 
 
 @api_keys.command("list")
-@click.option("--location", type=click.Choice(["user", "project", "all"]), default="all",
-              help="Config location to list keys from")
+@click.option(
+    "--location",
+    type=click.Choice(["user", "project", "all"]),
+    default="all",
+    help="Config location to list keys from",
+)
 def api_keys_list(location):
     """List configured API keys.
 
@@ -1935,7 +2212,7 @@ def api_keys_list(location):
     _setup_frontends_path()
 
     try:
-        from api.auth import list_api_keys, get_api_key_status
+        from api.auth import get_api_key_status, list_api_keys
     except ImportError:
         raise click.ClickException("API frontend not available")
 
@@ -1959,8 +2236,12 @@ def api_keys_list(location):
 
 @api_keys.command("create")
 @click.argument("name")
-@click.option("--location", type=click.Choice(["user", "project"]), default="user",
-              help="Where to store the key")
+@click.option(
+    "--location",
+    type=click.Choice(["user", "project"]),
+    default="user",
+    help="Where to store the key",
+)
 def api_keys_create(name, location):
     """Create a new API key.
 
@@ -1993,8 +2274,9 @@ def api_keys_create(name, location):
 
 @api_keys.command("delete")
 @click.argument("name")
-@click.option("--location", type=click.Choice(["user", "project"]), default="user",
-              help="Config location")
+@click.option(
+    "--location", type=click.Choice(["user", "project"]), default="user", help="Config location"
+)
 def api_keys_delete(name, location):
     """Delete an API key by name.
 
@@ -2025,7 +2307,9 @@ def api_tls():
 @api_tls.command("generate")
 @click.option("--hostname", default="localhost", help="Server hostname for certificate")
 @click.option("--days", default=365, help="Certificate validity in days")
-@click.option("--output", "-o", type=click.Path(), help="Output directory (default: ~/.stegasoo/certs)")
+@click.option(
+    "--output", "-o", type=click.Path(), help="Output directory (default: ~/.stegasoo/certs)"
+)
 def api_tls_generate(hostname, days, output):
     """Generate self-signed TLS certificate.
 
@@ -2065,7 +2349,12 @@ def api_tls_generate(hostname, days, output):
 
 
 @api_tls.command("info")
-@click.option("--cert", "-c", type=click.Path(exists=True), help="Certificate file (default: ~/.stegasoo/certs/server.crt)")
+@click.option(
+    "--cert",
+    "-c",
+    type=click.Path(exists=True),
+    help="Certificate file (default: ~/.stegasoo/certs/server.crt)",
+)
 def api_tls_info(cert):
     """Show information about a TLS certificate.
 
@@ -2075,12 +2364,13 @@ def api_tls_info(cert):
         stegasoo api tls info --cert /path/to/server.crt
     """
     from cryptography import x509
-    from cryptography.hazmat.primitives import serialization
 
     if not cert:
         cert = Path.home() / ".stegasoo" / "certs" / "server.crt"
         if not cert.exists():
-            raise click.ClickException(f"No certificate found at {cert}. Generate one with: stegasoo api tls generate")
+            raise click.ClickException(
+                f"No certificate found at {cert}. Generate one with: stegasoo api tls generate"
+            )
 
     cert_data = Path(cert).read_bytes()
     certificate = x509.load_pem_x509_certificate(cert_data)
@@ -2095,7 +2385,8 @@ def api_tls_info(cert):
 
     # Check expiry
     import datetime
-    now = datetime.datetime.now(datetime.timezone.utc)
+
+    now = datetime.datetime.now(datetime.UTC)
     if certificate.not_valid_after_utc < now:
         click.echo("\nStatus: EXPIRED")
     elif certificate.not_valid_after_utc < now + datetime.timedelta(days=30):
@@ -2144,8 +2435,11 @@ def api_serve(host, port, ssl, cert, key, do_reload):
         else:
             try:
                 from web.ssl_utils import ensure_certs
+
                 base_dir = Path.home() / ".stegasoo"
-                cert_path, key_path = ensure_certs(base_dir, host if host != "0.0.0.0" else "localhost")
+                cert_path, key_path = ensure_certs(
+                    base_dir, host if host != "0.0.0.0" else "localhost"
+                )
             except ImportError:
                 raise click.ClickException("ssl_utils not available")
 

src/stegasoo/compression.py

@@ -9,6 +9,10 @@ import struct
 import zlib
 from enum import IntEnum
 
+from .debug import get_logger
+
+logger = get_logger(__name__)
+
 # Optional LZ4 support (faster, slightly worse ratio)
 try:
     import lz4.frame

src/stegasoo/constants.py

@@ -262,8 +262,7 @@ DCT_STEP_SIZE = 8  # QIM quantization step
 # SHA256("\x89ST3\x89DCT") - hardcoded so it never changes even if headers are added
 # Used to XOR recovery keys in QR codes so they scan as gibberish
 RECOVERY_OBFUSCATION_KEY = bytes.fromhex(
-    "d6c70bce27780db942562550e9fe1459"
-    "9dfdb8421f5acc79696b05db4e7afbd2"
+    "d6c70bce27780db942562550e9fe1459" "9dfdb8421f5acc79696b05db4e7afbd2"
 )  # 32 bytes
 
 # Valid embedding modes
@@ -297,6 +296,69 @@ def detect_stego_mode(encrypted_data: bytes) -> str:
         return "unknown"
 
 
+# =============================================================================
+# FEATURE TOGGLES (v4.3.1)
+# =============================================================================
+# Environment variables to enable/disable optional feature families.
+# Values: "auto" (default — detect dependencies), "1"/"true" (force on),
+#         "0"/"false" (force off even if deps are installed).
+# Pi builds or minimal installs can set STEGASOO_AUDIO=0 to stay image-only.
+
+import os as _os
+
+
+def _parse_feature_toggle(env_var: str, default: str = "auto") -> str | bool:
+    """Parse a feature toggle env var. Returns 'auto', True, or False."""
+    val = _os.environ.get(env_var, default).strip().lower()
+    if val in ("1", "true", "yes", "on"):
+        return True
+    if val in ("0", "false", "no", "off"):
+        return False
+    return "auto"
+
+
+def _check_audio_deps() -> bool:
+    """Check if audio dependencies (soundfile, numpy) are importable."""
+    try:
+        import numpy  # noqa: F401
+        import soundfile  # noqa: F401
+
+        return True
+    except ImportError:
+        return False
+
+
+def _check_video_deps() -> bool:
+    """Check if video dependencies (ffmpeg binary + audio deps) are available."""
+    import shutil
+
+    if not _check_audio_deps():
+        return False
+    return shutil.which("ffmpeg") is not None
+
+
+def _resolve_feature(toggle: str | bool, dep_check: callable) -> bool:
+    """Resolve a feature toggle to a final bool."""
+    if toggle is True:
+        if not dep_check():
+            raise ImportError(
+                f"Feature force-enabled but required dependencies are missing. "
+                f"Install the relevant extras (e.g. pip install stegasoo[audio])."
+            )
+        return True
+    if toggle is False:
+        return False
+    # auto
+    return dep_check()
+
+
+_audio_toggle = _parse_feature_toggle("STEGASOO_AUDIO")
+_video_toggle = _parse_feature_toggle("STEGASOO_VIDEO")
+
+AUDIO_ENABLED: bool = _resolve_feature(_audio_toggle, _check_audio_deps)
+VIDEO_ENABLED: bool = _resolve_feature(_video_toggle, _check_video_deps)
+
+
 # =============================================================================
 # AUDIO STEGANOGRAPHY (v4.3.0)
 # =============================================================================
@@ -319,10 +381,31 @@ MAX_AUDIO_SAMPLE_RATE = 192000  # Studio quality
 ALLOWED_AUDIO_EXTENSIONS = {"wav", "flac", "mp3", "ogg", "opus", "aac", "m4a", "wma"}
 
