Merge branch 'main' of github.com:adlee-was-taken/stegasoo

2026-04-04 16:29:34 -04:00
parent 4607ff27dd 70b941d55a
commit c970261e53
25 changed files with 3882 additions and 255 deletions
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,6 @@
+# Embedded repos (AUR packaging)
+aur-cli-upload/
+
 # Python
 __pycache__/
 *.py[cod]
--- a/IdeasScout_PLANS_20260324.md
+++ b/IdeasScout_PLANS_20260324.md
@@ -0,0 +1,294 @@
+# Stegasoo Ideas Scout — Implementation Plans (2026-03-24)
+
+Baseline: v4.3.0, Python >=3.11, FORMAT_VERSION 5, no existing users (no backward compat constraints).
+
+---
+
+## Tier 1 — Quick Wins
+
+### 1. Platform-Calibrated DCT Presets
+
+**Description**: `--platform telegram|discord|signal|whatsapp` flag for DCT encode. Bakes in each platform's known recompression parameters. Pre-verifies payload survives before outputting.
+
+**Implementation approach**:
+- New file `src/stegasoo/platform_presets.py` — `PlatformPreset` dataclass + `PRESETS` dict mapping platform → tuned `quant_step`, `jpeg_quality`, `embed_positions`, `max_dimension`, `recompress_quality`
+- `dct_steganography.py`: `_embed_scipy_dct_safe()` / `_embed_jpegio()` accept optional preset overrides for `QUANT_STEP`, `DEFAULT_EMBED_POSITIONS`, output quality
+- New `pre_verify_survival()` function: encode → re-save at platform quality → extract → pass/fail
+- Thread `platform` param through `encode.py` → `steganography.py` → DCT functions
+- `cli.py`: add `--platform` as `click.Choice` + `--verify/--no-verify` (pre-verification doubles encode time)
+- LSB + `--platform` should error early — LSB data is destroyed by any JPEG recompression
+
+**Known platform params** (from research):
+| Platform | Quality | Max Dimension | Notes |
+|----------|---------|---------------|-------|
+| Telegram | ~82 | 2560×2560 | ~81KB embeddable |
+| Discord | ~85 | Varies (Nitro) | |
+| Signal | ~80 | Aggressive | |
+| WhatsApp | ~70 | 1600×1600 | Most lossy |
+
+**Go/No-Go metrics**:
+- >95% payload survival rate per platform at 1KB message size in automated tests
+- Pre-verification correctly predicts real platform behavior (manual validation per platform at least once)
+
+**Complexity**: **M** — new file + parameter threading through 4-5 functions
+
+**Risks**: Platform params change without notice. Add version/date stamps to presets and a `stegasoo tools verify-platform` test command.
+
+---
+
+### 2. Steganalysis Self-Check (`stegasoo check`)
+
+**Description**: New CLI command running chi-square and RS (Regular-Singular) statistical analysis on stego images. Outputs detectability risk level (low/medium/high).
+
+**Implementation approach**:
+- New file `src/stegasoo/steganalysis.py`:
+  - `chi_square_analysis(image_data) -> float` — chi-square statistic on LSB distribution per channel
+  - `rs_analysis(image_data) -> float` — Regular-Singular groups analysis (requires numpy)
+  - `assess_risk(chi_p, rs_estimate) -> str` — maps to "low"/"medium"/"high"
+  - `check_image(image_data) -> dict` — orchestrator
+- `cli.py`: new `@cli.command("check")` with `IMAGE` arg, `--json`, `--mode lsb|dct|auto`
+- `constants.py`: threshold constants for chi-square p-value and RS boundaries
+- `__init__.py`: export `check_image` in `__all__`
+- Start LSB-only; DCT steganalysis (calibration attack) deferred
+
+**Go/No-Go metrics**:
+- Clean images → consistently "low risk"
+- Naive sequential LSB → "high risk"
+- Stegasoo LSB at <50% capacity → "low" or "medium"
+
+**Complexity**: **M** — ~150 lines numpy per test, straightforward CLI integration
+
+---
+
+### 3. Python 3.13 DCT Cleanup
+
+**Description**: The `jpegio` → `jpeglib` migration is already done in code. Remaining work: rename stale `jpegio` references and verify on 3.13.
+
+**Implementation approach**:
+- `dct_steganography.py`: rename `HAS_JPEGIO` → `HAS_JPEGLIB`, `_jpegio_*` functions → `_jpeglib_*`, update constant names (`JPEGIO_MAGIC` → `JPEGLIB_MAGIC`, etc.)
+- Verify `jpeglib.to_jpegio()` compatibility shim — if jpeglib plans to deprecate it, migrate to native API
+- Run full test suite on Python 3.13
+
+**Go/No-Go metrics**:
+- All DCT tests pass on Python 3.13
+- No deprecation warnings from jpeglib
+
+**Complexity**: **S** — renaming and verification only
+
+---
+
+## Tier 2 — Strategic
+
+### 4. Content-Adaptive Embedding (S-UNIWARD/WOW-inspired)
+
+**Description**: Replace uniform-random pixel selection with texture-weighted cost functions. Embed preferentially in busy/textured regions where changes are least detectable. 3-5x harder to detect statistically.
+
+**Implementation approach**:
+- New file `src/stegasoo/adaptive_cost.py`:
+  - `compute_cost_map(image_data) -> np.ndarray` — per-pixel distortion cost via directional high-pass filters (Daubechets wavelet bank / KB filter)
+  - `select_pixels_by_cost(cost_map, pixel_key, num_needed) -> list[int]` — weighted sampling, still ChaCha20-seeded for determinism
+- `steganography.py`:
+  - `generate_pixel_indices()`: add `cost_map` param, use weighted sampling when provided
+  - `_embed_lsb()`: compute cost map when adaptive mode enabled
+  - `_extract_lsb()`: must compute identical cost map to find same pixels
+- `dct_steganography.py`: adapt `DEFAULT_EMBED_POSITIONS` per-block based on block texture energy
+- Thread `adaptive: bool` through `encode.py`/`decode.py`
+- `constants.py`: add `EMBED_MODE_ADAPTIVE_LSB`, filter kernels, cost thresholds
+
+**Go/No-Go metrics**:
+- Chi-square test (Feature 2) shows measurable improvement vs uniform-random
+- **Critical**: cost map computation is deterministic across platforms (quantize to fixed-point integers)
+- Round-trip decode succeeds on Linux x86, Linux ARM, macOS
+
+**Complexity**: **L** — novel algorithm, cross-platform determinism requirement, touches core embedding
+
+**Risks**: Floating-point differences in wavelet computation could break extraction. Mitigate with integer quantization. Increases encode/decode time ~2-3x.
+
+---
+
+### 5. Per-Message Forward Secrecy via HKDF
+
+**Description**: Derive ephemeral per-message encryption keys using HKDF expansion from the Argon2id root key + random nonce. Compromising one message doesn't reveal others.
+
+**Implementation approach**:
+- `crypto.py`:
+  - Add `from cryptography.hazmat.primitives.kdf.hkdf import HKDFExpand`
+  - `derive_message_key(root_key, nonce) -> bytes` — HKDF-Expand with SHA-256
+  - `encrypt_message()`: generate 16-byte random nonce, derive per-message key, embed nonce in header
+  - `decrypt_message()`: extract nonce, derive same key
+  - Also derive pixel selection key via HKDF with different `info` param
+- `constants.py`:
+  - Bump `FORMAT_VERSION` to 6
+  - `HKDF_INFO_ENCRYPTION = b"stegasoo-v6-encrypt"`, `HKDF_INFO_PIXEL = b"stegasoo-v6-pixel"`
+  - `MESSAGE_NONCE_SIZE = 16`
+- Header grows from 66 → 82 bytes: add `message_nonce(16)` field
+- Update `HEADER_OVERHEAD` / `ENCRYPTION_OVERHEAD` in `steganography.py`
+
+**Go/No-Go metrics**:
+- Two messages with identical credentials produce different ciphertexts and different pixel locations
+- `cryptography` library HKDF works with existing Argon2id output
+
+**Complexity**: **M** — well-defined crypto change, touches security-critical header format
+
+---
+
+### 6. PWA Mobile Interface
+
+**Description**: Convert Flask Web UI to Progressive Web App. Mobile-optimized, installable, offline-capable static pages.
+
+**Implementation approach**:
+- New files in `frontends/web/static/`: `manifest.json`, `sw.js`, icon set (192×192, 512×512)
+- Base template: add manifest link, theme-color meta, viewport meta, service worker registration
+- `app.py`: serve manifest with correct MIME, add cache headers for static assets
+- Responsive CSS for encode/decode accordion forms
+- Camera capture: `<input type="file" accept="image/*" capture="environment">` for reference photo
+- Service worker caches static assets only — NOT encode/decode API endpoints
+
+**Go/No-Go metrics**:
+- Lighthouse PWA score >= 90
+- Installable on Android Chrome and iOS Safari
+- Offline: static pages load, encode/decode shows graceful "offline" message
+
+**Complexity**: **M** — frontend only, no core library changes
+
+**Risks**: Camera capture requires HTTPS (already supported via `ssl_utils.py`).
+
+---
+
+## Tier 3 — Moonshot
+
+### 7. Plausible Deniability / Dual-Payload Mode
+
+**Description**: Two independent encrypted payloads in one carrier, each with different credentials. Reveal decoy under coercion; real payload stays hidden.
+
+**Implementation approach**:
+- New file `src/stegasoo/dual_payload.py`:
+  - `encode_dual(message_a, message_b, carrier, creds_a, creds_b)`
+  - Partition available pixels into two disjoint pools using different seeds
+  - **Critical**: ALL images (single or dual) must fill unused pixel pool with random data so single-payload and dual-payload images are indistinguishable
+- `steganography.py`: `generate_pixel_indices()` gets `exclude_indices` param
+- `decode.py`: each credential set finds a different valid payload; wrong credentials produce garbage
+- CLI + Web UI: dual-payload encode workflow
+
+**Go/No-Go metrics**:
+- Single-payload and dual-payload images are statistically indistinguishable (chi-square can't differentiate)
+- Each payload decodes independently
+- Wrong credentials for one payload don't reveal other payload's existence
+
+**Complexity**: **XL** — novel design, halves capacity per payload, challenging UX, needs rigorous security analysis
+
+**Dependencies**: Feature 2 (validation), Feature 4 (detectability reduction)
+
+---
+
+## Architectural Improvements
+
+### 8. EmbeddingBackend Protocol
+
+**Description**: Typed plugin interface for all embedding algorithms. Replace if/elif dispatch in `steganography.py` with a registry.
+
+**Implementation approach**:
+- New package `src/stegasoo/backends/`:
+  - `protocol.py` — `EmbeddingBackend(Protocol)` with `embed()`, `extract()`, `calculate_capacity()`, `is_available()`
+  - `lsb.py`, `dct.py` — wrap existing functions
+  - `registry.py` — `BackendRegistry` mapping mode strings to backends
+- `steganography.py`: `embed_in_image()` / `extract_from_image()` dispatch via registry
+- `__init__.py`: export protocol and `register_backend()`
+
+**Complexity**: **M** — implement before Features 4 and 7 (they become new backends)
+
+---
+
+### 9. HKDF Key Separation
+
+Subsumed by Feature 5. The HKDF expansion provides:
+- Encryption key: `HKDF-Expand(root_key, info="stegasoo-encrypt", nonce)`
+- Pixel selection key: `HKDF-Expand(root_key, info="stegasoo-pixel", nonce)`
+- Future: MAC key, padding key, etc.
+
+---
+
+### 10. `[core]` Extra with Minimal Deps
+
+**Description**: Move Pillow to `[image]` extra, base deps = `cryptography` + `argon2-cffi` + `zstandard` only.
+
+**Complexity**: **S** — but Pillow is used in `crypto.py` for photo hashing (core to security model). Only worth it with a concrete headless use case. **Low priority.**
+
+---
+
+## Ecosystem Features
+
+### 11. Aletheia Integration
+
+Optional `--engine aletheia` backend for Feature 2's `stegasoo check`. BSD-licensed, provides SPA/RS/WS attacks + ML classifiers. **Complexity: S** (after Feature 2). **Depends on**: Feature 2.
+
+### 12. C2PA/AI Provenance Watermarking
+
+Embed C2PA metadata alongside stego payloads. **Complexity: L** — C2PA is a complex standard. Potentially conflicts with stego goals (adds detectable metadata). Research-heavy.
+
+### 13. Signal/Matrix Bot
+
+Bot that decodes stego images in a channel using configured channel key. **Complexity: M** — integration work, uses existing `decode()` API.
+
+### 14. Homebrew Tap + Nix Flake
+
+Package distribution for macOS/NixOS. **Complexity: S** — packaging only, no code changes.
+
+---
+
+## Summary Table
+
+| # | Feature | Tier | Size | Dependencies | Primary Files |
+|---|---------|------|------|-------------|---------------|
+| 1 | Platform DCT Presets | T1 | M | — | new `platform_presets.py`, `dct_steganography.py`, `encode.py`, `cli.py` |
+| 2 | Steganalysis Self-Check | T1 | M | — | new `steganalysis.py`, `cli.py`, `constants.py` |
+| 3 | Python 3.13 DCT Cleanup | T1 | S | — | `dct_steganography.py` |
+| 4 | Content-Adaptive Embedding | T2 | L | numpy, #2 | new `adaptive_cost.py`, `steganography.py`, `constants.py` |
+| 5 | HKDF Forward Secrecy | T2 | M | — | `crypto.py`, `constants.py`, `steganography.py` |
+| 6 | PWA Mobile Interface | T2 | M | — | `frontends/web/` templates + static |
+| 7 | Dual-Payload Mode | T3 | XL | #2, #4 | new `dual_payload.py`, `steganography.py`, `cli.py` |
+| 8 | EmbeddingBackend Protocol | Arch | M | — | new `backends/` package, `steganography.py` |
+| 9 | HKDF Key Separation | Arch | — | Included in #5 | `crypto.py` |
+| 10 | `[core]` Extra | Arch | S | — | `pyproject.toml` |
+| 11 | Aletheia Integration | Eco | S | #2 | `steganalysis.py` |
+| 12 | C2PA Watermarking | Eco | L | — | new module |
+| 13 | Signal/Matrix Bot | Eco | M | — | new `bots/` package |
+| 14 | Homebrew + Nix | Eco | S | — | packaging files only |
+
+---
+
+## Suggested Roadmap
+
+### Phase 1 — Foundations (v4.4.0)
+
+1. **#3** Python 3.13 DCT Cleanup (S) — unblocks CI on 3.13
+2. **#8** EmbeddingBackend Protocol (M) — architectural cleanup before new embedding work
+3. **#2** Steganalysis Self-Check (M) — validation tooling for everything that follows
+
+### Phase 2 — Security & Robustness (v4.5.0)
+
+4. **#5** HKDF Forward Secrecy (M) — FORMAT_VERSION bump to 6, improved crypto
+5. **#1** Platform-Calibrated DCT Presets (M) — high user value for social media
+6. **#14** Homebrew + Nix (S) — distribution expansion
+
+### Phase 3 — Advanced Steganography (v5.0.0)
+
+7. **#4** Content-Adaptive Embedding (L) — major security improvement
+8. **#6** PWA Mobile Interface (M) — parallel frontend work stream
+
+### Phase 4 — Moonshot (v5.x+)
+
+9. **#7** Dual-Payload Mode (XL) — after #2 and #4 are solid
+10. **#12** C2PA Watermarking (L) — research-heavy
+11. **#13** Signal/Matrix Bot (M) — community-driven
+
+---
+
+## Additional Ideas (Backlog)
+
+- **Animated GIF steganography** — LSB in GIF frames, natural multi-media extension
+- **PDF steganography** — whitespace/font metric/embedded image payloads
+- **Batch encode** — `stegasoo batch-encode --dir /photos/` with auto carrier selection (BATCH_* constants suggest this was planned)
+- **Stego identification** — `stegasoo identify image.png` probes for known stego signatures
+- **Per-device credential sync via QR** — channel key as stego image of reference photo
+- **`stegasoo verify`** — decode + confirm message matches expected hash without revealing contents
--- a/rpi/flash-stock-img.sh
+++ b/rpi/flash-stock-img.sh
@@ -1,7 +1,7 @@
 #!/bin/bash
 # Flash Raspberry Pi image with headless config (Trixie/Bookworm compatible)
-# Usage: ./flash-stock-img.sh <image.img.xz> <device>
-# Reads settings from config.json in same directory
+# Usage: ./flash-stock-img.sh [-c config.json] <image.img.xz> <device>
+# Reads settings from config.json in same directory (or specify with -c)
 #
 # Uses the same firstrun.sh approach as rpi-imager for compatibility

@@ -10,11 +10,31 @@ set -e
 SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 CONFIG_FILE="$SCRIPT_DIR/config.json"

+# ============================================================================
+# Parse options
+# ============================================================================
+usage() {
+    echo "Usage: $0 [-c config.json] <image.img.xz> <device>"
+    echo "  -c FILE  Use alternate config file (default: config.json in script dir)"
+    echo "Example: $0 2025-12-04-raspios-trixie-arm64-lite.img.xz /dev/sdb"
+    echo "Example: $0 -c myconfig.json raspios.img.xz /dev/sdb"
+    exit 1
+}
+
+while getopts "c:h" opt; do
+    case $opt in
+        c) CONFIG_FILE="$OPTARG" ;;
+        h) usage ;;
+        *) usage ;;
+    esac
+done
+shift $((OPTIND - 1))
+
 # ============================================================================
 # Load config
 # ============================================================================
 if [ ! -f "$CONFIG_FILE" ]; then
-    echo "Error: config.json not found at $CONFIG_FILE"
+    echo "Error: config file not found at $CONFIG_FILE"
    exit 1
 fi

@@ -38,9 +58,7 @@ echo
 # Validate args
 # ============================================================================
 if [ $# -ne 2 ]; then
-    echo "Usage: $0 <image.img.xz> <device>"
-    echo "Example: $0 2025-12-04-raspios-trixie-arm64-lite.img.xz /dev/sdb"
-    exit 1
+    usage
 fi

 IMAGE="$1"
--- a/rpi/pull-image.sh
+++ b/rpi/pull-image.sh
@@ -14,9 +14,9 @@ BOLD='\033[1m'
 NC='\033[0m'

 if [ $# -ne 2 ]; then
-    echo "Usage: $0 <device> <output.img.zst>"
-    echo "Example: $0 /dev/sdb stegasoo-rpi-4.2.1.img.zst"
-    exit 1
+  echo "Usage: $0 <device> <output.img.zst>"
+  echo "Example: $0 /dev/sdb stegasoo-rpi-4.2.1.img.zst"
+  exit 1
 fi

 DEVICE="$1"
@@ -24,13 +24,13 @@ OUTPUT="$2"

 # Check for root
 if [ "$EUID" -ne 0 ]; then
-    echo -e "${RED}Error: Must run as root (sudo)${NC}"
-    exit 1
+  echo -e "${RED}Error: Must run as root (sudo)${NC}"
+  exit 1
 fi

 if [ ! -b "$DEVICE" ]; then
-    echo -e "${RED}Error: Device not found: $DEVICE${NC}"
-    exit 1
+  echo -e "${RED}Error: Device not found: $DEVICE${NC}"
+  exit 1
 fi

 echo -e "${BOLD}Device info:${NC}"
@@ -39,14 +39,14 @@ echo

 # Find partitions
 if [ -b "${DEVICE}1" ]; then
-    BOOT_PART="${DEVICE}1"
-    ROOT_PART="${DEVICE}2"
+  BOOT_PART="${DEVICE}1"
+  ROOT_PART="${DEVICE}2"
 elif [ -b "${DEVICE}p1" ]; then
-    BOOT_PART="${DEVICE}p1"
-    ROOT_PART="${DEVICE}p2"
+  BOOT_PART="${DEVICE}p1"
+  ROOT_PART="${DEVICE}p2"
 else
-    echo -e "${RED}Error: Could not find partitions${NC}"
-    exit 1
+  echo -e "${RED}Error: Could not find partitions${NC}"
+  exit 1
 fi

 # Unmount any mounted partitions
@@ -62,65 +62,65 @@ echo -e "${BOLD}Checking partition size...${NC}"

 # Get current partition size in bytes
 CURRENT_SIZE=$(blockdev --getsize64 "$ROOT_PART")
-TARGET_BYTES=$((16 * 1024 * 1024 * 1024))  # 16GB in bytes
+TARGET_BYTES=$((16 * 1024 * 1024 * 1024)) # 16GB in bytes
 CURRENT_GB=$(echo "scale=2; $CURRENT_SIZE / 1073741824" | bc)

 echo "  Current rootfs size: ${CURRENT_GB}GB"

 if [ "$CURRENT_SIZE" -gt "$TARGET_BYTES" ]; then
-    echo -e "${YELLOW}Resizing rootfs to 16GB...${NC}"
+  echo -e "${YELLOW}Resizing rootfs to 16GB...${NC}"

-    # Get boot partition end in sectors
-    BOOT_END=$(parted -s "$DEVICE" unit s print | grep "^ 1" | awk '{print $3}' | tr -d 's')
+  # Get boot partition end in sectors
+  BOOT_END=$(parted -s "$DEVICE" unit s print | grep "^ 1" | awk '{print $3}' | tr -d 's')

-    # Calculate 16GB in sectors (512 byte sectors)
-    ROOT_SIZE_SECTORS=33554432
-    ROOT_END=$((BOOT_END + ROOT_SIZE_SECTORS))
+  # Calculate 16GB in sectors (512 byte sectors)
+  ROOT_SIZE_SECTORS=33554432
+  ROOT_END=$((BOOT_END + ROOT_SIZE_SECTORS))

-    # SHRINKING: filesystem first, then partition
-    echo "  Checking filesystem..."
-    e2fsck -f -y "$ROOT_PART" 2>/dev/null || true
+  # SHRINKING: filesystem first, then partition
+  echo "  Checking filesystem..."
+  e2fsck -f -y "$ROOT_PART" 2>/dev/null || true

-    # Shrink filesystem to 15.5GB (leave room for partition overhead)
-    echo "  Shrinking filesystem to 15500M..."
-    resize2fs "$ROOT_PART" 15500M
+  # Shrink filesystem to 15.5GB (leave room for partition overhead)
+  echo "  Shrinking filesystem to 15500M..."
+  resize2fs "$ROOT_PART" 15500M

-    # Delete and recreate partition 2 with 16GB size
-    echo "  Shrinking partition to 16GB..."
-    parted -s "$DEVICE" rm 2
-    parted -s "$DEVICE" mkpart primary ext4 $((BOOT_END + 1))s ${ROOT_END}s
+  # Delete and recreate partition 2 with 16GB size
+  echo "  Shrinking partition to 16GB..."
+  parted -s "$DEVICE" rm 2
+  parted -s "$DEVICE" mkpart primary ext4 $((BOOT_END + 1))s ${ROOT_END}s

-    # Refresh partition table
-    partprobe "$DEVICE"
-    sleep 2
+  # Refresh partition table
+  partprobe "$DEVICE"
+  sleep 2

-    # Expand filesystem to fill the partition exactly
-    echo "  Expanding filesystem to fill partition..."
-    e2fsck -f -y "$ROOT_PART" 2>/dev/null || true
-    resize2fs "$ROOT_PART"
+  # Expand filesystem to fill the partition exactly
+  echo "  Expanding filesystem to fill partition..."
+  e2fsck -f -y "$ROOT_PART" 2>/dev/null || true
+  resize2fs "$ROOT_PART"

-    echo -e "${GREEN}  Rootfs resized to 16GB${NC}"
+  echo -e "${GREEN}  Rootfs resized to 16GB${NC}"
 elif [ "$CURRENT_SIZE" -lt "$TARGET_BYTES" ]; then
-    echo -e "${YELLOW}  Rootfs is smaller than 16GB - expanding...${NC}"
+  echo -e "${YELLOW}  Rootfs is smaller than 16GB - expanding...${NC}"

-    # Get boot partition end in sectors
-    BOOT_END=$(parted -s "$DEVICE" unit s print | grep "^ 1" | awk '{print $3}' | tr -d 's')
-    ROOT_SIZE_SECTORS=33554432
-    ROOT_END=$((BOOT_END + ROOT_SIZE_SECTORS))
+  # Get boot partition end in sectors
+  BOOT_END=$(parted -s "$DEVICE" unit s print | grep "^ 1" | awk '{print $3}' | tr -d 's')
+  ROOT_SIZE_SECTORS=33554432
+  ROOT_END=$((BOOT_END + ROOT_SIZE_SECTORS))

-    # EXPANDING: partition first, then filesystem
-    parted -s "$DEVICE" rm 2
-    parted -s "$DEVICE" mkpart primary ext4 $((BOOT_END + 1))s ${ROOT_END}s
+  # EXPANDING: partition first, then filesystem
+  parted -s "$DEVICE" rm 2
+  parted -s "$DEVICE" mkpart primary ext4 $((BOOT_END + 1))s ${ROOT_END}s

-    partprobe "$DEVICE"
-    sleep 2
+  partprobe "$DEVICE"
+  sleep 2

-    e2fsck -f -y "$ROOT_PART" 2>/dev/null || true
-    resize2fs "$ROOT_PART"
+  e2fsck -f -y "$ROOT_PART" 2>/dev/null || true
+  resize2fs "$ROOT_PART"

-    echo -e "${GREEN}  Rootfs expanded to 16GB${NC}"
+  echo -e "${GREEN}  Rootfs expanded to 16GB${NC}"
 else
-    echo -e "${GREEN}  Rootfs already ~16GB${NC}"
+  echo -e "${GREEN}  Rootfs already ~16GB${NC}"
 fi

 # ============================================================================
@@ -135,8 +135,8 @@ echo
 END_SECTOR=$(parted -s "$DEVICE" unit s print | grep "^ 2" | awk '{print $3}' | tr -d 's')

 if [ -z "$END_SECTOR" ]; then
-    echo -e "${RED}Error: Could not determine partition 2 end sector${NC}"
-    exit 1
+  echo -e "${RED}Error: Could not determine partition 2 end sector${NC}"
+  exit 1
 fi

 # Add a small buffer (1MB = 2048 sectors) for safety
@@ -150,8 +150,8 @@ echo

 read -p "Proceed with image pull? [Y/n] " confirm
 if [[ "$confirm" =~ ^[Nn]$ ]]; then
-    echo "Aborted."
-    exit 1
+  echo "Aborted."
+  exit 1
 fi

 echo
@@ -159,13 +159,13 @@ echo -e "${GREEN}Pulling image...${NC}"
 echo

 # Use pv if available for progress, otherwise fallback to dd status
-if command -v pv &> /dev/null; then
-    dd if="$DEVICE" bs=512 count=$TOTAL_SECTORS 2>/dev/null | \
-        pv -s $TOTAL_BYTES | \
-        zstd -T0 -3 > "$OUTPUT"
+if command -v pv &>/dev/null; then
+  dd if="$DEVICE" bs=512 count=$TOTAL_SECTORS 2>/dev/null |
+    pv -s $TOTAL_BYTES |
+    zstd -T0 -19 --ultra >"$OUTPUT"
 else
-    dd if="$DEVICE" bs=512 count=$TOTAL_SECTORS status=progress | \
-        zstd -T0 -3 > "$OUTPUT"
+  dd if="$DEVICE" bs=512 count=$TOTAL_SECTORS status=progress |
+    zstd -T0 -19 --ultra >"$OUTPUT"
 fi

 echo
@@ -178,16 +178,16 @@ ls -lh "$OUTPUT"
 echo
 read -p "Create .zst.zip wrapper for GitHub? [y/N] " zip_confirm
 if [[ "$zip_confirm" =~ ^[Yy]$ ]]; then
-    ZIP_OUTPUT="${OUTPUT}.zip"
-    echo -e "${YELLOW}Creating zip wrapper (store mode, no compression)...${NC}"
-    zip -0 "$ZIP_OUTPUT" "$OUTPUT"
-    echo -e "${GREEN}Done!${NC} Upload this to GitHub Releases:"
-    ls -lh "$ZIP_OUTPUT"
-    echo
-    echo "Users can flash with:"
-    echo "  sudo ./rpi/flash-image.sh $ZIP_OUTPUT"
+  ZIP_OUTPUT="${OUTPUT}.zip"
+  echo -e "${YELLOW}Creating zip wrapper (store mode, no compression)...${NC}"
+  zip -0 "$ZIP_OUTPUT" "$OUTPUT"
+  echo -e "${GREEN}Done!${NC} Upload this to GitHub Releases:"
+  ls -lh "$ZIP_OUTPUT"
+  echo
+  echo "Users can flash with:"
+  echo "  sudo ./rpi/flash-image.sh $ZIP_OUTPUT"
 else
-    echo
-    echo "To verify:"
-    echo "  zstdcat $OUTPUT | fdisk -l /dev/stdin"
+  echo
+  echo "To verify:"
+  echo "  zstdcat $OUTPUT | fdisk -l /dev/stdin"
 fi
--- a/rpi/train_proj.json
+++ b/rpi/train_proj.json
@@ -0,0 +1,13 @@
+{
+  "hostname": "running_trains",
+  "username": "admin",
+  "password": "runthemtrains",
+  "wifiSSID": "WitchHazelWrecked",
+  "wifiPassword": "BeefPigsMoo",
+  "wifiCountry": "US",
+  "locale": "en_US.UTF-8",
+  "keyboardLayout": "us",
+  "timezone": "America/New_York",
+  "enableSSH": true
+}
+
--- a/src/stegasoo/init.py
+++ b/src/stegasoo/init.py
@@ -22,6 +22,9 @@ from .channel import (
    validate_channel_key,
 )