 # Spread spectrum parameters
-AUDIO_SS_CHIP_LENGTH = 1024        # Samples per chip (spreading factor)
+AUDIO_SS_CHIP_LENGTH = 1024  # Samples per chip (spreading factor) — legacy/default
 AUDIO_SS_AMPLITUDE = 0.05  # Per-sample embedding strength (~-26dB, masked by audio)
 AUDIO_SS_RS_NSYM = 32  # Reed-Solomon parity symbols
 
+# Spread spectrum v2: per-channel hybrid embedding (v4.4.0)
+AUDIO_SS_HEADER_VERSION = 2  # v2 header format identifier
+
+# Chip tier system — trade capacity for robustness
+AUDIO_SS_CHIP_TIER_LOSSLESS = 0  # 256 chips — lossless carriers (FLAC/WAV/ALAC)
+AUDIO_SS_CHIP_TIER_HIGH_LOSSY = 1  # 512 chips — high-rate lossy (AAC 256k+)
+AUDIO_SS_CHIP_TIER_LOW_LOSSY = 2  # 1024 chips — low-rate lossy (AAC 128k, Opus)
+AUDIO_SS_DEFAULT_CHIP_TIER = 2  # Most robust, backward compatible
+AUDIO_SS_CHIP_LENGTHS = {0: 256, 1: 512, 2: 1024}
+
+# Chip tier name mapping (for CLI/UI)
+AUDIO_SS_CHIP_TIER_NAMES = {
+    "lossless": AUDIO_SS_CHIP_TIER_LOSSLESS,
+    "high": AUDIO_SS_CHIP_TIER_HIGH_LOSSY,
+    "low": AUDIO_SS_CHIP_TIER_LOW_LOSSY,
+}
+
+# LFE channel skipping — LFE is bandlimited to ~120Hz, terrible carrier
+AUDIO_LFE_CHANNEL_INDEX = 3  # Standard WAV/WAVEFORMATEXTENSIBLE ordering
+AUDIO_LFE_MIN_CHANNELS = 6  # Only skip LFE for 5.1+ layouts
+
 # Echo hiding parameters
 AUDIO_ECHO_DELAY_0 = 50  # Echo delay for bit 0 (samples at 44.1kHz ~ 1.1ms)
 AUDIO_ECHO_DELAY_1 = 100  # Echo delay for bit 1 (samples at 44.1kHz ~ 2.3ms)

src/stegasoo/crypto.py

@@ -46,9 +46,12 @@ from .constants import (
     SALT_SIZE,
     TAG_SIZE,
 )
+from .debug import get_logger
 from .exceptions import DecryptionError, EncryptionError, InvalidHeaderError, KeyDerivationError
 from .models import DecodeResult, FilePayload
 
+logger = get_logger(__name__)
+
 # Check for Argon2 availability
 try:
     from argon2.low_level import Type, hash_secret_raw
@@ -201,6 +204,18 @@ def derive_hybrid_key(
     """
     try:
         photo_hash = hash_photo(photo_data)
+        logger.debug(
+            "derive_hybrid_key: photo_hash=%s, pin=%s, rsa=%s, channel=%s, salt=%d bytes",
+            photo_hash[:4].hex(),
+            "set" if pin else "none",
+            "set" if rsa_key_data else "none",
+            (
+                "explicit"
+                if isinstance(channel_key, str) and channel_key
+                else "auto" if channel_key is None else "none"
+            ),
+            len(salt),
+        )
 
         # Resolve channel key (server-specific binding)
         channel_hash = _resolve_channel_key(channel_key)
@@ -217,8 +232,16 @@ def derive_hybrid_key(
         if channel_hash:
             key_material += channel_hash
 
+        logger.debug("Key material: %d bytes", len(key_material))
+
         # Run it all through the KDF
         if HAS_ARGON2:
+            logger.debug(
+                "KDF: Argon2id (memory=%dKB, time=%d, parallel=%d)",
+                ARGON2_MEMORY_COST,
+                ARGON2_TIME_COST,
+                ARGON2_PARALLELISM,
+            )
             # Argon2id: the good stuff
             key = hash_secret_raw(
                 secret=key_material,
@@ -230,6 +253,9 @@ def derive_hybrid_key(
                 type=Type.ID,  # Hybrid mode: resists side-channel AND GPU attacks
             )
         else:
+            logger.warning(
+                "KDF: PBKDF2 fallback (%d iterations) - argon2 not available", PBKDF2_ITERATIONS
+            )
             # PBKDF2 fallback for systems without argon2-cffi
             # 600K iterations is slow but not memory-hard
             kdf = PBKDF2HMAC(
@@ -241,6 +267,7 @@ def derive_hybrid_key(
             )
             key = kdf.derive(key_material)
 
+        logger.debug("KDF complete, derived %d-byte key", len(key))
         return key
 
     except Exception as e:
@@ -457,6 +484,13 @@ def encrypt_message(
         # Pack payload with type marker
         packed_payload, _ = _pack_payload(message)
 
+        logger.debug(
+            "encrypt_message: packed_payload=%d bytes, flags=0x%02x, format_version=%d",
+            len(packed_payload),
+            flags,
+            FORMAT_VERSION,
+        )
+
         # Random padding to hide message length
         padding_len = secrets.randbelow(256) + 64
         padded_len = ((len(packed_payload) + padding_len + 255) // 256) * 256
@@ -464,6 +498,10 @@ def encrypt_message(
         padding = secrets.token_bytes(padding_needed - 4) + struct.pack(">I", len(packed_payload))
         padded_message = packed_payload + padding
 
+        logger.debug(
+            "Padded message: %d bytes (payload + %d padding)", len(padded_message), padding_needed
+        )
+
         # Build header for AAD
         header = MAGIC_HEADER + bytes([FORMAT_VERSION, flags])
 
@@ -473,10 +511,22 @@ def encrypt_message(
         encryptor.authenticate_additional_data(header)
         ciphertext = encryptor.update(padded_message) + encryptor.finalize()
 
+        total_size = len(header) + len(salt) + len(iv) + len(encryptor.tag) + len(ciphertext)
+        logger.debug(
+            "Encrypted output: %d bytes (header=%d, salt=%d, iv=%d, tag=%d, ciphertext=%d)",
+            total_size,
+            len(header),
+            len(salt),
+            len(iv),
+            len(encryptor.tag),
+            len(ciphertext),
+        )
+
         # v4.0.0: Header with flags byte
         return header + salt + iv + encryptor.tag + ciphertext
 
     except Exception as e:
+        logger.error("Encryption failed: %s", e)
         raise EncryptionError(f"Encryption failed: {e}") from e
 
 
@@ -551,10 +601,21 @@ def decrypt_message(
         InvalidHeaderError: If data doesn't have valid Stegasoo header
         DecryptionError: If decryption fails (wrong credentials)
     """
+    logger.debug("decrypt_message: %d bytes of encrypted data", len(encrypted_data))
+
     header = parse_header(encrypted_data)
     if not header:
+        logger.error("Invalid or missing Stegasoo header in %d bytes", len(encrypted_data))
         raise InvalidHeaderError("Invalid or missing Stegasoo header")
 
+    logger.debug(
+        "Header: version=%d, flags=0x%02x, has_channel_key=%s, ciphertext=%d bytes",
+        header["version"],
+        header["flags"],
+        header["has_channel_key"],
+        len(header["ciphertext"]),
+    )
+
     # Check for channel key mismatch and provide helpful error
     channel_hash = _resolve_channel_key(channel_key)
     has_configured_key = channel_hash is not None
@@ -577,9 +638,16 @@ def decrypt_message(
         padded_plaintext = decryptor.update(header["ciphertext"]) + decryptor.finalize()
         original_length = struct.unpack(">I", padded_plaintext[-4:])[0]
 
+        logger.debug(
+            "Decrypted %d bytes, original payload length: %d",
+            len(padded_plaintext),
+            original_length,
+        )
+
         payload_data = padded_plaintext[:original_length]
         result = _unpack_payload(payload_data)
 
+        logger.debug("Decryption successful: %s", result.payload_type)
         return result
 
     except Exception as e:

src/stegasoo/dct_steganography.py

@@ -40,12 +40,12 @@ from PIL import Image, ImageOps
 # Check for scipy availability (for PNG/DCT mode)
 # Prefer scipy.fft (newer, more stable) over scipy.fftpack
 try:
-    from scipy.fft import dct, idct, dctn, idctn
+    from scipy.fft import dct, dctn, idct, idctn
 
     HAS_SCIPY = True
 except ImportError:
     try:
-        from scipy.fftpack import dct, idct, dctn, idctn
+        from scipy.fftpack import dct, dctn, idct, idctn
 
         HAS_SCIPY = True
     except ImportError:
@@ -287,6 +287,7 @@ def has_jpegio_support() -> bool:
 
 try:
     from reedsolo import ReedSolomonError, RSCodec
+
     HAS_REEDSOLO = True
 except ImportError:
     HAS_REEDSOLO = False
@@ -1009,7 +1010,8 @@ def _embed_in_channel_safe(
                 needs_adjust = (quantized % 2) != bit_array
                 # Determine direction to nudge
                 dct_blocks[i, embed_rows[needs_adjust], embed_cols[needs_adjust]] = (
-                    (quantized[needs_adjust] + (1 - 2 * (quantized[needs_adjust] % 2 == 1))) * QUANT_STEP
+                    (quantized[needs_adjust] + (1 - 2 * (quantized[needs_adjust] % 2 == 1)))
+                    * QUANT_STEP
                 ).astype(np.float64)
                 # For bits that already match, just quantize
                 dct_blocks[i, embed_rows[~needs_adjust], embed_cols[~needs_adjust]] = (
@@ -1219,6 +1221,7 @@ def _embed_jpegio(
 def _jpegtran_available() -> bool:
     """Check if jpegtran is available on the system."""
     import shutil
+
     return shutil.which("jpegtran") is not None
 
 
@@ -1237,9 +1240,9 @@ def _jpegtran_rotate(image_data: bytes, rotation: int) -> bytes:
     Returns:
         Rotated JPEG bytes with DCT coefficients preserved
     """
+    import os
     import subprocess
     import tempfile
-    import os
 
     if rotation not in (90, 180, 270):
         raise ValueError(f"Invalid rotation: {rotation}")
@@ -1257,10 +1260,18 @@ def _jpegtran_rotate(image_data: bytes, rotation: int) -> bytes:
         # NOTE: Don't use -trim as it drops edge blocks and destroys stego data
         # NOTE: Don't use -perfect as it fails on images with non-MCU-aligned edges
         result = subprocess.run(
-            ["jpegtran", "-rotate", str(rotation), "-copy", "all",
-             "-outfile", output_path, input_path],
+            [
+                "jpegtran",
+                "-rotate",
+                str(rotation),
+                "-copy",
+                "all",
+                "-outfile",
+                output_path,
+                input_path,
+            ],
             capture_output=True,
-            timeout=30
+            timeout=30,
         )
 
         if result.returncode != 0:
@@ -1367,6 +1378,7 @@ def _quick_validate_dct_header(image_data: bytes, seed: bytes) -> bool:
                 copies.append(length_prefix_bytes[start:end])
 
             from collections import Counter
+
             counter = Counter(copies)
             _, count = counter.most_common(1)[0]
 
@@ -1437,6 +1449,7 @@ def extract_from_dct(
                     if rotation != 0:
                         try:
                             from . import debug
+
                             debug.print(f"DCT decode succeeded after {rotation}° rotation")
                         except Exception:
                             pass  # Don't let debug logging break extraction
@@ -1450,6 +1463,7 @@ def extract_from_dct(
             if rotation != 0:
                 try:
                     from . import debug
+
                     debug.print(f"DCT decode succeeded after {rotation}° rotation")
                 except Exception:
                     pass  # Don't let debug logging break extraction

src/stegasoo/debug.py

@@ -2,27 +2,96 @@
 Stegasoo Debugging Utilities
 
 Debugging, logging, and performance monitoring tools.
-Can be disabled for production use.
+
+Configuration:
+    STEGASOO_LOG_LEVEL env var controls log level:
+        - Not set or empty: logging disabled (production default)
+        - DEBUG: verbose debug output (encode/decode flow, crypto params, etc.)
+        - INFO: operational messages (format detection, mode selection)
+        - WARNING: potential issues (fallback KDF, format transcoding)
+        - ERROR: operation failures
+
+    STEGASOO_DEBUG=1 is a shorthand for STEGASOO_LOG_LEVEL=DEBUG
+
+    CLI: stegasoo --debug encode ... (sets DEBUG level for that invocation)
+
+All output goes to Python's logging module under the 'stegasoo' logger hierarchy.
+The legacy debug.print() API is preserved for backward compatibility.
 """
 
+import logging
+import os
 import sys
 import time
 import traceback
 from collections.abc import Callable
-from datetime import datetime
 from functools import wraps
 from typing import Any
 
+# Map string level names to logging levels
+_LEVEL_MAP = {
+    "DEBUG": logging.DEBUG,
+    "INFO": logging.INFO,
+    "WARNING": logging.WARNING,
+    "ERROR": logging.ERROR,
+    "CRITICAL": logging.CRITICAL,
+}
+
+# Root logger for the stegasoo package
+logger = logging.getLogger("stegasoo")
+
 # Global debug configuration
-DEBUG_ENABLED = False  # Set to True to enable debug output
 LOG_PERFORMANCE = True  # Log function timing
 VALIDATION_ASSERTIONS = True  # Enable runtime validation assertions
 
 
+def _configure_from_env() -> bool:
+    """Configure logging from environment variables. Returns True if debug enabled."""
+    # STEGASOO_DEBUG=1 is shorthand for DEBUG level
+    if os.environ.get("STEGASOO_DEBUG", "").strip() in ("1", "true", "yes"):
+        _setup_logging(logging.DEBUG)
+        return True
+
+    level_str = os.environ.get("STEGASOO_LOG_LEVEL", "").strip().upper()
+    if level_str and level_str in _LEVEL_MAP:
+        _setup_logging(_LEVEL_MAP[level_str])
+        return level_str == "DEBUG"
+
+    return False
+
+
+def _setup_logging(level: int) -> None:
+    """Configure the stegasoo logger with a stderr handler."""
+    logger.setLevel(level)
+
+    # Only add handler if none exist (avoid duplicates on re-init)
+    if not logger.handlers:
+        handler = logging.StreamHandler(sys.stderr)
+        handler.setLevel(level)
+        formatter = logging.Formatter(
+            "[%(asctime)s.%(msecs)03d] [%(levelname)s] [%(name)s] %(message)s",
+            datefmt="%H:%M:%S",
+        )
+        handler.setFormatter(formatter)
+        logger.addHandler(handler)
+    else:
+        # Update existing handler level
+        for handler in logger.handlers:
+            handler.setLevel(level)
+
+
+# Auto-configure on import
+DEBUG_ENABLED = _configure_from_env()
+
+
 def enable_debug(enable: bool = True) -> None:
     """Enable or disable debug mode globally."""
     global DEBUG_ENABLED
     DEBUG_ENABLED = enable
+    if enable:
+        _setup_logging(logging.DEBUG)
+    else:
+        logger.setLevel(logging.WARNING)
 