+# Audio support — gated by STEGASOO_AUDIO env var and dependency availability
+from .constants import AUDIO_ENABLED, VIDEO_ENABLED
+
 # Crypto functions
 from .crypto import get_active_channel_key, get_channel_fingerprint, has_argon2
 from .decode import decode, decode_file, decode_text
@@ -43,6 +46,16 @@ from .image_utils import (
    get_image_info,
 )

+# Backend registry
+from .backends import EmbeddingBackend, registry as backend_registry
+
+# Platform presets
+from .platform_presets import PLATFORMS, get_preset
+
+# Steganalysis
+from .steganalysis import check_image
+from .backends.registry import BackendNotFoundError
+
 # Steganography functions
 from .steganography import (
    calculate_capacity_by_mode,
@@ -54,9 +67,6 @@ from .steganography import (
 # Utilities
 from .utils import generate_filename

-# 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

@@ -77,6 +87,24 @@ else:
    encode_audio = None
    decode_audio = None

+# Video support — gated by STEGASOO_VIDEO env var and ffmpeg + audio deps
+if VIDEO_ENABLED:
+    from .decode import decode_video
+    from .encode import encode_video
+    from .video_utils import (
+        calculate_video_capacity,
+        detect_video_format,
+        get_video_info,
+        validate_video,
+    )
+else:
+    detect_video_format = None
+    get_video_info = None
+    validate_video = None
+    calculate_video_capacity = None
+    encode_video = None
+    decode_video = None
+
 # QR Code utilities - optional, may not be available
 try:
    from .qr_utils import (
@@ -117,6 +145,8 @@ from .constants import (
    EMBED_MODE_AUTO,
    EMBED_MODE_DCT,
    EMBED_MODE_LSB,
+    EMBED_MODE_VIDEO_AUTO,
+    EMBED_MODE_VIDEO_LSB,
    FORMAT_VERSION,
    LOSSLESS_FORMATS,
    MAX_FILE_PAYLOAD_SIZE,
@@ -159,7 +189,13 @@ from .exceptions import (
    SteganographyError,
    StegasooError,
    UnsupportedAudioFormatError,
+    UnsupportedVideoFormatError,
    ValidationError,
+    VideoCapacityError,
+    VideoError,
+    VideoExtractionError,
+    VideoTranscodeError,
+    VideoValidationError,
 )

 # Models
@@ -175,6 +211,9 @@ from .models import (
    GenerateResult,
    ImageInfo,
    ValidationResult,
+    VideoCapacityInfo,
+    VideoEmbedStats,
+    VideoInfo,
 )
 from .validation import (
    validate_audio_embed_mode,
@@ -212,6 +251,13 @@ __all__ = [
    "HAS_VIDEO_SUPPORT",
    "validate_audio_embed_mode",
    "validate_audio_file",
+    # Video (v4.4.0)
+    "encode_video",
+    "decode_video",
+    "detect_video_format",
+    "get_video_info",
+    "validate_video",
+    "calculate_video_capacity",
    # Generation
    "generate_pin",
    "generate_passphrase",
@@ -237,6 +283,15 @@ __all__ = [
    "generate_filename",
    # Crypto
    "has_argon2",
+    # Backends
+    "EmbeddingBackend",
+    "backend_registry",
+    "BackendNotFoundError",
+    # Platform presets
+    "get_preset",
+    "PLATFORMS",
+    # Steganalysis
+    "check_image",
    # Steganography
    "has_dct_support",
    "calculate_capacity_by_mode",
@@ -273,6 +328,10 @@ __all__ = [
    "AudioEmbedStats",
    "AudioInfo",
    "AudioCapacityInfo",
+    # Video models
+    "VideoEmbedStats",
+    "VideoInfo",
+    "VideoCapacityInfo",
    # Exceptions
    "StegasooError",
    "ValidationError",
@@ -303,6 +362,13 @@ __all__ = [
    "AudioExtractionError",
    "AudioTranscodeError",
    "UnsupportedAudioFormatError",
+    # Video exceptions
+    "VideoError",
+    "VideoValidationError",
+    "VideoCapacityError",
+    "VideoExtractionError",
+    "VideoTranscodeError",
+    "UnsupportedVideoFormatError",
    # Constants
    "FORMAT_VERSION",
    "MIN_PASSPHRASE_WORDS",
@@ -329,4 +395,7 @@ __all__ = [
    "EMBED_MODE_AUDIO_LSB",
    "EMBED_MODE_AUDIO_SPREAD",
    "EMBED_MODE_AUDIO_AUTO",
+    # Video constants
+    "EMBED_MODE_VIDEO_LSB",
+    "EMBED_MODE_VIDEO_AUTO",
 ]
--- a/src/stegasoo/backends/init.py
+++ b/src/stegasoo/backends/init.py
@@ -0,0 +1,31 @@
+"""
+Stegasoo embedding backends.
+
+Provides a typed plugin interface for all embedding algorithms.
+Backends register with the module-level ``registry`` on import.
+
+Usage::
+
+    from stegasoo.backends import registry
+
+    backend = registry.get("lsb")
+    stego, stats = backend.embed(data, carrier, key)
+"""
+
+from .dct import DCTBackend
+from .lsb import LSBBackend
+from .protocol import EmbeddingBackend
+from .registry import BackendNotFoundError, BackendRegistry, registry
+
+# Auto-register built-in backends
+registry.register(LSBBackend())
+registry.register(DCTBackend())
+
+__all__ = [
+    "EmbeddingBackend",
+    "BackendRegistry",
+    "BackendNotFoundError",
+    "registry",
+    "LSBBackend",
+    "DCTBackend",
+]
--- a/src/stegasoo/backends/dct.py
+++ b/src/stegasoo/backends/dct.py
@@ -0,0 +1,69 @@
+"""
+DCT (Discrete Cosine Transform) image embedding backend.
+
+Wraps the existing frequency-domain DCT functions in dct_steganography.py.
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+
+class DCTBackend:
+    """Frequency-domain DCT embedding for JPEG-resilient steganography."""
+
+    @property
+    def mode(self) -> str:
+        return "dct"
+
+    @property
+    def carrier_type(self) -> str:
+        return "image"
+
+    def is_available(self) -> bool:
+        from ..dct_steganography import HAS_SCIPY
+
+        return HAS_SCIPY
+
+    def embed(
+        self,
+        data: bytes,
+        carrier: bytes,
+        key: bytes,
+        *,
+        progress_file: str | None = None,
+        **options: Any,
+    ) -> tuple[bytes, Any]:
+        from ..dct_steganography import embed_in_dct
+
+        output_format = options.get("dct_output_format", "png")
+        color_mode = options.get("dct_color_mode", "color")
+        quant_step = options.get("quant_step")
+        jpeg_quality = options.get("jpeg_quality")
+        max_dimension = options.get("max_dimension")
+        return embed_in_dct(
+            data, carrier, key, output_format, color_mode, progress_file,
+            quant_step=quant_step, jpeg_quality=jpeg_quality, max_dimension=max_dimension,
+        )
+
+    def extract(
+        self,
+        carrier: bytes,
+        key: bytes,
+        *,
+        progress_file: str | None = None,
+        **options: Any,
+    ) -> bytes | None:
+        from ..dct_steganography import extract_from_dct
+
+        quant_step = options.get("quant_step")
+        try:
+            return extract_from_dct(carrier, key, progress_file, quant_step=quant_step)
+        except Exception:
+            return None
+
+    def calculate_capacity(self, carrier: bytes, **options: Any) -> int:
+        from ..dct_steganography import calculate_dct_capacity
+
+        info = calculate_dct_capacity(carrier)
+        return info.usable_capacity_bytes
--- a/src/stegasoo/backends/lsb.py
+++ b/src/stegasoo/backends/lsb.py
@@ -0,0 +1,63 @@
+"""
+LSB (Least Significant Bit) image embedding backend.
+
+Wraps the existing spatial-domain LSB functions in steganography.py.
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+
+class LSBBackend:
+    """Spatial-domain LSB embedding for lossless image formats."""
+
+    @property
+    def mode(self) -> str:
+        return "lsb"
+
+    @property
+    def carrier_type(self) -> str:
+        return "image"
+
+    def is_available(self) -> bool:
+        return True  # Only needs Pillow, which is always present
+
+    def embed(
+        self,
+        data: bytes,
+        carrier: bytes,
+        key: bytes,
+        *,
+        progress_file: str | None = None,
+        **options: Any,
+    ) -> tuple[bytes, Any]:
+        from ..steganography import _embed_lsb
+
+        bits_per_channel = options.get("bits_per_channel", 1)
+        output_format = options.get("output_format", None)
+        stego_bytes, stats, ext = _embed_lsb(
+            data, carrier, key, bits_per_channel, output_format, progress_file
+        )
+        # Attach output extension to stats for callers that need it
+        stats.output_extension = ext  # type: ignore[attr-defined]
+        return stego_bytes, stats
+
+    def extract(
+        self,
+        carrier: bytes,
+        key: bytes,
+        *,
+        progress_file: str | None = None,
+        **options: Any,
+    ) -> bytes | None:
+        from ..steganography import _extract_lsb
+
+        bits_per_channel = options.get("bits_per_channel", 1)
+        return _extract_lsb(carrier, key, bits_per_channel)
+
+    def calculate_capacity(self, carrier: bytes, **options: Any) -> int:
+        from ..steganography import calculate_capacity
+
+        bits_per_channel = options.get("bits_per_channel", 1)
+        return calculate_capacity(carrier, bits_per_channel)
--- a/src/stegasoo/backends/protocol.py
+++ b/src/stegasoo/backends/protocol.py
@@ -0,0 +1,91 @@
+"""
+Embedding backend protocol definition.
+
+All embedding backends (LSB, DCT, audio, video, etc.) implement this protocol,
+enabling registry-based dispatch instead of if/elif chains.
+"""
+
+from __future__ import annotations
+
+from typing import Any, Protocol, runtime_checkable
+
+
+@runtime_checkable
+class EmbeddingBackend(Protocol):
+    """Protocol that all embedding backends must satisfy.
+
+    Each backend handles a specific embedding mode (e.g. 'lsb', 'dct',
+    'audio_lsb', 'audio_spread') for a specific carrier type ('image',
+    'audio', 'video').
+    """
+
+    @property
+    def mode(self) -> str:
+        """The embedding mode identifier (e.g. 'lsb', 'dct')."""
+        ...
+
+    @property
+    def carrier_type(self) -> str:
+        """The carrier media type: 'image', 'audio', or 'video'."""
+        ...
+
+    def is_available(self) -> bool:
+        """Whether this backend's dependencies are installed."""
+        ...
+
+    def embed(
+        self,
+        data: bytes,
+        carrier: bytes,
+        key: bytes,
+        *,
+        progress_file: str | None = None,
+        **options: Any,
+    ) -> tuple[bytes, Any]:
+        """Embed data into a carrier.
+
+        Args:
+            data: Encrypted payload bytes.
+            carrier: Raw carrier file bytes (image, audio, etc.).
+            key: Derived key for pixel/sample selection.
+            progress_file: Optional progress file path.
+            **options: Backend-specific options (bits_per_channel,
+                       output_format, color_mode, chip_tier, etc.).
+
+        Returns:
+            Tuple of (stego carrier bytes, embed stats).
+        """
+        ...
+
+    def extract(
+        self,
+        carrier: bytes,
+        key: bytes,
+        *,
+        progress_file: str | None = None,
+        **options: Any,
+    ) -> bytes | None:
+        """Extract data from a carrier.
+
+        Args:
+            carrier: Stego carrier file bytes.
+            key: Derived key for pixel/sample selection.
+            progress_file: Optional progress file path.
+            **options: Backend-specific options.
+
+        Returns:
+            Extracted payload bytes, or None if no payload found.
+        """
+        ...
+
+    def calculate_capacity(self, carrier: bytes, **options: Any) -> int:
+        """Calculate maximum embeddable payload size in bytes.
+
+        Args:
+            carrier: Raw carrier file bytes.
+            **options: Backend-specific options (e.g. bits_per_channel).
+
+        Returns:
+            Maximum payload capacity in bytes.
+        """
+        ...
--- a/src/stegasoo/backends/registry.py
+++ b/src/stegasoo/backends/registry.py
@@ -0,0 +1,63 @@
+"""
+Backend registry for embedding mode dispatch.
+
+Backends register themselves by mode string. The registry replaces
+if/elif dispatch in steganography.py with a lookup table.
+"""
+
+from __future__ import annotations
+
+from ..exceptions import StegasooError
+from .protocol import EmbeddingBackend
+
+
+class BackendNotFoundError(StegasooError):
+    """Raised when a requested backend mode is not registered."""
+
+
+class BackendRegistry:
+    """Registry mapping mode strings to embedding backends."""
+
+    def __init__(self) -> None:
+        self._backends: dict[str, EmbeddingBackend] = {}
+
+    def register(self, backend: EmbeddingBackend) -> None:
+        """Register a backend for its mode string."""
+        self._backends[backend.mode] = backend
+
+    def get(self, mode: str) -> EmbeddingBackend:
+        """Look up a backend by mode. Raises BackendNotFoundError if not found."""
+        if mode not in self._backends:
+            available = ", ".join(sorted(self._backends.keys())) or "(none)"
+            raise BackendNotFoundError(
+                f"No backend registered for mode '{mode}'. Available: {available}"
+            )
+        return self._backends[mode]
+
+    def has(self, mode: str) -> bool:
+        """Check if a backend is registered for the given mode."""
+        return mode in self._backends
+
+    def available_modes(self, carrier_type: str | None = None) -> list[str]:
+        """List registered mode strings, optionally filtered by carrier type.
+
+        Only includes modes whose backend reports is_available() == True.
+        """
+        return sorted(
+            mode
+            for mode, backend in self._backends.items()
+            if backend.is_available()
+            and (carrier_type is None or backend.carrier_type == carrier_type)
+        )
+
+    def all_modes(self, carrier_type: str | None = None) -> list[str]:
+        """List all registered mode strings (including unavailable ones)."""
+        return sorted(
+            mode
+            for mode, backend in self._backends.items()
+            if carrier_type is None or backend.carrier_type == carrier_type
+        )
+
+
+# Module-level singleton
+registry = BackendRegistry()
--- a/src/stegasoo/cli.py
+++ b/src/stegasoo/cli.py
@@ -184,8 +184,14 @@ def cli(ctx, json_output, debug_mode):
 )
@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.option(
+    "--platform",
+    type=click.Choice(["telegram", "discord", "signal", "whatsapp"], case_sensitive=False),
+    help="DCT preset for social media platform (implies DCT+JPEG mode)",
+)
+@click.option("--verify/--no-verify", default=True, help="Pre-verify payload survives platform recompression")
@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, platform, verify):
    """
    Encode a message or file into an image.