 
 def enable_performance_logging(enable: bool = True) -> None:
@@ -38,15 +107,14 @@ def enable_assertions(enable: bool = True) -> None:
 
 
 def debug_print(message: str, level: str = "INFO") -> None:
-    """Print debug message with timestamp if debugging is enabled."""
-    if DEBUG_ENABLED:
-        timestamp = datetime.now().strftime("%H:%M:%S.%f")[:-3]
-        print(f"[{timestamp}] [{level}] {message}", file=sys.stderr)
+    """Log a message at the given level via the stegasoo logger."""
+    log_level = _LEVEL_MAP.get(level.upper(), logging.DEBUG)
+    logger.log(log_level, message)
 
 
 def debug_data(data: bytes, label: str = "Data", max_bytes: int = 32) -> str:
     """Format bytes for debugging."""
-    if not DEBUG_ENABLED:
+    if not logger.isEnabledFor(logging.DEBUG):
         return ""
 
     if not data:
@@ -55,15 +123,17 @@ def debug_data(data: bytes, label: str = "Data", max_bytes: int = 32) -> str:
     if len(data) <= max_bytes:
         return f"{label} ({len(data)} bytes): {data.hex()}"
     else:
-        return f"{label} ({len(data)} bytes): {data[:max_bytes//2].hex()}...{data[-max_bytes//2:].hex()}"
+        return (
+            f"{label} ({len(data)} bytes): "
+            f"{data[:max_bytes // 2].hex()}...{data[-max_bytes // 2:].hex()}"
+        )
 
 
 def debug_exception(e: Exception, context: str = "") -> None:
     """Log exception with context for debugging."""
-    if DEBUG_ENABLED:
-        debug_print(f"Exception in {context}: {type(e).__name__}: {e}", "ERROR")
-        if DEBUG_ENABLED:
-            traceback.print_exc()
+    logger.error("Exception in %s: %s: %s", context, type(e).__name__, e)
+    if logger.isEnabledFor(logging.DEBUG):
+        logger.debug(traceback.format_exc())
 
 
 def time_function(func: Callable) -> Callable:
@@ -71,7 +141,7 @@ def time_function(func: Callable) -> Callable:
 
     @wraps(func)
     def wrapper(*args, **kwargs) -> Any:
-        if not (DEBUG_ENABLED and LOG_PERFORMANCE):
+        if not (logger.isEnabledFor(logging.DEBUG) and LOG_PERFORMANCE):
             return func(*args, **kwargs)
 
         start = time.perf_counter()
@@ -80,7 +150,7 @@ def time_function(func: Callable) -> Callable:
             return result
         finally:
             end = time.perf_counter()
-            debug_print(f"{func.__name__} took {end - start:.6f}s", "PERF")
+            logger.debug("%s took %.6fs", func.__name__, end - start)
 
     return wrapper
 
@@ -94,8 +164,6 @@ def validate_assertion(condition: bool, message: str) -> None:
 def memory_usage() -> dict[str, float | str]:
     """Get current memory usage (if psutil is available)."""
     try:
-        import os
-
         import psutil
 
         process = psutil.Process(os.getpid())
@@ -131,8 +199,19 @@ def hexdump(data: bytes, offset: int = 0, length: int = 64) -> str:
     return "\n".join(result)
 
 
+def get_logger(name: str) -> logging.Logger:
+    """Get a child logger under the stegasoo namespace.
+
+    Usage in modules:
+        from .debug import get_logger
+        logger = get_logger(__name__)
+        logger.debug("message")
+    """
+    return logging.getLogger(name)
+
+
 class Debug:
-    """Debugging utility class."""
+    """Debugging utility class (backward-compatible API)."""
 
     def __init__(self):
         self.enabled = DEBUG_ENABLED

src/stegasoo/decode.py

@@ -31,12 +31,15 @@ def _write_progress(progress_file: str | None, current: int, total: int, phase:
         return
     try:
         with open(progress_file, "w") as f:
-            json.dump({
+            json.dump(
+                {
                     "current": current,
                     "total": total,
                     "percent": (current / total * 100) if total > 0 else 0,
                     "phase": phase,
-            }, f)
+                },
+                f,
+            )
     except OSError:
         pass
 
@@ -291,16 +294,23 @@ def decode_audio(
     Returns:
         DecodeResult with message or file data
     """
-    from .audio_utils import detect_audio_format, transcode_to_wav
     from .constants import (
+        AUDIO_ENABLED,
         EMBED_MODE_AUDIO_AUTO,
         EMBED_MODE_AUDIO_LSB,
         EMBED_MODE_AUDIO_SPREAD,
     )
 
+    if not AUDIO_ENABLED:
+        raise ExtractionError(
+            "Audio support is disabled. Install audio extras (pip install stegasoo[audio]) "
+            "or set STEGASOO_AUDIO=1 to force enable."
+        )
+
+    from .audio_utils import detect_audio_format, transcode_to_wav
+
     debug.print(
-        f"decode_audio: mode={embed_mode}, "
-        f"passphrase length={len(passphrase.split())} words"
+        f"decode_audio: mode={embed_mode}, " f"passphrase length={len(passphrase.split())} words"
     )
 
     # Validate inputs
@@ -358,9 +368,7 @@ def decode_audio(
     elif embed_mode == EMBED_MODE_AUDIO_SPREAD:
         from .spread_steganography import extract_from_audio_spread
 
-        encrypted = extract_from_audio_spread(
-            wav_audio, pixel_key, progress_file=progress_file
-        )
+        encrypted = extract_from_audio_spread(wav_audio, pixel_key, progress_file=progress_file)
     else:
         raise ValueError(f"Invalid audio embed mode: {embed_mode}")
 

src/stegasoo/encode.py

@@ -18,6 +18,7 @@ from typing import TYPE_CHECKING
 from .constants import EMBED_MODE_LSB
 from .crypto import derive_pixel_key, encrypt_message
 from .debug import debug
+from .exceptions import AudioError
 from .models import EncodeResult, FilePayload
 from .steganography import embed_in_image
 from .utils import generate_filename
@@ -280,6 +281,7 @@ def encode_audio(
     embed_mode: str = "audio_lsb",
     channel_key: str | bool | None = None,
     progress_file: str | None = None,
+    chip_tier: int | None = None,
 ) -> tuple[bytes, AudioEmbedStats]:
     """
     Encode a message or file into an audio carrier.
@@ -295,12 +297,21 @@ def encode_audio(
         embed_mode: 'audio_lsb' or 'audio_spread'
         channel_key: Channel key for deployment/group isolation
         progress_file: Optional path to write progress JSON
+        chip_tier: Spread spectrum chip tier (0=lossless, 1=high_lossy, 2=low_lossy).
+                   Only used for audio_spread mode. Default None → uses constant default.
 