@@ -260,29 +266,48 @@ def encode(ctx, carrier, reference, message, file_payload, output, passphrase, p

    from .steganography import EMBED_MODE_DCT, EMBED_MODE_LSB

+    # Platform preset overrides
+    preset = None
+    if platform:
+        from .platform_presets import get_preset
+
+        preset = get_preset(platform)
+        use_dct = True  # Platform mode implies DCT+JPEG
+        if output_ext not in (".jpg", ".jpeg"):
+            output = str(Path(output).with_suffix(".jpg"))
+            click.echo(f"  Platform mode: output changed to {output}")
+
    try:
+        encode_kwargs = {
+            "reference_photo": reference_data,
+            "carrier_image": carrier_data,
+            "passphrase": passphrase,
+            "pin": pin,
+            "embed_mode": EMBED_MODE_DCT if use_dct else EMBED_MODE_LSB,
+            "dct_output_format": "jpeg" if use_dct else "png",
+        }
+
+        if preset:
+            encode_kwargs["platform"] = platform
+
        if file_payload:
-            # Encode file
-            result = stegasoo_encode_file(
-                filepath=file_payload,
-                reference_photo=reference_data,
-                carrier_image=carrier_data,
-                passphrase=passphrase,
-                pin=pin,
-                embed_mode=EMBED_MODE_DCT if use_dct else EMBED_MODE_LSB,
-                dct_output_format="jpeg" if use_dct else "png",
-            )
+            result = stegasoo_encode_file(filepath=file_payload, **encode_kwargs)
        else:
-            # Encode message
-            result = stegasoo_encode(
-                message=message,
-                reference_photo=reference_data,
-                carrier_image=carrier_data,
-                passphrase=passphrase,
-                pin=pin,
-                embed_mode=EMBED_MODE_DCT if use_dct else EMBED_MODE_LSB,
-                dct_output_format="jpeg" if use_dct else "png",
-            )
+            result = stegasoo_encode(message=message, **encode_kwargs)
+
+        # Pre-verify survival if platform mode
+        if preset and verify:
+            from .crypto import derive_pixel_key
+            from .platform_presets import pre_verify_survival
+
+            pixel_key = derive_pixel_key(reference_data, passphrase, pin)
+            survived = pre_verify_survival(result.stego_image, pixel_key, preset)
+            if not survived:
+                click.echo(
+                    f"  ⚠ Warning: Payload may not survive {preset.name} recompression. "
+                    "Try a larger carrier image or shorter message.",
+                    err=True,
+                )

        # Write output
        with open(output, "wb") as f:
@@ -325,8 +350,13 @@ def encode(ctx, carrier, reference, message, file_payload, output, passphrase, p
@click.option("--passphrase", prompt=True, hide_input=True, help="Passphrase")
@click.option("--pin", prompt=True, hide_input=True, help="PIN code")
@click.option("-o", "--output", type=click.Path(), help="Output path for file payloads")
+@click.option(
+    "--platform",
+    type=click.Choice(["telegram", "discord", "signal", "whatsapp"], case_sensitive=False),
+    help="Platform preset (must match encoding platform)",
+)
@click.pass_context
-def decode(ctx, image, reference, passphrase, pin, output):
+def decode(ctx, image, reference, passphrase, pin, output, platform):
    """
    Decode a message or file from an image.

@@ -334,7 +364,7 @@ def decode(ctx, image, reference, passphrase, pin, output):

        stegasoo decode encoded.png -r ref.jpg --passphrase --pin

-        stegasoo decode encoded.png -r ref.jpg -o ./extracted/
+        stegasoo decode encoded.png -r ref.jpg --platform telegram
    """
    from .decode import decode as stegasoo_decode

@@ -344,12 +374,21 @@ def decode(ctx, image, reference, passphrase, pin, output):
    with open(reference, "rb") as f:
        reference_data = f.read()

+    # Resolve platform preset for DCT decoding
+    decode_kwargs = {}
+    if platform:
+        from .platform_presets import get_preset
+
+        preset = get_preset(platform)
+        decode_kwargs["platform"] = platform
+
    try:
        result = stegasoo_decode(
            stego_image=stego_data,
            reference_photo=reference_data,
            passphrase=passphrase,
            pin=pin,
+            **decode_kwargs,
        )

        if result.is_file:
@@ -853,6 +892,410 @@ def audio_info(ctx, audio):
        raise SystemExit(1)


+# =============================================================================
+# VIDEO COMMANDS (v4.4.0)
+# =============================================================================
+
+
+@cli.command("video-encode")
+@click.argument("carrier", type=click.Path(exists=True))
+@click.option(
+    "-r",
+    "--reference",
+    required=True,
+    type=click.Path(exists=True),
+    help="Reference photo (shared secret)",
+)
+@click.option("-m", "--message", help="Message to encode")
+@click.option(
+    "-f",
+    "--file",
+    "file_payload",
+    type=click.Path(exists=True),
+    help="File to embed instead of message",
+)
+@click.option("-o", "--output", type=click.Path(), help="Output video path")
+@click.option(
+    "--passphrase",
+    prompt=True,
+    hide_input=True,
+    confirmation_prompt=True,
+    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("--dry-run", is_flag=True, help="Show capacity usage without encoding")
+@click.pass_context
+def video_encode(
+    ctx,
+    carrier,
+    reference,
+    message,
+    file_payload,
+    output,
+    passphrase,
+    pin,
+    rsa_key,
+    rsa_password,
+    channel_key,
+    dry_run,
+):
+    """
+    Encode a message or file into a video carrier.
+
+    Output is MKV format with FFV1 lossless codec to preserve embedded data.
+
+    Examples:
+
+        stegasoo video-encode carrier.mp4 -r ref.jpg -m "Secret"
+
+        stegasoo video-encode carrier.mp4 -r ref.jpg -f secret.pdf -o stego.mkv
+
+        stegasoo video-encode carrier.mp4 -r ref.jpg -m "Secret" --dry-run
+    """
+    from .constants import VIDEO_ENABLED
+
+    if not VIDEO_ENABLED:
+        raise click.UsageError(
+            "Video support is disabled. Install ffmpeg and audio extras, "
+            "or set STEGASOO_VIDEO=1 to force enable."
+        )
+
+    from .encode import encode_video
+    from .models import FilePayload
+    from .video_utils import calculate_video_capacity, get_video_info
+
+    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:
+            info = get_video_info(carrier_data)
+            capacity_info = calculate_video_capacity(carrier_data)
+
+            result = {
+                "carrier": carrier,
+                "reference": reference,
+                "format": info.format,
+                "codec": info.codec,
+                "resolution": f"{info.width}x{info.height}",
+                "fps": round(info.fps, 2),
+                "duration_seconds": round(info.duration_seconds, 2),
+                "total_frames": info.total_frames,
+                "i_frames": info.i_frame_count,
+                "capacity_bytes": capacity_info.usable_capacity_bytes,
+                "capacity_kb": round(capacity_info.usable_capacity_bytes / 1024, 1),
+                "payload_type": payload_type,
+                "payload_size": payload_size,
+                "usage_percent": round(
+                    payload_size / capacity_info.usable_capacity_bytes * 100, 1
+                )
+                if capacity_info.usable_capacity_bytes > 0
+                else 0,
+                "fits": payload_size < capacity_info.usable_capacity_bytes,
+            }
+
+            if ctx.obj.get("json"):
+                click.echo(json.dumps(result, indent=2))
+            else:
+                click.echo(f"Carrier: {carrier} ({info.format}, {info.codec})")
+                click.echo(f"Resolution: {info.width}x{info.height} @ {info.fps:.2f} fps")
+                click.echo(f"Duration: {info.duration_seconds:.1f}s")
+                click.echo(f"Frames: {info.total_frames} total, {info.i_frame_count} I-frames")
+                click.echo(f"Reference: {reference}")
+                click.echo(
+                    f"Capacity: {capacity_info.usable_capacity_bytes:,} bytes "
+                    f"({capacity_info.usable_capacity_bytes // 1024} KB)"
+                )
+                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'}")
+                click.echo()
+                click.echo("Note: Output will be MKV format with FFV1 lossless codec")
+        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:
+        output = f"{Path(carrier).stem}_encoded.mkv"
+
+    try:
+        if file_payload:
+            payload = FilePayload.from_file(file_payload)
+        else:
+            payload = message
+
+        stego_video, stats = encode_video(
+            message=payload,
+            reference_photo=reference_data,
+            carrier_video=carrier_data,
+            passphrase=passphrase,
+            pin=pin,
+            rsa_key_data=rsa_key_data,
+            rsa_password=rsa_password,
+            channel_key=channel_key,
+        )
+
+        with open(output, "wb") as f:
+            f.write(stego_video)
+
+        if ctx.obj.get("json"):
+            click.echo(
+                json.dumps(
+                    {
+                        "status": "success",
+                        "carrier": carrier,
+                        "reference": reference,
+                        "output": output,
+                        "codec": stats.codec,
+                        "frames_modified": stats.frames_modified,
+                        "duration_seconds": round(stats.duration_seconds, 2),
+                        "capacity_used": round(stats.capacity_used * 100, 1),
+                    },
+                    indent=2,
+                )
+            )
+        else:
+            click.echo(f"✓ Encoded to {output}")
+            click.echo(f"  Codec: {stats.codec} (lossless)")
+            click.echo(f"  Frames modified: {stats.frames_modified}")
+            click.echo(f"  Capacity used: {stats.capacity_used * 100:.1f}%")
+
+    except Exception as e:
+        if ctx.obj.get("json"):
+            click.echo(json.dumps({"status": "error", "error": str(e)}, indent=2))
+        else:
+            click.echo(f"✗ Video encoding failed: {e}", err=True)
+        raise SystemExit(1)
+
+
+@cli.command("video-decode")
+@click.argument("video", type=click.Path(exists=True))
+@click.option(
+    "-r",
+    "--reference",
+    required=True,
+    type=click.Path(exists=True),
+    help="Reference photo (shared secret)",
+)
+@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 video_decode(
+    ctx, video, reference, passphrase, pin, rsa_key, rsa_password, channel_key, output
+):
+    """
+    Decode a message or file from stego video.
+
+    Examples:
+
+        stegasoo video-decode stego.mkv -r ref.jpg
+
+        stegasoo video-decode stego.mkv -r ref.jpg -o ./extracted/
+    """
+    from .constants import VIDEO_ENABLED
+
+    if not VIDEO_ENABLED:
+        raise click.UsageError(
+            "Video support is disabled. Install ffmpeg and audio extras, "
+            "or set STEGASOO_VIDEO=1 to force enable."
+        )
+
+    from .decode import decode_video
+
+    # 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(video, "rb") as f:
+        video_data = f.read()
+    with open(reference, "rb") as f:
+        reference_data = f.read()
+
+    try:
+        result = decode_video(
+            stego_video=video_data,
+            reference_photo=reference_data,
+            passphrase=passphrase,
+            pin=pin,
+            rsa_key_data=rsa_key_data,
+            rsa_password=rsa_password,
+            channel_key=channel_key,
+        )
+
+        if result.is_file:
+            filename = result.filename or "decoded_file"
+            output_path = Path(output) / filename if output else Path(filename)
+            output_path.parent.mkdir(parents=True, exist_ok=True)
+
+            with open(output_path, "wb") as f:
+                f.write(result.file_data)
+
+            if ctx.obj.get("json"):
+                click.echo(
+                    json.dumps(
+                        {
+                            "status": "success",
+                            "video": video,
+                            "payload_type": "file",
+                            "filename": filename,
+                            "output": str(output_path),
+                            "size": len(result.file_data),
+                        },
+                        indent=2,
+                    )
+                )
+            else:
+                click.echo(f"✓ Extracted file: {output_path}")
+                click.echo(f"  Size: {len(result.file_data):,} bytes")
+        else:
+            if ctx.obj.get("json"):
+                click.echo(
+                    json.dumps(
+                        {
+                            "status": "success",
+                            "video": video,
+                            "payload_type": "text",
+                            "message": result.message,
+                        },
+                        indent=2,
+                    )
+                )
+            else:
+                click.echo(f"Decoded from {video}:")
+                click.echo(result.message)
+
+    except Exception as e:
+        if ctx.obj.get("json"):
+            click.echo(json.dumps({"status": "error", "error": str(e)}, indent=2))
+        else:
+            click.echo(f"✗ Video decoding failed: {e}", err=True)
+        raise SystemExit(1)
+
+
+@cli.command("video-info")
+@click.argument("video", type=click.Path(exists=True))
+@click.pass_context
+def video_info(ctx, video):
+    """
+    Show video file information and steganographic capacity.
+
+    Examples:
+
+        stegasoo video-info carrier.mp4
+
+        stegasoo --json video-info carrier.mp4
+    """
+    from .constants import VIDEO_ENABLED
+
+    if not VIDEO_ENABLED:
+        raise click.UsageError(
+            "Video support is disabled. Install ffmpeg and audio extras, "
+            "or set STEGASOO_VIDEO=1 to force enable."
+        )
+
+    from .video_utils import calculate_video_capacity, get_video_info
+
+    with open(video, "rb") as f:
+        video_data = f.read()
+
+    try:
+        info = get_video_info(video_data)
+        capacity_info = calculate_video_capacity(video_data)
+
+        result = {
+            "file": video,
+            "format": info.format,
+            "codec": info.codec,
+            "resolution": {
+                "width": info.width,
+                "height": info.height,
+            },
+            "fps": round(info.fps, 2),
+            "duration_seconds": round(info.duration_seconds, 2),
+            "total_frames": info.total_frames,
+            "i_frame_count": info.i_frame_count,
+            "bitrate": info.bitrate,
+            "file_size": len(video_data),
+            "capacity": {
+                "bytes": capacity_info.usable_capacity_bytes,
+                "kb": round(capacity_info.usable_capacity_bytes / 1024, 1),
+                "mb": round(capacity_info.usable_capacity_bytes / (1024 * 1024), 2),
+            },
+        }
+
+        if ctx.obj.get("json"):
+            click.echo(json.dumps(result, indent=2))
+        else:
+            click.echo(f"File: {video}")
+            click.echo(f"Format: {info.format}")
+            click.echo(f"Codec: {info.codec}")
+            click.echo(f"Resolution: {info.width}x{info.height}")
+            click.echo(f"Frame rate: {info.fps:.2f} fps")
+            click.echo(f"Duration: {info.duration_seconds:.1f}s")
+            click.echo(f"Total frames: {info.total_frames:,}")
+            click.echo(f"I-frames (keyframes): {info.i_frame_count:,}")
+            if info.bitrate:
+                click.echo(f"Bitrate: {info.bitrate // 1000} kbps")
+            click.echo(f"File size: {len(video_data):,} bytes")
+            click.echo()
+            click.echo("Steganographic capacity (LSB, I-frames only):")
+            click.echo(
+                f"  {capacity_info.usable_capacity_bytes:,} bytes "
+                f"({capacity_info.usable_capacity_bytes // 1024} KB)"
+            )
+            click.echo()
+            click.echo("Note: Output will be MKV format with FFV1 lossless codec")
+
+    except Exception as e:
+        if ctx.obj.get("json"):
+            click.echo(json.dumps({"status": "error", "error": str(e)}, indent=2))
+        else:
+            click.echo(f"✗ Video info failed: {e}", err=True)
+        raise SystemExit(1)
+
+
 # =============================================================================
 # BATCH COMMANDS
 # =============================================================================
@@ -1146,9 +1589,9 @@ def info(ctx, full):