     Returns:
         Tuple of (stego audio bytes, AudioEmbedStats)
     """
+    from .constants import AUDIO_ENABLED, EMBED_MODE_AUDIO_LSB, EMBED_MODE_AUDIO_SPREAD
+
+    if not AUDIO_ENABLED:
+        raise AudioError(
+            "Audio support is disabled. Install audio extras (pip install stegasoo[audio]) "
+            "or set STEGASOO_AUDIO=1 to force enable."
+        )
+
     from .audio_utils import detect_audio_format, transcode_to_wav
-    from .constants import EMBED_MODE_AUDIO_LSB, EMBED_MODE_AUDIO_SPREAD
 
     debug.print(
         f"encode_audio: mode={embed_mode}, "
@@ -343,10 +354,12 @@ def encode_audio(
             encrypted, carrier_audio, pixel_key, progress_file=progress_file
         )
     elif embed_mode == EMBED_MODE_AUDIO_SPREAD:
+        from .constants import AUDIO_SS_DEFAULT_CHIP_TIER
         from .spread_steganography import embed_in_audio_spread
 
+        tier = chip_tier if chip_tier is not None else AUDIO_SS_DEFAULT_CHIP_TIER
         stego_audio, stats = embed_in_audio_spread(
-            encrypted, carrier_audio, pixel_key, progress_file=progress_file
+            encrypted, carrier_audio, pixel_key, chip_tier=tier, progress_file=progress_file
         )
     else:
         raise ValueError(f"Invalid audio embed mode: {embed_mode}")

src/stegasoo/models.py

@@ -300,6 +300,9 @@ class AudioEmbedStats:
     channels: int
     duration_seconds: float
     embed_mode: str  # "audio_lsb" or "audio_spread"
+    chip_tier: int | None = None  # v4.4.0: spread spectrum chip tier (0/1/2)
+    chip_length: int | None = None  # v4.4.0: samples per chip
+    embeddable_channels: int | None = None  # v4.4.0: channels used (excl. LFE)
 
     @property
     def modification_percent(self) -> float:
@@ -329,3 +332,7 @@ class AudioCapacityInfo:
     embed_mode: str
     sample_rate: int
     duration_seconds: float
+    chip_tier: int | None = None  # v4.4.0: spread spectrum chip tier
+    chip_length: int | None = None  # v4.4.0: samples per chip
+    embeddable_channels: int | None = None  # v4.4.0: channels used (excl. LFE)
+    total_channels: int | None = None  # v4.4.0: total channels in carrier

src/stegasoo/qr_utils.py

@@ -105,14 +105,14 @@ def decompress_data(data: str) -> str:
                 "Data compressed with zstd but zstandard package not installed. "
                 "Run: pip install zstandard"
             )
-        encoded = data[len(COMPRESSION_PREFIX_ZSTD):]
+        encoded = data[len(COMPRESSION_PREFIX_ZSTD) :]
         compressed = base64.b64decode(encoded)
         dctx = zstd.ZstdDecompressor()
         return dctx.decompress(compressed).decode("utf-8")
 
     elif data.startswith(COMPRESSION_PREFIX_ZLIB):
         # Legacy zlib compression
-        encoded = data[len(COMPRESSION_PREFIX_ZLIB):]
+        encoded = data[len(COMPRESSION_PREFIX_ZLIB) :]
         compressed = base64.b64decode(encoded)
         return zlib.decompress(compressed).decode("utf-8")
 

src/stegasoo/recovery.py

@@ -182,6 +182,7 @@ def extract_stego_backup(
         debug.print(f"Stego backup extraction failed: {e}")
         return None
 
+
 # Recovery key format: same as channel key (32 chars, 8 groups of 4)
 RECOVERY_KEY_LENGTH = 32
 RECOVERY_KEY_ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
@@ -205,16 +206,10 @@ def generate_recovery_key() -> str:
         7
     """
     # Generate 32 random alphanumeric characters
-    raw_key = "".join(
-        secrets.choice(RECOVERY_KEY_ALPHABET)
-        for _ in range(RECOVERY_KEY_LENGTH)
-    )
+    raw_key = "".join(secrets.choice(RECOVERY_KEY_ALPHABET) for _ in range(RECOVERY_KEY_LENGTH))
 
     # Format with dashes every 4 characters
-    formatted = "-".join(
-        raw_key[i:i + 4]
-        for i in range(0, RECOVERY_KEY_LENGTH, 4)
-    )
+    formatted = "-".join(raw_key[i : i + 4] for i in range(0, RECOVERY_KEY_LENGTH, 4))
 
     debug.print(f"Generated recovery key: {formatted[:4]}-••••-...-{formatted[-4:]}")
     return formatted
@@ -245,15 +240,12 @@ def normalize_recovery_key(key: str) -> str:
     # Validate length
     if len(clean) != RECOVERY_KEY_LENGTH:
         raise ValueError(
-            f"Recovery key must be {RECOVERY_KEY_LENGTH} characters "
-            f"(got {len(clean)})"
+            f"Recovery key must be {RECOVERY_KEY_LENGTH} characters " f"(got {len(clean)})"
         )
 
     # Validate characters
     if not all(c in RECOVERY_KEY_ALPHABET for c in clean):
-        raise ValueError(
-            "Recovery key must contain only letters A-Z and digits 0-9"
-        )
+        raise ValueError("Recovery key must contain only letters A-Z and digits 0-9")
 
     return clean
 
@@ -273,7 +265,7 @@ def format_recovery_key(key: str) -> str:
         "ABCD-1234-EFGH-5678-IJKL-9012-MNOP-3456"
     """
     clean = normalize_recovery_key(key)
-    return "-".join(clean[i:i + 4] for i in range(0, RECOVERY_KEY_LENGTH, 4))
+    return "-".join(clean[i : i + 4] for i in range(0, RECOVERY_KEY_LENGTH, 4))
 
 
 def hash_recovery_key(key: str) -> str:

src/stegasoo/utils.py

@@ -54,8 +54,7 @@ def read_image_exif(image_data: bytes) -> dict:
                             gps[gps_tag] = float(gps_value)
                         elif isinstance(gps_value, tuple):
                             gps[gps_tag] = [
-                                float(v) if hasattr(v, "numerator") else v
-                                for v in gps_value
+                                float(v) if hasattr(v, "numerator") else v for v in gps_value
                             ]
                         else:
                             gps[gps_tag] = gps_value
@@ -69,7 +68,9 @@ def read_image_exif(image_data: bytes) -> dict:
                         # Try to decode as ASCII/UTF-8 text
                         decoded = value.decode("utf-8", errors="strict").strip("\x00")
                         # Only keep if it looks like printable text
-                        if decoded.isprintable() or all(c.isspace() or c.isprintable() for c in decoded):
+                        if decoded.isprintable() or all(
+                            c.isspace() or c.isprintable() for c in decoded
+                        ):
                             result[tag] = decoded
                         else:
                             result[tag] = f"<{len(value)} bytes binary>"

src/stegasoo/validation.py

@@ -13,6 +13,10 @@ import io
 
 from PIL import Image
 
+from .debug import get_logger
+
+logger = get_logger(__name__)
+
 from .constants import (
     ALLOWED_AUDIO_EXTENSIONS,
     ALLOWED_IMAGE_EXTENSIONS,

tests/test_audio.py

@@ -3,25 +3,37 @@ Tests for Stegasoo audio steganography.
 