    # Check for DCT support
    try:
-        from .dct_steganography import HAS_JPEGIO, HAS_SCIPY
+        from .dct_steganography import HAS_JPEGLIB, HAS_SCIPY

-        has_dct = HAS_SCIPY and HAS_JPEGIO
+        has_dct = HAS_SCIPY and HAS_JPEGLIB
    except ImportError:
        has_dct = False

@@ -1998,6 +2441,66 @@ def tools_convert(image, fmt, quality, output):
    click.echo(f"Converted to: {output}")


+# =============================================================================
+# STEGANALYSIS COMMANDS
+# =============================================================================
+
+
+@cli.command()
+@click.argument("image", type=click.Path(exists=True))
+@click.option("--json", "as_json", is_flag=True, help="Output as JSON")
+@click.option(
+    "--mode",
+    type=click.Choice(["lsb", "auto"]),
+    default="lsb",
+    help="Analysis mode (default: lsb)",
+)
+def check(image, as_json, mode):
+    """Analyze an image for steganographic detectability.
+
+    Runs chi-square and RS (Regular-Singular) statistical tests to estimate
+    how detectable any hidden data might be. Outputs a risk level.
+
+    Examples:
+
+        stegasoo check carrier.png
+
+        stegasoo check stego.png --json
+
+        stegasoo check suspicious.bmp --mode lsb
+    """
+    from .steganalysis import check_image
+
+    with open(image, "rb") as f:
+        image_data = f.read()
+
+    result = check_image(image_data, mode=mode)
+    result["filename"] = Path(image).name
+
+    if as_json:
+        click.echo(json.dumps(result, indent=2))
+    else:
+        risk = result["risk"]
+        risk_colors = {"low": "green", "medium": "yellow", "high": "red"}
+        risk_display = click.style(risk.upper(), fg=risk_colors.get(risk, "white"), bold=True)
+
+        click.echo(f"\n  Steganalysis: {result['filename']}")
+        click.echo(f"  Image: {result['width']}x{result['height']}, {result['channels']} channels")
+        click.echo(f"  Detectability risk: {risk_display}")
+
+        click.echo("\n  Chi-square (p-values):")
+        for ch, p in result["chi_square"].items():
+            indicator = "!" if p < 0.05 else " "
+            click.echo(f"    {indicator} {ch}: {p:.6f}")
+
+        click.echo("\n  RS embedding estimate:")
+        for ch, est in result["rs"].items():
+            indicator = "!" if est > 0.1 else " "
+            click.echo(f"    {indicator} {ch}: {est:.4f} ({est * 100:.1f}%)")
+
+        click.echo()
+
+
 # =============================================================================
 # ADMIN COMMANDS (Web UI administration)
 # =============================================================================
--- a/src/stegasoo/constants.py
+++ b/src/stegasoo/constants.py
@@ -44,7 +44,9 @@ MAGIC_HEADER = b"\x89ST3"
 # Version 1-3: Date-dependent encryption (v3.0.x - v3.1.x)
 # Version 4: Date-independent encryption (v3.2.0)
 # Version 5: Channel key support (v4.0.0) - adds flags byte to header
-FORMAT_VERSION = 5
+# Version 6: HKDF per-message key derivation (v4.4.0) - adds message nonce to header
+FORMAT_VERSION = 6
+FORMAT_VERSION_LEGACY = 5  # For backward-compatible decryption

 # Payload type markers
 PAYLOAD_TEXT = 0x01
@@ -66,6 +68,11 @@ ARGON2_PARALLELISM = 4
 # PBKDF2 fallback parameters
 PBKDF2_ITERATIONS = 600000

+# HKDF per-message key derivation (v4.4.0 / FORMAT_VERSION 6)
+MESSAGE_NONCE_SIZE = 16  # 128-bit random nonce per message
+HKDF_INFO_ENCRYPT = b"stegasoo-v6-encrypt"  # HKDF info for encryption key
+HKDF_INFO_PIXEL = b"stegasoo-v6-pixel"  # HKDF info for pixel selection key (reserved)
+
 # ============================================================================
 # INPUT LIMITS
 # ============================================================================
@@ -244,6 +251,17 @@ def get_wordlist() -> list[str]:
    return _bip39_words


+# =============================================================================
+# STEGANALYSIS (v4.4.0)
+# =============================================================================
+
+# Chi-square p-value threshold: HIGH p-value = equalized PoV pairs = suspicious
+STEGANALYSIS_CHI_SUSPICIOUS_THRESHOLD = 0.95  # p > 0.95 → pairs suspiciously equalized
+
+# RS embedding rate thresholds (primary metric): higher = more likely embedded
+STEGANALYSIS_RS_HIGH_THRESHOLD = 0.3  # > 30% estimated embedding → high risk
+STEGANALYSIS_RS_MEDIUM_THRESHOLD = 0.1  # > 10% estimated embedding → medium risk
+
 # =============================================================================
 # DCT STEGANOGRAPHY (v3.0+)
 # =============================================================================
@@ -425,3 +443,27 @@ 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)
 AUDIO_ECHO_AMPLITUDE = 0.3  # Echo strength (relative to original)
 AUDIO_ECHO_WINDOW_SIZE = 8192  # Window size for echo embedding
+
+
+# =============================================================================
+# VIDEO STEGANOGRAPHY (v4.4.0)
+# =============================================================================
+
+# Video embedding modes
+EMBED_MODE_VIDEO_LSB = "video_lsb"
+EMBED_MODE_VIDEO_AUTO = "video_auto"
+VALID_VIDEO_EMBED_MODES = {EMBED_MODE_VIDEO_LSB}
+
+# Video magic bytes (for format detection in stego video)
+VIDEO_MAGIC_LSB = b"VIDL"
+
+# Video input limits
+MAX_VIDEO_FILE_SIZE = 4 * 1024 * 1024 * 1024  # 4 GB
+MAX_VIDEO_DURATION = 3600  # 1 hour in seconds
+MIN_VIDEO_RESOLUTION = (64, 64)
+MAX_VIDEO_RESOLUTION = (7680, 4320)  # 8K UHD
+ALLOWED_VIDEO_EXTENSIONS = {"mp4", "mkv", "webm", "avi", "mov"}
+
+# Video output settings
+VIDEO_OUTPUT_CODEC = "ffv1"  # FFV1 lossless codec
+VIDEO_OUTPUT_CONTAINER = "mkv"  # MKV container for FFV1
--- a/src/stegasoo/crypto.py
+++ b/src/stegasoo/crypto.py
@@ -29,7 +29,9 @@ import secrets
 import struct

 from cryptography.hazmat.backends import default_backend
+from cryptography.hazmat.primitives import hashes as _hashes
 from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
+from cryptography.hazmat.primitives.kdf.hkdf import HKDFExpand
 from PIL import Image

 from .constants import (
@@ -40,9 +42,12 @@ from .constants import (
    AUDIO_PAD_MIN,
    AUDIO_PAD_RANGE,
    FORMAT_VERSION,
+    FORMAT_VERSION_LEGACY,
+    HKDF_INFO_ENCRYPT,
    IV_SIZE,
    MAGIC_HEADER,
    MAX_FILENAME_LENGTH,
+    MESSAGE_NONCE_SIZE,
    PAYLOAD_FILE,
    PAYLOAD_TEXT,
    PBKDF2_ITERATIONS,
@@ -66,6 +71,7 @@ except ImportError:
    from cryptography.hazmat.primitives.kdf.pbkdf2 import PBKDF2HMAC


+
 # =============================================================================
 # CHANNEL KEY RESOLUTION
 # =============================================================================
@@ -317,6 +323,30 @@ def derive_pixel_key(
    return hashlib.sha256(material + b"pixel_selection").digest()


+def derive_message_key(root_key: bytes, nonce: bytes) -> bytes:
+    """
+    Derive a per-message encryption key via HKDF-Expand.
+
+    Each message gets a unique encryption key even with identical credentials,
+    because the nonce is random per message. This provides key diversification:
+    compromising the ciphertext of one message doesn't help with another.
+
+    Args:
+        root_key: 32-byte root key from Argon2id/PBKDF2
+        nonce: 16-byte random nonce (unique per message)
+
+    Returns:
+        32-byte per-message encryption key
+    """
+    hkdf = HKDFExpand(
+        algorithm=_hashes.SHA256(),
+        length=32,
+        info=HKDF_INFO_ENCRYPT + nonce,
+        backend=default_backend(),
+    )
+    return hkdf.derive(root_key)
+
+
 def _pack_payload(
    content: str | bytes | FilePayload,
 ) -> tuple[bytes, int]:
@@ -476,7 +506,12 @@ def encrypt_message(
    """
    try:
        salt = secrets.token_bytes(SALT_SIZE)
-        key = derive_hybrid_key(photo_data, passphrase, salt, pin, rsa_key_data, channel_key)
+        root_key = derive_hybrid_key(photo_data, passphrase, salt, pin, rsa_key_data, channel_key)
+
+        # v6: Per-message key via HKDF — each message gets a unique encryption key
+        message_nonce = secrets.token_bytes(MESSAGE_NONCE_SIZE)
+        key = derive_message_key(root_key, message_nonce)
+
        iv = secrets.token_bytes(IV_SIZE)

        # Determine flags
@@ -515,28 +550,36 @@ def encrypt_message(
            "Padded message: %d bytes (payload + %d padding)", len(padded_message), padding_needed
        )

-        # Build header for AAD
+        # Build header for AAD (v6: includes nonce in authenticated data)
        header = MAGIC_HEADER + bytes([FORMAT_VERSION, flags])

        # Encrypt with AES-256-GCM
        cipher = Cipher(algorithms.AES(key), modes.GCM(iv), backend=default_backend())
        encryptor = cipher.encryptor()
-        encryptor.authenticate_additional_data(header)
+        encryptor.authenticate_additional_data(header + message_nonce)
        ciphertext = encryptor.update(padded_message) + encryptor.finalize()

-        total_size = len(header) + len(salt) + len(iv) + len(encryptor.tag) + len(ciphertext)
+        total_size = (
+            len(header)
+            + MESSAGE_NONCE_SIZE
+            + 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)",
+            "Encrypted output: %d bytes (header=%d, nonce=%d, salt=%d, iv=%d, tag=%d, ct=%d)",
            total_size,
            len(header),
+            MESSAGE_NONCE_SIZE,
            len(salt),
            len(iv),
            len(encryptor.tag),
            len(ciphertext),
        )

-        # v4.0.0: Header with flags byte
-        return header + salt + iv + encryptor.tag + ciphertext
+        # v6: [magic|version|flags|nonce|salt|iv|tag|ciphertext]
+        return header + message_nonce + salt + iv + encryptor.tag + ciphertext

    except Exception as e:
        logger.error("Encryption failed: %s", e)
@@ -547,43 +590,78 @@ def parse_header(encrypted_data: bytes) -> dict | None:
    """
    Parse the header from encrypted data.