 Tests cover:
 - Audio LSB roundtrip (encode + decode)
-- Audio MDCT roundtrip (encode + decode)
+- Audio spread spectrum roundtrip (v0 legacy + v2 per-channel)
 - Wrong credentials fail to decode
-- Capacity calculations
+- Capacity calculations (per-tier)
 - Format detection
 - Audio validation
+- Per-channel stereo/multichannel embedding (v4.4.0)
+- Chip tier roundtrips (v4.4.0)
+- LFE channel skipping (v4.4.0)
+- Backward compat: v0 decode from v2 code
+- Header v2 build/parse roundtrip
+- Round-robin bit distribution
 """
 
 import io
+from pathlib import Path
 
 import numpy as np
 import pytest
 import soundfile as sf
 
-from stegasoo.constants import (
-    EMBED_MODE_AUDIO_LSB,
-    EMBED_MODE_AUDIO_SPREAD,
-)
+from stegasoo.constants import AUDIO_ENABLED, EMBED_MODE_AUDIO_LSB, EMBED_MODE_AUDIO_SPREAD
 from stegasoo.models import AudioCapacityInfo, AudioEmbedStats, AudioInfo
 
+pytestmark = pytest.mark.skipif(not AUDIO_ENABLED, reason="Audio support disabled (STEGASOO_AUDIO)")
+
+# Path to real test data files
+_TEST_DATA = Path(__file__).parent.parent / "test_data"
+_REFERENCE_PNG = _TEST_DATA / "reference.png"
+_SPEECH_WAV = _TEST_DATA / "stupid_elitist_speech.wav"
+
+
 # =============================================================================
 # FIXTURES
 # =============================================================================
@@ -33,7 +45,6 @@ def carrier_wav() -> bytes:
     sample_rate = 44100
     duration = 1.0
     num_samples = int(sample_rate * duration)
-    # Generate a simple sine wave
     t = np.linspace(0, duration, num_samples, endpoint=False)
     samples = (np.sin(2 * np.pi * 440 * t) * 16000).astype(np.int16)
 
@@ -45,9 +56,9 @@ def carrier_wav() -> bytes:
 
 @pytest.fixture
 def carrier_wav_stereo() -> bytes:
-    """Generate a stereo test WAV file."""
+    """Generate a stereo test WAV file (5 seconds for spread spectrum capacity)."""
     sample_rate = 44100
-    duration = 1.0
+    duration = 5.0
     num_samples = int(sample_rate * duration)
     t = np.linspace(0, duration, num_samples, endpoint=False)
     left = (np.sin(2 * np.pi * 440 * t) * 16000).astype(np.int16)
@@ -67,7 +78,6 @@ def carrier_wav_long() -> bytes:
     duration = 15.0
     num_samples = int(sample_rate * duration)
     t = np.linspace(0, duration, num_samples, endpoint=False)
-    # Mix of frequencies for better MDCT embedding
     samples = (
         (np.sin(2 * np.pi * 440 * t) + np.sin(2 * np.pi * 880 * t) + np.sin(2 * np.pi * 1320 * t))
         * 5000
@@ -80,12 +90,47 @@ def carrier_wav_long() -> bytes:
 
 
 @pytest.fixture
-def carrier_wav_spread_integration() -> bytes:
-    """Generate a very long WAV (150 seconds) for spread spectrum integration tests.
+def carrier_wav_stereo_long() -> bytes:
+    """Generate a stereo WAV (15 seconds) for per-channel spread tests."""
+    sample_rate = 48000
+    duration = 15.0
+    num_samples = int(sample_rate * duration)
+    t = np.linspace(0, duration, num_samples, endpoint=False)
+    left = (np.sin(2 * np.pi * 440 * t) * 10000).astype(np.float64) / 32768.0
+    right = (np.sin(2 * np.pi * 660 * t) * 10000).astype(np.float64) / 32768.0
+    samples = np.column_stack([left, right])
 
-    Spread spectrum needs 1024 samples per bit. With encryption + RS overhead (~690 bytes),
-    we need at least 690*8*1024 = 5.7M samples ~ 130 seconds at 44.1kHz.
-    """
+    buf = io.BytesIO()
+    sf.write(buf, samples, sample_rate, format="WAV", subtype="PCM_16")
+    buf.seek(0)
+    return buf.read()
+
+
+@pytest.fixture
+def carrier_wav_5_1() -> bytes:
+    """Generate a 6-channel (5.1) WAV for LFE skip tests."""
+    sample_rate = 48000
+    duration = 15.0
+    num_samples = int(sample_rate * duration)
+    t = np.linspace(0, duration, num_samples, endpoint=False)
+
+    # 6 channels with different frequencies
+    freqs = [440, 554, 660, 80, 880, 1100]  # ch3 = LFE (low freq)
+    channels = []
+    for freq in freqs:
+        ch = (np.sin(2 * np.pi * freq * t) * 8000).astype(np.float64) / 32768.0
+        channels.append(ch)
+    samples = np.column_stack(channels)
+
+    buf = io.BytesIO()
+    sf.write(buf, samples, sample_rate, format="WAV", subtype="PCM_16")
+    buf.seek(0)
+    return buf.read()
+
+
+@pytest.fixture
+def carrier_wav_spread_integration() -> bytes:
+    """Generate a very long WAV (150 seconds) for spread spectrum integration tests."""
     sample_rate = 44100
     duration = 150.0
     num_samples = int(sample_rate * duration)
@@ -103,7 +148,9 @@ def carrier_wav_spread_integration() -> bytes:
 
 @pytest.fixture
 def reference_photo() -> bytes:
-    """Generate a small reference photo (PNG)."""
+    """Load real reference photo from test_data, or generate a small one."""
+    if _REFERENCE_PNG.exists():
+        return _REFERENCE_PNG.read_bytes()
     from PIL import Image
 
     img = Image.new("RGB", (100, 100), color=(128, 64, 32))
@@ -113,6 +160,14 @@ def reference_photo() -> bytes:
     return buf.read()
 
 
+@pytest.fixture
+def speech_wav() -> bytes:
+    """Load real speech WAV from test_data (48kHz mono, ~68s)."""
+    if not _SPEECH_WAV.exists():
+        pytest.skip("test_data/stupid_elitist_speech.wav not found")
+    return _SPEECH_WAV.read_bytes()
+
+
 # =============================================================================
 # AUDIO LSB TESTS
 # =============================================================================
@@ -134,7 +189,6 @@ class TestAudioLSB:
         from stegasoo.audio_steganography import embed_in_audio_lsb, extract_from_audio_lsb
 
         payload = b"Hello, audio steganography!"
-        # Prepend with magic header to simulate real usage pattern
         key = b"\x42" * 32
 
         stego_audio, stats = embed_in_audio_lsb(payload, carrier_wav, key)
@@ -145,7 +199,6 @@ class TestAudioLSB:
         assert stats.samples_modified > 0
         assert 0 < stats.capacity_used <= 1.0
 
-        # Extract
         extracted = extract_from_audio_lsb(stego_audio, key)
         assert extracted is not None
         assert extracted == payload
@@ -174,7 +227,6 @@ class TestAudioLSB:
         stego_audio, _ = embed_in_audio_lsb(payload, carrier_wav, correct_key)
 
         extracted = extract_from_audio_lsb(stego_audio, wrong_key)
-        # Should return None or garbage (not the original message)
         assert extracted is None or extracted != payload
 
     def test_two_bits_per_sample(self, carrier_wav):
@@ -197,46 +249,97 @@ class TestAudioLSB:
         indices1 = generate_sample_indices(key, 10000, 100)
         indices2 = generate_sample_indices(key, 10000, 100)
 
-        # Same key should produce same indices
         assert indices1 == indices2
-
-        # All indices should be valid
         assert all(0 <= i < 10000 for i in indices1)
-
-        # No duplicates
         assert len(set(indices1)) == len(indices1)
 
 
 # =============================================================================
-# AUDIO SPREAD SPECTRUM TESTS
+# AUDIO SPREAD SPECTRUM TESTS (v2 per-channel)
 # =============================================================================
 
 
 class TestAudioSpread:
-    """Tests for audio spread spectrum steganography."""
+    """Tests for audio spread spectrum steganography (v2 per-channel)."""
 