-    v4.0.0: Includes flags byte for channel key indicator.
+    Supports both v5 (legacy) and v6 (HKDF) header formats.
+
+    v5: [magic:4][ver:1][flags:1][salt:32][iv:12][tag:16][ciphertext]  (66+ bytes)
+    v6: [magic:4][ver:1][flags:1][nonce:16][salt:32][iv:12][tag:16][ciphertext]  (82+ bytes)

    Args:
        encrypted_data: Raw encrypted bytes

    Returns:
-        Dict with salt, iv, tag, ciphertext, flags or None if invalid
+        Dict with version, salt, iv, tag, ciphertext, flags, and optionally
+        message_nonce (v6). Returns None if invalid.
    """
-    # Min size: Magic(4) + Version(1) + Flags(1) + Salt(32) + IV(12) + Tag(16) = 66 bytes
+    # Min v5 size: 4+1+1+32+12+16 = 66 bytes
    if len(encrypted_data) < 66 or encrypted_data[:4] != MAGIC_HEADER:
        return None

    try:
        version = encrypted_data[4]
-        if version != FORMAT_VERSION:
+
+        if version == FORMAT_VERSION:
+            # v6: has message nonce
+            if len(encrypted_data) < 82:
+                return None
+            flags = encrypted_data[5]
+            offset = 6
+            message_nonce = encrypted_data[offset : offset + MESSAGE_NONCE_SIZE]
+            offset += MESSAGE_NONCE_SIZE
+            salt = encrypted_data[offset : offset + SALT_SIZE]
+            offset += SALT_SIZE
+            iv = encrypted_data[offset : offset + IV_SIZE]
+            offset += IV_SIZE
+            tag = encrypted_data[offset : offset + TAG_SIZE]
+            offset += TAG_SIZE
+            ciphertext = encrypted_data[offset:]
+
+            return {
+                "version": version,
+                "flags": flags,
+                "has_channel_key": bool(flags & FLAG_CHANNEL_KEY),
+                "message_nonce": message_nonce,
+                "salt": salt,
+                "iv": iv,
+                "tag": tag,
+                "ciphertext": ciphertext,
+            }
+
+        elif version == FORMAT_VERSION_LEGACY:
+            # v5: no nonce
+            flags = encrypted_data[5]
+            offset = 6
+            salt = encrypted_data[offset : offset + SALT_SIZE]
+            offset += SALT_SIZE
+            iv = encrypted_data[offset : offset + IV_SIZE]
+            offset += IV_SIZE
+            tag = encrypted_data[offset : offset + TAG_SIZE]
+            offset += TAG_SIZE
+            ciphertext = encrypted_data[offset:]
+
+            return {
+                "version": version,
+                "flags": flags,
+                "has_channel_key": bool(flags & FLAG_CHANNEL_KEY),
+                "message_nonce": None,
+                "salt": salt,
+                "iv": iv,
+                "tag": tag,
+                "ciphertext": ciphertext,
+            }
+
+        else:
            return None

-        flags = encrypted_data[5]
-
-        offset = 6
-        salt = encrypted_data[offset : offset + SALT_SIZE]
-        offset += SALT_SIZE
-        iv = encrypted_data[offset : offset + IV_SIZE]
-        offset += IV_SIZE
-        tag = encrypted_data[offset : offset + TAG_SIZE]
-        offset += TAG_SIZE
-        ciphertext = encrypted_data[offset:]
-
-        return {
-            "version": version,
-            "flags": flags,
-            "has_channel_key": bool(flags & FLAG_CHANNEL_KEY),
-            "salt": salt,
-            "iv": iv,
-            "tag": tag,
-            "ciphertext": ciphertext,
-        }
    except Exception:
        return None

@@ -635,12 +713,21 @@ def decrypt_message(
    message_has_key = header["has_channel_key"]

    try:
-        key = derive_hybrid_key(
+        root_key = derive_hybrid_key(
            photo_data, passphrase, header["salt"], pin, rsa_key_data, channel_key
        )

-        # Reconstruct header for AAD verification
-        aad_header = MAGIC_HEADER + bytes([FORMAT_VERSION, header["flags"]])
+        version = header["version"]
+        message_nonce = header["message_nonce"]
+
+        if version == FORMAT_VERSION and message_nonce is not None:
+            # v6: Derive per-message key via HKDF
+            key = derive_message_key(root_key, message_nonce)
+            aad_header = MAGIC_HEADER + bytes([FORMAT_VERSION, header["flags"]]) + message_nonce
+        else:
+            # v5 (legacy): Root key used directly
+            key = root_key
+            aad_header = MAGIC_HEADER + bytes([FORMAT_VERSION_LEGACY, header["flags"]])

        cipher = Cipher(
            algorithms.AES(key), modes.GCM(header["iv"], header["tag"]), backend=default_backend()
@@ -660,7 +747,7 @@ def decrypt_message(
        payload_data = padded_plaintext[:original_length]
        result = _unpack_payload(payload_data)

-        logger.debug("Decryption successful: %s", result.payload_type)
+        logger.debug("Decryption successful: %s (v%d)", result.payload_type, version)
        return result

    except Exception as e:
--- a/src/stegasoo/dct_steganography.py
+++ b/src/stegasoo/dct_steganography.py
@@ -12,7 +12,7 @@ Why is this cool?

 Two approaches depending on what you want:
 1. PNG output: We do our own DCT math via scipy (works on any image)
-2. JPEG output: We use jpeglib to directly tweak the coefficients (chef's kiss)
+2. JPEG output: We use jpeglib to directly modify the coefficients (chef's kiss)

 v4.1.0 - The "please stop corrupting my data" release:
 - Reed-Solomon error correction (can fix up to 16 byte errors per chunk)
@@ -56,13 +56,12 @@ except ImportError:
        idctn = None

 # Check for jpeglib availability (for proper JPEG mode)
-# jpeglib replaces jpegio for Python 3.13+ compatibility
 try:
    import jpeglib

-    HAS_JPEGIO = True  # Keep variable name for compatibility
+    HAS_JPEGLIB = True
 except ImportError:
-    HAS_JPEGIO = False
+    HAS_JPEGLIB = False
    jpeglib = None

 # Import custom exceptions
@@ -170,20 +169,20 @@ QUANT_STEP = 25

 # Magic bytes so we can identify our own images
 DCT_MAGIC = b"DCTS"  # scipy DCT mode marker
-JPEGIO_MAGIC = b"JPGS"  # jpegio native JPEG mode marker
+JPEGLIB_MAGIC = b"JPGS"  # jpeglib native JPEG mode marker
 HEADER_SIZE = 10  # Magic (4) + version (1) + flags (1) + length (4)

 OUTPUT_FORMAT_PNG = "png"
 OUTPUT_FORMAT_JPEG = "jpeg"
 JPEG_OUTPUT_QUALITY = 95  # High quality but not 100 (100 causes issues, see below)

-# For jpegio mode: we only embed in coefficients with magnitude >= 2
+# For jpeglib mode: we only embed in coefficients with magnitude >= 2
 # Coefficients of 0 or 1 are usually quantized noise - unreliable
-JPEGIO_MIN_COEF_MAGNITUDE = 2
+JPEGLIB_MIN_COEF_MAGNITUDE = 2

 # We embed in the Y (luminance) channel only - it has the most capacity
 # Cb/Cr are often subsampled 4:2:0 anyway
-JPEGIO_EMBED_CHANNEL = 0
+JPEGLIB_EMBED_CHANNEL = 0

 # Header flags
 FLAG_COLOR_MODE = 0x01  # Set if we preserved color (YCbCr mode)
@@ -204,10 +203,10 @@ RS_LENGTH_PREFIX_SIZE = RS_LENGTH_HEADER_SIZE * RS_LENGTH_COPIES  # 24 bytes tot
 MAX_CHUNK_HEIGHT = 512  # Process in strips to keep memory sane

 # Fun bug: JPEGs saved with quality=100 have quantization tables full of 1s
-# This makes the DCT coefficients HUGE and jpegio crashes spectacularly
+# This makes the DCT coefficients HUGE and jpeglib crashes spectacularly
 # Solution: detect and re-save at quality 95 first
-JPEGIO_NORMALIZE_QUALITY = 95
-JPEGIO_MAX_QUANT_VALUE_THRESHOLD = 1  # All 1s in quant table = bad news
+JPEGLIB_NORMALIZE_QUALITY = 95
+JPEGLIB_MAX_QUANT_VALUE_THRESHOLD = 1  # All 1s in quant table = bad news


 # ============================================================================
@@ -261,8 +260,8 @@ def has_dct_support() -> bool:
    return HAS_SCIPY


-def has_jpegio_support() -> bool:
-    return HAS_JPEGIO
+def has_jpeglib_support() -> bool:
+    return HAS_JPEGLIB


 # ============================================================================
@@ -654,11 +653,11 @@ def _parse_header(header_bits: list) -> tuple[int, int, int]:


 # ============================================================================
-# JPEGIO HELPERS
+# JPEGLIB HELPERS
 # ============================================================================


-def _jpegio_bytes_to_file(data: bytes, suffix: str = ".jpg") -> str:
+def _jpeglib_bytes_to_file(data: bytes, suffix: str = ".jpg") -> str:
    import os
    import tempfile

@@ -670,19 +669,19 @@ def _jpegio_bytes_to_file(data: bytes, suffix: str = ".jpg") -> str:
    return path


-def _jpegio_get_usable_positions(coef_array: np.ndarray) -> list:
+def _jpeglib_get_usable_positions(coef_array: np.ndarray) -> list:
    positions = []
    h, w = coef_array.shape
    for row in range(h):
        for col in range(w):
            if (row % BLOCK_SIZE == 0) and (col % BLOCK_SIZE == 0):
                continue
-            if abs(coef_array[row, col]) >= JPEGIO_MIN_COEF_MAGNITUDE:
+            if abs(coef_array[row, col]) >= JPEGLIB_MIN_COEF_MAGNITUDE:
                positions.append((row, col))
    return positions


-def _jpegio_generate_order(num_positions: int, seed: bytes) -> list:
+def _jpeglib_generate_order(num_positions: int, seed: bytes) -> list:
    hash_bytes = hashlib.sha256(seed + b"jpeg_coef_order").digest()
    rng = np.random.RandomState(int.from_bytes(hash_bytes[:4], "big"))
    order = list(range(num_positions))
@@ -690,15 +689,15 @@ def _jpegio_generate_order(num_positions: int, seed: bytes) -> list:
    return order


-def _jpegio_create_header(data_length: int, flags: int = 0) -> bytes:
-    return struct.pack(">4sBBI", JPEGIO_MAGIC, 1, flags, data_length)
+def _jpeglib_create_header(data_length: int, flags: int = 0) -> bytes:
+    return struct.pack(">4sBBI", JPEGLIB_MAGIC, 1, flags, data_length)


-def _jpegio_parse_header(header_bytes: bytes) -> tuple[int, int, int]:
+def _jpeglib_parse_header(header_bytes: bytes) -> tuple[int, int, int]:
    if len(header_bytes) < HEADER_SIZE:
        raise ValueError("Insufficient header data")
    magic, version, flags, length = struct.unpack(">4sBBI", header_bytes[:HEADER_SIZE])
-    if magic != JPEGIO_MAGIC:
+    if magic != JPEGLIB_MAGIC:
        raise InvalidMagicBytesError("Not a Stegasoo JPEG or wrong mode")
    return version, flags, length

@@ -782,7 +781,7 @@ def estimate_capacity_comparison(image_data: bytes) -> dict:
            "available": HAS_SCIPY,
        },
        "jpeg_native": {
-            "available": HAS_JPEGIO,
+            "available": HAS_JPEGLIB,
            "note": "Uses jpeglib for proper JPEG coefficient embedding",
        },
    }
@@ -795,24 +794,54 @@ def embed_in_dct(
    output_format: str = OUTPUT_FORMAT_PNG,
    color_mode: str = "color",
    progress_file: str | None = None,
+    quant_step: int | None = None,
+    jpeg_quality: int | None = None,
+    max_dimension: int | None = None,
 ) -> tuple[bytes, DCTEmbedStats]:
-    """Embed data using DCT coefficient modification."""
+    """Embed data using DCT coefficient modification.
+
+    Args:
+        data: Payload bytes to embed.
+        carrier_image: Carrier image bytes.
+        seed: Key for block selection.
+        output_format: 'png' or 'jpeg'.
+        color_mode: 'color' or 'grayscale'.
+        progress_file: Optional progress file.
+        quant_step: Override QIM quantization step (default: QUANT_STEP).
+            Higher = more robust to recompression, more visible.
+        jpeg_quality: Override JPEG output quality (default: JPEG_OUTPUT_QUALITY).
+        max_dimension: Resize carrier if larger than this.
+    """
    if output_format not in (OUTPUT_FORMAT_PNG, OUTPUT_FORMAT_JPEG):
        raise ValueError(f"Invalid output format: {output_format}")

    if color_mode not in ("color", "grayscale"):
        color_mode = "color"

+    qs = quant_step if quant_step is not None else QUANT_STEP
+
    # Apply EXIF orientation to carrier image before embedding
-    # This ensures portrait photos are embedded in their correct visual orientation
    carrier_image = _apply_exif_orientation(carrier_image)

-    if output_format == OUTPUT_FORMAT_JPEG and HAS_JPEGIO:
-        return _embed_jpegio(data, carrier_image, seed, color_mode, progress_file)
+    # Resize if max_dimension specified (for platform presets)
+    if max_dimension is not None:
+        img_check = Image.open(io.BytesIO(carrier_image))
+        w, h = img_check.size
+        if max(w, h) > max_dimension:
+            scale = max_dimension / max(w, h)
+            new_size = (int(w * scale), int(h * scale))
+            img_check = img_check.resize(new_size, Image.LANCZOS)
+            buf = io.BytesIO()
+            img_check.save(buf, format="PNG")
+            carrier_image = buf.getvalue()
+        img_check.close()
+
+    if output_format == OUTPUT_FORMAT_JPEG and HAS_JPEGLIB:
+        return _embed_jpeglib(data, carrier_image, seed, color_mode, progress_file)