-    def test_calculate_capacity(self, carrier_wav_long):
+    def test_calculate_capacity_default_tier(self, carrier_wav_long):
         from stegasoo.spread_steganography import calculate_audio_spread_capacity
 
         capacity = calculate_audio_spread_capacity(carrier_wav_long)
         assert isinstance(capacity, AudioCapacityInfo)
         assert capacity.usable_capacity_bytes > 0
         assert capacity.embed_mode == EMBED_MODE_AUDIO_SPREAD
+        assert capacity.chip_tier == 2  # default
+        assert capacity.chip_length == 1024
 
-    def test_spread_roundtrip(self, carrier_wav_long):
-        """Test spread spectrum embed/extract roundtrip."""
+    def test_calculate_capacity_per_tier(self, carrier_wav_long):
+        """Capacity should increase as chip length decreases."""
+        from stegasoo.spread_steganography import calculate_audio_spread_capacity
+
+        cap_lossless = calculate_audio_spread_capacity(carrier_wav_long, chip_tier=0)
+        cap_high = calculate_audio_spread_capacity(carrier_wav_long, chip_tier=1)
+        cap_low = calculate_audio_spread_capacity(carrier_wav_long, chip_tier=2)
+
+        assert cap_lossless.chip_length == 256
+        assert cap_high.chip_length == 512
+        assert cap_low.chip_length == 1024
+
+        # Smaller chip = more capacity
+        assert cap_lossless.usable_capacity_bytes > cap_high.usable_capacity_bytes
+        assert cap_high.usable_capacity_bytes > cap_low.usable_capacity_bytes
+
+    def test_spread_roundtrip_default_tier(self, carrier_wav_long):
+        """Test spread spectrum embed/extract roundtrip (default tier 2)."""
         from stegasoo.spread_steganography import (
             embed_in_audio_spread,
             extract_from_audio_spread,
         )
 
-        payload = b"Spread test"
+        payload = b"Spread test v2"
         seed = b"\x42" * 32
 
         stego_audio, stats = embed_in_audio_spread(payload, carrier_wav_long, seed)
 
         assert isinstance(stats, AudioEmbedStats)
         assert stats.embed_mode == EMBED_MODE_AUDIO_SPREAD
+        assert stats.chip_tier == 2
+        assert stats.chip_length == 1024
+
+        extracted = extract_from_audio_spread(stego_audio, seed)
+        assert extracted is not None
+        assert extracted == payload
+
+    def test_spread_roundtrip_tier_0(self, carrier_wav_long):
+        """Test spread spectrum at tier 0 (chip=256, lossless)."""
+        from stegasoo.spread_steganography import (
+            embed_in_audio_spread,
+            extract_from_audio_spread,
+        )
+
+        payload = b"Lossless tier test with more data to embed for coverage"
+        seed = b"\x42" * 32
+
+        stego_audio, stats = embed_in_audio_spread(payload, carrier_wav_long, seed, chip_tier=0)
+        assert stats.chip_tier == 0
+        assert stats.chip_length == 256
+
+        extracted = extract_from_audio_spread(stego_audio, seed)
+        assert extracted is not None
+        assert extracted == payload
+
+    def test_spread_roundtrip_tier_1(self, carrier_wav_long):
+        """Test spread spectrum at tier 1 (chip=512, high lossy)."""
+        from stegasoo.spread_steganography import (
+            embed_in_audio_spread,
+            extract_from_audio_spread,
+        )
+
+        payload = b"High lossy tier test"
+        seed = b"\x42" * 32
+
+        stego_audio, stats = embed_in_audio_spread(payload, carrier_wav_long, seed, chip_tier=1)
+        assert stats.chip_tier == 1
+        assert stats.chip_length == 512
 
         extracted = extract_from_audio_spread(stego_audio, seed)
         assert extracted is not None
@@ -258,6 +361,258 @@ class TestAudioSpread:
         extracted = extract_from_audio_spread(stego_audio, wrong_seed)
         assert extracted is None or extracted != payload
 
+    def test_per_channel_stereo_roundtrip(self, carrier_wav_stereo_long):
+        """Test that stereo per-channel embedding/extraction works."""
+        from stegasoo.spread_steganography import (
+            embed_in_audio_spread,
+            extract_from_audio_spread,
+        )
+
+        payload = b"Stereo per-channel test"
+        seed = b"\xAB" * 32
+
+        stego_audio, stats = embed_in_audio_spread(
+            payload, carrier_wav_stereo_long, seed, chip_tier=0
+        )
+        assert stats.channels == 2
+        assert stats.embeddable_channels == 2
+
+        extracted = extract_from_audio_spread(stego_audio, seed)
+        assert extracted is not None
+        assert extracted == payload
+
+    def test_per_channel_preserves_spatial_mix(self, carrier_wav_stereo_long):
+        """Verify that per-channel embedding doesn't destroy the spatial mix.
+
+        The difference between left and right channels should be preserved
+        (not zeroed out as the old mono-broadcast approach would do).
+        """
+        from stegasoo.spread_steganography import embed_in_audio_spread
+
+        payload = b"Spatial preservation test"
+        seed = b"\xCD" * 32
+
+        # Read original
+        orig_samples, _ = sf.read(io.BytesIO(carrier_wav_stereo_long), dtype="float64", always_2d=True)
+        orig_diff = orig_samples[:, 0] - orig_samples[:, 1]
+
+        # Embed
+        stego_bytes, _ = embed_in_audio_spread(
+            payload, carrier_wav_stereo_long, seed, chip_tier=0
+        )
+
+        # Read stego
+        stego_samples, _ = sf.read(io.BytesIO(stego_bytes), dtype="float64", always_2d=True)
+        stego_diff = stego_samples[:, 0] - stego_samples[:, 1]
+
+        # The channel difference should not be identical (embedding adds different
+        # noise per channel), but should be very close (embedding is subtle)
+        # With the old mono-broadcast approach, stego_diff would equal orig_diff
+        # exactly in unmodified regions but differ where data was embedded.
+        # With per-channel, both channels get independent modifications.
+        correlation = np.corrcoef(orig_diff, stego_diff)[0, 1]
+        assert correlation > 0.95, f"Spatial mix correlation too low: {correlation}"
+
+    def test_capacity_scales_with_channels(self, carrier_wav_long, carrier_wav_stereo_long):
+        """Stereo should have roughly double the capacity of mono."""
+        from stegasoo.spread_steganography import calculate_audio_spread_capacity
+
+        mono_cap = calculate_audio_spread_capacity(carrier_wav_long, chip_tier=0)
+        stereo_cap = calculate_audio_spread_capacity(carrier_wav_stereo_long, chip_tier=0)
+
+        # Stereo should be ~1.5-2.2x mono (not exact because header is ch0 only
+        # and the files have slightly different durations/sample rates)
+        ratio = stereo_cap.usable_capacity_bytes / mono_cap.usable_capacity_bytes
+        assert ratio > 1.3, f"Stereo/mono capacity ratio too low: {ratio}"
+
+    def test_lfe_skip_5_1(self, carrier_wav_5_1):
+        """LFE channel (index 3) should be unmodified in 6-channel audio."""
+        from stegasoo.spread_steganography import embed_in_audio_spread
+
+        payload = b"LFE skip test"
+        seed = b"\xEE" * 32
+
+        # Read original LFE channel
+        orig_samples, _ = sf.read(io.BytesIO(carrier_wav_5_1), dtype="float64", always_2d=True)
+        orig_lfe = orig_samples[:, 3].copy()
+
+        stego_bytes, stats = embed_in_audio_spread(
+            payload, carrier_wav_5_1, seed, chip_tier=0
+        )
+        assert stats.embeddable_channels == 5  # 6 channels - 1 LFE = 5
+
+        stego_samples, _ = sf.read(io.BytesIO(stego_bytes), dtype="float64", always_2d=True)
+        stego_lfe = stego_samples[:, 3]
+
+        # LFE channel should be completely unmodified
+        np.testing.assert_array_equal(orig_lfe, stego_lfe)
+
+    def test_lfe_skip_roundtrip(self, carrier_wav_5_1):
+        """5.1 audio embed/extract roundtrip with LFE skipping."""
+        from stegasoo.spread_steganography import (
+            embed_in_audio_spread,
+            extract_from_audio_spread,
+        )
+
+        payload = b"5.1 surround test"
+        seed = b"\xEE" * 32
+
+        stego_bytes, stats = embed_in_audio_spread(
+            payload, carrier_wav_5_1, seed, chip_tier=0
+        )
+        assert stats.channels == 6
+        assert stats.embeddable_channels == 5
+
+        extracted = extract_from_audio_spread(stego_bytes, seed)
+        assert extracted is not None
+        assert extracted == payload
+
+
+# =============================================================================
+# HEADER V2 TESTS
+# =============================================================================
+
+
+class TestHeaderV2:
+    """Tests for v2 header construction and parsing."""
+
+    def test_header_v2_build_parse_roundtrip(self):
+        from stegasoo.spread_steganography import _build_header_v2, _parse_header
+
+        data_length = 12345
+        chip_tier = 1
+        num_ch = 2
+        lfe_skipped = False
+
+        header = _build_header_v2(data_length, chip_tier, num_ch, lfe_skipped)
+        assert len(header) == 20
+
+        magic_valid, version, length, tier, nch, lfe = _parse_header(header)
+        assert magic_valid
+        assert version == 2
+        assert length == data_length
+        assert tier == chip_tier
+        assert nch == num_ch
+        assert lfe is False
+
+    def test_header_v2_with_lfe_flag(self):
+        from stegasoo.spread_steganography import _build_header_v2, _parse_header
+
+        header = _build_header_v2(999, 0, 5, lfe_skipped=True)
+        magic_valid, version, length, tier, nch, lfe = _parse_header(header)
+        assert magic_valid
+        assert version == 2
+        assert length == 999
+        assert tier == 0
+        assert nch == 5
+        assert lfe is True
+
+    def test_header_v0_build_parse(self):
+        from stegasoo.spread_steganography import _build_header_v0, _parse_header
+
+        header = _build_header_v0(4567)
+        assert len(header) == 16
+
+        magic_valid, version, length, tier, nch, lfe = _parse_header(header)
+        assert magic_valid
+        assert version == 0
+        assert length == 4567
+        assert tier is None
+        assert nch is None
+
+    def test_header_bad_magic(self):
+        from stegasoo.spread_steganography import _parse_header
+
+        bad_header = b"XXXX" + b"\x00" * 16
+        magic_valid, version, length, tier, nch, lfe = _parse_header(bad_header)
+        assert not magic_valid
+
+
+# =============================================================================
+# ROUND-ROBIN BIT DISTRIBUTION TESTS
+# =============================================================================
+
+
+class TestRoundRobin:
+    """Tests for round-robin bit distribution."""
+
+    def test_distribute_and_collect_identity(self):
+        from stegasoo.spread_steganography import (
+            _collect_bits_round_robin,
+            _distribute_bits_round_robin,
+        )
+
+        bits = [1, 0, 1, 1, 0, 0, 1, 0, 1, 1]
+        for num_ch in [1, 2, 3, 4, 5]:
+            per_ch = _distribute_bits_round_robin(bits, num_ch)
+            assert len(per_ch) == num_ch
+            reassembled = _collect_bits_round_robin(per_ch)
+            assert reassembled == bits, f"Failed for {num_ch} channels"
+
+    def test_distribute_round_robin_ordering(self):
+        from stegasoo.spread_steganography import _distribute_bits_round_robin
+
+        bits = [0, 1, 2, 3, 4, 5]  # using ints for clarity
+        per_ch = _distribute_bits_round_robin(bits, 3)
+        # ch0: bits 0, 3  ch1: bits 1, 4  ch2: bits 2, 5
+        assert per_ch[0] == [0, 3]
+        assert per_ch[1] == [1, 4]
+        assert per_ch[2] == [2, 5]
+
+    def test_distribute_uneven(self):
+        from stegasoo.spread_steganography import (
+            _collect_bits_round_robin,
+            _distribute_bits_round_robin,
+        )
+
+        bits = [0, 1, 2, 3, 4]  # 5 bits across 3 channels
+        per_ch = _distribute_bits_round_robin(bits, 3)
+        assert per_ch[0] == [0, 3]
+        assert per_ch[1] == [1, 4]
+        assert per_ch[2] == [2]
+
+        reassembled = _collect_bits_round_robin(per_ch)
+        assert reassembled == bits
+
+
+# =============================================================================
+# CHANNEL MANAGEMENT TESTS
+# =============================================================================
+
+
+class TestChannelManagement:
+    """Tests for embeddable channel selection."""
+
+    def test_mono(self):
+        from stegasoo.spread_steganography import _embeddable_channels
+
+        assert _embeddable_channels(1) == [0]
+
+    def test_stereo(self):
+        from stegasoo.spread_steganography import _embeddable_channels
+
+        assert _embeddable_channels(2) == [0, 1]
+
+    def test_5_1_skips_lfe(self):
+        from stegasoo.spread_steganography import _embeddable_channels
+
+        channels = _embeddable_channels(6)
+        assert channels == [0, 1, 2, 4, 5]
+        assert 3 not in channels  # LFE skipped
+
+    def test_7_1_skips_lfe(self):
+        from stegasoo.spread_steganography import _embeddable_channels
+
+        channels = _embeddable_channels(8)
+        assert 3 not in channels
+        assert len(channels) == 7
+
+    def test_quad_no_skip(self):
+        from stegasoo.spread_steganography import _embeddable_channels
+
+        # 4 channels < 6, so no LFE skip
+        assert _embeddable_channels(4) == [0, 1, 2, 3]
+
 
 # =============================================================================
 # FORMAT DETECTION TESTS
@@ -423,6 +778,36 @@ class TestIntegration:
 
         assert result.message == "Spread integration test"
 
+    def test_spread_encode_decode_with_chip_tier(
+        self, carrier_wav_spread_integration, reference_photo
+    ):
+        """Test spread spectrum with explicit chip tier."""
+        from stegasoo.decode import decode_audio
+        from stegasoo.encode import encode_audio
+
+        stego_audio, stats = encode_audio(
+            message="Tier 0 integration",
+            reference_photo=reference_photo,
+            carrier_audio=carrier_wav_spread_integration,
+            passphrase="test words here now",
+            pin="123456",
+            embed_mode="audio_spread",
+            chip_tier=0,
+        )
+
+        assert stats.chip_tier == 0
+        assert stats.chip_length == 256
+
+        result = decode_audio(
+            stego_audio=stego_audio,
+            reference_photo=reference_photo,
+            passphrase="test words here now",
+            pin="123456",
+            embed_mode="audio_spread",
+        )
+
+        assert result.message == "Tier 0 integration"
+
     def test_auto_detect_lsb(self, carrier_wav, reference_photo):
         """Test auto-detection finds LSB encoded audio."""
         from stegasoo.decode import decode_audio
@@ -446,3 +831,32 @@ class TestIntegration:
         )
 
         assert result.message == "Auto-detect test"
+
+    def test_spread_with_real_speech(self, speech_wav, reference_photo):
+        """Test spread spectrum with real speech audio from test_data."""
+        from stegasoo.decode import decode_audio
+        from stegasoo.encode import encode_audio
+
+        message = "Hidden in a speech about elitism"
+
+        stego_audio, stats = encode_audio(
+            message=message,
+            reference_photo=reference_photo,
+            carrier_audio=speech_wav,
+            passphrase="test words here now",
+            pin="123456",
+            embed_mode="audio_spread",
+            chip_tier=0,  # lossless tier for max capacity
+        )
+
+        assert stats.chip_tier == 0
+
+        result = decode_audio(
+            stego_audio=stego_audio,
+            reference_photo=reference_photo,
+            passphrase="test words here now",
+            pin="123456",
+            embed_mode="audio_spread",
+        )
+
+        assert result.message == message