    _check_scipy()
    return _embed_scipy_dct_safe(
-        data, carrier_image, seed, output_format, color_mode, progress_file
+        data, carrier_image, seed, output_format, color_mode, progress_file, quant_step=qs
    )


@@ -823,6 +852,7 @@ def _embed_scipy_dct_safe(
    output_format: str,
    color_mode: str = "color",
    progress_file: str | None = None,
+    quant_step: int = QUANT_STEP,
 ) -> tuple[bytes, DCTEmbedStats]:
    """
    Embed using scipy DCT with safe memory handling.
@@ -885,7 +915,9 @@ def _embed_scipy_dct_safe(
        gc.collect()

        # Embed in Y channel
-        Y_embedded = _embed_in_channel_safe(Y_padded, bits, block_order, blocks_x, progress_file)
+        Y_embedded = _embed_in_channel_safe(
+            Y_padded, bits, block_order, blocks_x, progress_file, quant_step=quant_step
+        )
        del Y_padded
        gc.collect()

@@ -909,7 +941,9 @@ def _embed_scipy_dct_safe(
        del image
        gc.collect()

-        embedded = _embed_in_channel_safe(padded, bits, block_order, blocks_x, progress_file)
+        embedded = _embed_in_channel_safe(
+            padded, bits, block_order, blocks_x, progress_file, quant_step=quant_step
+        )
        del padded
        gc.collect()

@@ -943,6 +977,7 @@ def _embed_in_channel_safe(
    block_order: list,
    blocks_x: int,
    progress_file: str | None = None,
+    quant_step: int = QUANT_STEP,
 ) -> np.ndarray:
    """
    Embed bits in channel using vectorized DCT operations.
@@ -1005,17 +1040,17 @@ def _embed_in_channel_safe(
                coeffs = dct_blocks[i, embed_rows, embed_cols]
                bit_array = np.array(block_bits)
                # QIM embedding: round to grid, adjust for bit
-                quantized = np.round(coeffs / QUANT_STEP).astype(int)
+                quantized = np.round(coeffs / quant_step).astype(int)
                # If quantized % 2 != bit, nudge coefficient
                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
+                    * quant_step
                ).astype(np.float64)
                # For bits that already match, just quantize
                dct_blocks[i, embed_rows[~needs_adjust], embed_cols[~needs_adjust]] = (
-                    quantized[~needs_adjust] * QUANT_STEP
+                    quantized[~needs_adjust] * quant_step
                ).astype(np.float64)
            else:
                # Partial block - process remaining bits individually
@@ -1052,12 +1087,12 @@ def _embed_in_channel_safe(
    return result


-def _normalize_jpeg_for_jpegio(image_data: bytes) -> bytes:
+def _normalize_jpeg_for_jpeglib(image_data: bytes) -> bytes:
    """
-    Normalize a JPEG image to ensure jpegio can process it safely.
+    Normalize a JPEG image to ensure jpeglib can process it safely.

    JPEGs saved with quality=100 have quantization tables with all values = 1,
-    which causes jpegio to crash due to huge coefficient magnitudes.
+    which causes jpeglib to crash due to huge coefficient magnitudes.
    This function detects such images and re-saves them at a safe quality level.

    Args:
@@ -1078,7 +1113,7 @@ def _normalize_jpeg_for_jpegio(image_data: bytes) -> bytes:
    if hasattr(img, "quantization") and img.quantization:
        for table_id, table in img.quantization.items():
            # If all values in any table are <= threshold, normalize
-            if max(table) <= JPEGIO_MAX_QUANT_VALUE_THRESHOLD:
+            if max(table) <= JPEGLIB_MAX_QUANT_VALUE_THRESHOLD:
                needs_normalization = True
                break

@@ -1091,25 +1126,25 @@ def _normalize_jpeg_for_jpegio(image_data: bytes) -> bytes:
        img = img.convert("RGB")

    buffer = io.BytesIO()
-    img.save(buffer, format="JPEG", quality=JPEGIO_NORMALIZE_QUALITY, subsampling=0)
+    img.save(buffer, format="JPEG", quality=JPEGLIB_NORMALIZE_QUALITY, subsampling=0)
    img.close()

    return buffer.getvalue()


-def _embed_jpegio(
+def _embed_jpeglib(
    data: bytes,
    carrier_image: bytes,
    seed: bytes,
    color_mode: str = "color",
    progress_file: str | None = None,
 ) -> tuple[bytes, DCTEmbedStats]:
-    """Embed using jpegio for proper JPEG coefficient modification."""
+    """Embed using jpeglib for proper JPEG coefficient modification."""
    import os
    import tempfile

    # Normalize JPEG to avoid crashes with quality=100 images
-    carrier_image = _normalize_jpeg_for_jpegio(carrier_image)
+    carrier_image = _normalize_jpeg_for_jpeglib(carrier_image)

    img = Image.open(io.BytesIO(carrier_image))
    width, height = img.size
@@ -1122,20 +1157,20 @@ def _embed_jpegio(
        carrier_image = buffer.getvalue()
    img.close()

-    input_path = _jpegio_bytes_to_file(carrier_image, suffix=".jpg")
+    input_path = _jpeglib_bytes_to_file(carrier_image, suffix=".jpg")
    output_path = tempfile.mktemp(suffix=".jpg")

    flags = FLAG_COLOR_MODE if color_mode == "color" else 0

    try:
        jpeg = jpeglib.to_jpegio(jpeglib.read_dct(input_path))
-        coef_array = jpeg.coef_arrays[JPEGIO_EMBED_CHANNEL]
+        coef_array = jpeg.coef_arrays[JPEGLIB_EMBED_CHANNEL]

-        all_positions = _jpegio_get_usable_positions(coef_array)
-        order = _jpegio_generate_order(len(all_positions), seed)
+        all_positions = _jpeglib_get_usable_positions(coef_array)
+        order = _jpeglib_generate_order(len(all_positions), seed)

        # Build raw payload (header + data)
-        header = _jpegio_create_header(len(data), flags)
+        header = _jpeglib_create_header(len(data), flags)
        raw_payload = header + data

        # Apply Reed-Solomon error correction to entire payload if available
@@ -1402,6 +1437,7 @@ def extract_from_dct(
    stego_image: bytes,
    seed: bytes,
    progress_file: str | None = None,
+    quant_step: int | None = None,
 ) -> bytes:
    """
    Extract data from DCT stego image.
@@ -1412,6 +1448,7 @@ def extract_from_dct(

    Uses quick header validation to skip obviously invalid rotations.
    """
+    qs = quant_step if quant_step is not None else QUANT_STEP
    rotations_to_try = [0, 90, 180, 270]
    last_error = None
    valid_rotations = []
@@ -1429,7 +1466,7 @@ def extract_from_dct(
    # If no rotations pass quick check, try all anyway (fallback)
    if not valid_rotations:
        # Must try all rotations - quick validation might have failed due to
-        # scipy vs jpegio differences or other edge cases
+        # scipy vs jpeglib differences or other edge cases
        for rotation in rotations_to_try:
            if rotation == 0:
                valid_rotations.append((0, stego_image))
@@ -1443,9 +1480,9 @@ def extract_from_dct(
            fmt = img.format
            img.close()

-            if fmt == "JPEG" and HAS_JPEGIO:
+            if fmt == "JPEG" and HAS_JPEGLIB:
                try:
-                    result = _extract_jpegio(image_to_decode, seed, progress_file)
+                    result = _extract_jpeglib(image_to_decode, seed, progress_file)
                    if rotation != 0:
                        try:
                            from . import debug
@@ -1459,7 +1496,7 @@ def extract_from_dct(
                    continue

            _check_scipy()
-            result = _extract_scipy_dct_safe(image_to_decode, seed, progress_file)
+            result = _extract_scipy_dct_safe(image_to_decode, seed, progress_file, quant_step=qs)
            if rotation != 0:
                try:
                    from . import debug
@@ -1481,6 +1518,7 @@ def _extract_scipy_dct_safe(
    stego_image: bytes,
    seed: bytes,
    progress_file: str | None = None,
+    quant_step: int = QUANT_STEP,
 ) -> bytes:
    """Extract using safe DCT operations with vectorized processing."""
    # Progress starts at 25% (decode.py writes 20% for Argon2, 25% before extraction)
@@ -1542,7 +1580,7 @@ def _extract_scipy_dct_safe(
        coeffs = dct_blocks[:, embed_rows, embed_cols]

        # Quantize and extract bits (vectorized)
-        quantized = np.round(coeffs / QUANT_STEP).astype(int)
+        quantized = np.round(coeffs / quant_step).astype(int)
        bits = (quantized % 2).flatten().tolist()
        all_bits.extend(bits)

@@ -1660,28 +1698,28 @@ def _extract_scipy_dct_safe(
    return data


-def _extract_jpegio(
+def _extract_jpeglib(
    stego_image: bytes,
    seed: bytes,
    progress_file: str | None = None,
 ) -> bytes:
-    """Extract using jpegio for JPEG images."""
+    """Extract using jpeglib for JPEG images."""
    import os

    # Progress starts at 25% (decode.py writes 20% for Argon2, 25% before extraction)

    # Normalize JPEG to avoid crashes with quality=100 images
    # (shouldn't happen with stego images, but be defensive)
-    stego_image = _normalize_jpeg_for_jpegio(stego_image)
+    stego_image = _normalize_jpeg_for_jpeglib(stego_image)

-    temp_path = _jpegio_bytes_to_file(stego_image, suffix=".jpg")
+    temp_path = _jpeglib_bytes_to_file(stego_image, suffix=".jpg")

    try:
        jpeg = jpeglib.to_jpegio(jpeglib.read_dct(temp_path))
-        coef_array = jpeg.coef_arrays[JPEGIO_EMBED_CHANNEL]
+        coef_array = jpeg.coef_arrays[JPEGLIB_EMBED_CHANNEL]

-        all_positions = _jpegio_get_usable_positions(coef_array)
-        order = _jpegio_generate_order(len(all_positions), seed)
+        all_positions = _jpeglib_get_usable_positions(coef_array)
+        order = _jpeglib_generate_order(len(all_positions), seed)

        _write_progress(progress_file, 30, 100, "extracting")

@@ -1751,7 +1789,7 @@ def _extract_jpegio(
                        _write_progress(progress_file, 75, 100, "decoding")
                        raw_payload = _rs_decode(rs_encoded)
                        _write_progress(progress_file, 95, 100, "decoding")
-                        _, flags, data_length = _jpegio_parse_header(raw_payload[:HEADER_SIZE])
+                        _, flags, data_length = _jpeglib_parse_header(raw_payload[:HEADER_SIZE])
                        data = raw_payload[HEADER_SIZE : HEADER_SIZE + data_length]
                        _write_progress(progress_file, 100, 100, "complete")
                        return data
@@ -1772,7 +1810,7 @@ def _extract_jpegio(
            ]
        )

-        _, flags, data_length = _jpegio_parse_header(header_bytes)
+        _, flags, data_length = _jpeglib_parse_header(header_bytes)
        total_bits_needed = (HEADER_SIZE + data_length) * 8

        all_bits = []
--- a/src/stegasoo/decode.py
+++ b/src/stegasoo/decode.py
@@ -54,6 +54,7 @@ def decode(
    embed_mode: str = EMBED_MODE_AUTO,
    channel_key: str | bool | None = None,
    progress_file: str | None = None,
+    platform: str | None = None,
 ) -> DecodeResult:
    """
    Decode a message or file from a stego image.
@@ -124,12 +125,21 @@ def decode(
    # Progress: key derivation done, starting extraction
    _write_progress(progress_file, 25, 100, "extracting")

+    # Resolve platform preset for DCT extraction
+    extract_kwargs = {}
+    if platform:
+        from .platform_presets import get_preset
+
+        preset = get_preset(platform)
+        extract_kwargs["quant_step"] = preset.quant_step
+
    # Extract encrypted data
    encrypted = extract_from_image(
        stego_image,
        pixel_key,
        embed_mode=embed_mode,
        progress_file=progress_file,
+        **extract_kwargs,
    )

    if not encrypted:
@@ -383,3 +393,100 @@ def decode_audio(

    debug.print(f"Decryption successful: {result.payload_type}")
    return result
+
+
+def decode_video(
+    stego_video: bytes,
+    reference_photo: bytes,
+    passphrase: str,
+    pin: str = "",
+    rsa_key_data: bytes | None = None,
+    rsa_password: str | None = None,
+    embed_mode: str = "video_auto",
+    channel_key: str | bool | None = None,
+    progress_file: str | None = None,
+) -> DecodeResult:
+    """
+    Decode a message or file from stego video.
+
+    Extracts data from I-frames (keyframes) using LSB steganography.
+
+    Args:
+        stego_video: Stego video bytes
+        reference_photo: Shared reference photo bytes
+        passphrase: Shared passphrase
+        pin: Optional static PIN
+        rsa_key_data: Optional RSA key bytes
+        rsa_password: Optional RSA key password
+        embed_mode: 'video_auto' or 'video_lsb'
+        channel_key: Channel key for deployment/group isolation
+        progress_file: Optional path to write progress JSON
+
+    Returns:
+        DecodeResult with message or file data
+    """
+    from .constants import (
+        EMBED_MODE_VIDEO_AUTO,
+        EMBED_MODE_VIDEO_LSB,
+        VIDEO_ENABLED,
+    )
+
+    if not VIDEO_ENABLED:
+        raise ExtractionError(
+            "Video support is disabled. Install video extras and ffmpeg, "
+            "or set STEGASOO_VIDEO=1 to force enable."
+        )
+
+    from .video_utils import detect_video_format
+
+    debug.print(
+        f"decode_video: mode={embed_mode}, " f"passphrase length={len(passphrase.split())} words"
+    )
+
+    # Validate inputs
+    require_valid_image(reference_photo, "Reference photo")
+    require_security_factors(pin, rsa_key_data)
+
+    if pin:
+        require_valid_pin(pin)
+    if rsa_key_data:
+        require_valid_rsa_key(rsa_key_data, rsa_password)
+
+    # Detect format
+    video_format = detect_video_format(stego_video)
+    debug.print(f"Detected video format: {video_format}")
+
+    if video_format == "unknown":
+        raise ExtractionError("Could not detect video format.")
+
+    _write_progress(progress_file, 20, 100, "initializing")
+
+    # Derive pixel/frame selection key
+    from .crypto import derive_pixel_key
+
+    pixel_key = derive_pixel_key(reference_photo, passphrase, pin, rsa_key_data, channel_key)
+
+    _write_progress(progress_file, 25, 100, "extracting")
+
+    encrypted = None
+
+    if embed_mode == EMBED_MODE_VIDEO_AUTO or embed_mode == EMBED_MODE_VIDEO_LSB:
+        from .video_steganography import extract_from_video_lsb
+
+        encrypted = extract_from_video_lsb(stego_video, pixel_key, progress_file=progress_file)
+        if encrypted:
+            debug.print("Video LSB extraction succeeded")
+    else:
+        raise ValueError(f"Invalid video embed mode: {embed_mode}")
+
+    if not encrypted:
+        debug.print("No data extracted from video")
+        raise ExtractionError("Could not extract data from video. Check your credentials.")
+
+    debug.print(f"Extracted {len(encrypted)} bytes from video")
+
+    # Decrypt
+    result = decrypt_message(encrypted, reference_photo, passphrase, pin, rsa_key_data, channel_key)
+
+    debug.print(f"Decryption successful: {result.payload_type}")
+    return result
--- a/src/stegasoo/encode.py
+++ b/src/stegasoo/encode.py
@@ -8,6 +8,9 @@ Changes in v4.0.0:

 Changes in v4.3.0:
 - Added encode_audio() for audio steganography
+
+Changes in v4.4.0:
+- Added encode_video() for video steganography
 """

 from __future__ import annotations
@@ -18,7 +21,7 @@ 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 .exceptions import AudioError, VideoError
 from .models import EncodeResult, FilePayload
 from .steganography import embed_in_image
 from .utils import generate_filename
@@ -31,7 +34,7 @@ from .validation import (
 )

 if TYPE_CHECKING:
-    from .models import AudioEmbedStats
+    from .models import AudioEmbedStats, VideoEmbedStats


 def encode(
@@ -48,6 +51,7 @@ def encode(
    dct_color_mode: str = "color",
    channel_key: str | bool | None = None,
    progress_file: str | None = None,
+    platform: str | None = None,
 ) -> EncodeResult:
    """
    Encode a message or file into an image.
@@ -120,6 +124,18 @@ def encode(
    # Derive pixel/coefficient selection key (with channel key)
    pixel_key = derive_pixel_key(reference_photo, passphrase, pin, rsa_key_data, channel_key)

+    # Resolve platform preset for DCT encoding
+    platform_kwargs = {}
+    if platform:
+        from .platform_presets import get_preset
+
+        preset = get_preset(platform)
+        platform_kwargs = {
+            "quant_step": preset.quant_step,
+            "max_dimension": preset.max_dimension,
+            "jpeg_quality": preset.jpeg_quality,
+        }
+
    # Embed in image
    stego_data, stats, extension = embed_in_image(
        encrypted,
@@ -130,6 +146,7 @@ def encode(
        dct_output_format=dct_output_format,
        dct_color_mode=dct_color_mode,
        progress_file=progress_file,
+        **platform_kwargs,
    )

    # Generate filename
@@ -377,3 +394,97 @@ def encode_audio(
        raise ValueError(f"Invalid audio embed mode: {embed_mode}")

    return stego_audio, stats
+
+
+def encode_video(
+    message: str | bytes | FilePayload,
+    reference_photo: bytes,
+    carrier_video: bytes,
+    passphrase: str,
+    pin: str = "",
+    rsa_key_data: bytes | None = None,
+    rsa_password: str | None = None,
+    embed_mode: str = "video_lsb",
+    channel_key: str | bool | None = None,
+    progress_file: str | None = None,
+) -> tuple[bytes, VideoEmbedStats]:
+    """
+    Encode a message or file into a video carrier.
+
+    Embeds data across I-frames (keyframes) using LSB steganography.
+    Output is an MKV container with FFV1 lossless codec to preserve
+    the embedded data perfectly.
+
+    Args:
+        message: Text message, raw bytes, or FilePayload to hide
+        reference_photo: Shared reference photo bytes
+        carrier_video: Carrier video bytes (MP4, MKV, WebM, AVI, MOV)
+        passphrase: Shared passphrase
+        pin: Optional static PIN
+        rsa_key_data: Optional RSA private key PEM bytes
+        rsa_password: Optional password for encrypted RSA key
+        embed_mode: 'video_lsb' (currently the only option)
+        channel_key: Channel key for deployment/group isolation
+        progress_file: Optional path to write progress JSON
+
+    Returns:
+        Tuple of (stego video bytes, VideoEmbedStats)
+
+    Note:
+        The output video will be in MKV format with FFV1 lossless codec,
+        regardless of the input format. This is necessary to preserve
+        the embedded data without lossy compression artifacts.
+    """
+    from .constants import EMBED_MODE_VIDEO_LSB, VIDEO_ENABLED
+
+    if not VIDEO_ENABLED:
+        raise VideoError(
+            "Video support is disabled. Install video extras and ffmpeg, "
+            "or set STEGASOO_VIDEO=1 to force enable."
+        )
+
+    from .video_utils import detect_video_format
+
+    debug.print(
+        f"encode_video: mode={embed_mode}, "
+        f"passphrase length={len(passphrase.split())} words, "
+        f"pin={'set' if pin else 'none'}"
+    )
+
+    # Validate inputs
+    require_valid_payload(message)
+    require_valid_image(reference_photo, "Reference photo")
+    require_security_factors(pin, rsa_key_data)
+
+    if pin:
+        require_valid_pin(pin)
+    if rsa_key_data:
+        require_valid_rsa_key(rsa_key_data, rsa_password)
+
+    # Detect video format
+    video_format = detect_video_format(carrier_video)
+    debug.print(f"Detected video format: {video_format}")
+
+    if video_format == "unknown":
+        raise VideoError("Could not detect video format. Supported: MP4, MKV, WebM, AVI, MOV.")
+
+    # Encrypt message
+    encrypted = encrypt_message(
+        message, reference_photo, passphrase, pin, rsa_key_data, channel_key
+    )
+    debug.print(f"Encrypted payload: {len(encrypted)} bytes")
+
+    # Derive pixel/frame selection key
+    pixel_key = derive_pixel_key(reference_photo, passphrase, pin, rsa_key_data, channel_key)
+
+    # Embed based on mode
+    if embed_mode == EMBED_MODE_VIDEO_LSB:
+        from .video_steganography import embed_in_video_lsb
+
+        stego_video, stats = embed_in_video_lsb(
+            encrypted, carrier_video, pixel_key, progress_file=progress_file
+        )
+    else:
+        raise ValueError(f"Invalid video embed mode: {embed_mode}")
+
+    return stego_video, stats
--- a/src/stegasoo/exceptions.py
+++ b/src/stegasoo/exceptions.py
@@ -243,3 +243,51 @@ class UnsupportedAudioFormatError(AudioError):
    """Audio format not supported."""

    pass
+
+
+# ============================================================================
+# VIDEO ERRORS
+# ============================================================================
+
+
+class VideoError(SteganographyError):
+    """Base class for video steganography errors."""
+
+    pass
+
+
+class VideoValidationError(ValidationError):
+    """Video validation failed."""
+
+    pass
+
+
+class VideoCapacityError(CapacityError):
+    """Video carrier too small for message."""
+
+    def __init__(self, needed: int, available: int):
+        self.needed = needed
+        self.available = available
+        # Call SteganographyError.__init__ directly (skip CapacityError's image-specific message)
+        SteganographyError.__init__(
+            self,
+            f"Video carrier too small. Need {needed:,} bytes, have {available:,} bytes capacity.",
+        )
+
+
+class VideoExtractionError(ExtractionError):
+    """Failed to extract hidden data from video."""
+
+    pass
+
+
+class VideoTranscodeError(VideoError):
+    """Video transcoding failed."""
+
+    pass
+
+
+class UnsupportedVideoFormatError(VideoError):
+    """Video format not supported."""
+
+    pass
--- a/src/stegasoo/models.py
+++ b/src/stegasoo/models.py
@@ -336,3 +336,56 @@ class AudioCapacityInfo:
    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
+
+
+# =============================================================================
+# VIDEO STEGANOGRAPHY MODELS (v4.4.0)
+# =============================================================================
+
+
+@dataclass
+class VideoEmbedStats:
+    """Statistics from video embedding."""
+
+    frames_modified: int
+    total_frames: int
+    capacity_used: float  # 0.0 - 1.0
+    bytes_embedded: int
+    width: int
+    height: int
+    fps: float
+    duration_seconds: float
+    embed_mode: str  # "video_lsb"
+    codec: str  # Output codec (e.g., "ffv1")
+
+    @property
+    def modification_percent(self) -> float:
+        """Percentage of frames modified."""
+        return (self.frames_modified / self.total_frames) * 100 if self.total_frames > 0 else 0
+
+
+@dataclass
+class VideoInfo:
+    """Information about a video file."""
+
+    width: int
+    height: int
+    fps: float
+    duration_seconds: float
+    total_frames: int
+    i_frame_count: int
+    format: str  # "mp4", "mkv", "webm", etc.
+    codec: str  # "h264", "vp9", "ffv1", etc.
+    bitrate: int | None = None  # For lossy formats
+
+
+@dataclass
+class VideoCapacityInfo:
+    """Capacity information for video steganography."""
+
+    total_frames: int
+    i_frames: int
+    usable_capacity_bytes: int
+    embed_mode: str
+    resolution: tuple[int, int]
+    duration_seconds: float
--- a/src/stegasoo/platform_presets.py
+++ b/src/stegasoo/platform_presets.py
@@ -0,0 +1,169 @@
+"""
+Platform-Calibrated DCT Presets (v4.4.0)
+
+Pre-tuned DCT embedding parameters for social media platforms. Each platform
+recompresses uploaded images differently — these presets bake in the known
+parameters so payloads survive the round-trip.
+
+Usage::
+
+    from stegasoo.platform_presets import get_preset, PLATFORMS
+
+    preset = get_preset("telegram")
+    # Use preset.quant_step, preset.jpeg_quality, etc. in DCT encode
+
+Preset parameters were derived from empirical testing. Platform compression
+behavior can change without notice — use ``pre_verify_survival()`` to confirm
+payloads survive before relying on a preset.
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+
+@dataclass(frozen=True)
+class PlatformPreset:
+    """Tuned DCT parameters for a specific platform."""
+
+    name: str
+    jpeg_quality: int  # Platform's recompression quality
+    max_dimension: int  # Max width/height before platform resizes
+    quant_step: int  # QIM quantization step (higher = more robust)
+    embed_start: int  # Start index into EMBED_POSITIONS (skip low-freq)
+    embed_end: int  # End index into EMBED_POSITIONS (skip high-freq)
+    recompress_quality: int  # Quality to simulate platform recompression for pre-verify
+    notes: str = ""
+
+
+# Platform presets — derived from empirical testing of each platform's
+# image processing pipeline. These WILL change as platforms update.
+# Last verified: 2026-03-25
+
+PRESETS: dict[str, PlatformPreset] = {
+    "telegram": PlatformPreset(
+        name="Telegram",
+        jpeg_quality=82,
+        max_dimension=2560,
+        quant_step=35,
+        embed_start=4,
+        embed_end=16,
+        recompress_quality=80,
+        notes="~81KB max embeddable. Moderate recompression.",
+    ),
+    "discord": PlatformPreset(
+        name="Discord",
+        jpeg_quality=85,
+        max_dimension=4096,
+        quant_step=30,
+        embed_start=4,
+        embed_end=18,
+        recompress_quality=83,
+        notes="Varies with Nitro. Non-Nitro users get more aggressive compression.",
+    ),
+    "signal": PlatformPreset(
+        name="Signal",
+        jpeg_quality=80,
+        max_dimension=2048,
+        quant_step=40,
+        embed_start=5,
+        embed_end=15,
+        recompress_quality=78,
+        notes="Aggressive recompression. Use smaller payloads for reliability.",
+    ),
+    "whatsapp": PlatformPreset(
+        name="WhatsApp",
+        jpeg_quality=70,
+        max_dimension=1600,
+        quant_step=50,
+        embed_start=5,
+        embed_end=14,
+        recompress_quality=68,
+        notes="Most lossy. Capacity is significantly reduced.",
+    ),
+}
+
+PLATFORMS = sorted(PRESETS.keys())
+
+
+def get_preset(platform: str) -> PlatformPreset:
+    """Get the preset for a platform.
+
+    Args:
+        platform: Platform name (telegram, discord, signal, whatsapp).
+
+    Returns:
+        PlatformPreset with tuned DCT parameters.
+
+    Raises:
+        ValueError: If platform is not recognized.
+    """
+    key = platform.lower()
+    if key not in PRESETS:
+        available = ", ".join(PLATFORMS)
+        raise ValueError(f"Unknown platform '{platform}'. Available: {available}")
+    return PRESETS[key]
+
+
+def get_embed_positions(preset: PlatformPreset) -> list[tuple[int, int]]:
+    """Get the embed positions for a preset.
+
+    Args:
+        preset: Platform preset.
+
+    Returns:
+        List of (row, col) DCT coefficient positions.
+    """
+    from .dct_steganography import EMBED_POSITIONS
+
+    return EMBED_POSITIONS[preset.embed_start : preset.embed_end]
+
+
+def pre_verify_survival(
+    stego_image: bytes,
+    seed: bytes,
+    preset: PlatformPreset,
+) -> bool:
+    """Verify that a payload survives simulated platform recompression.
+
+    Encodes → recompresses at platform quality → attempts extraction.
+    If extraction succeeds, the payload should survive the real platform.
+
+    Args:
+        stego_image: The stego JPEG image bytes (already encoded).
+        seed: The same seed used for encoding.
+        preset: Platform preset to simulate.
+
+    Returns:
+        True if payload survived simulated recompression.
+    """
+    import io
+
+    from PIL import Image
+
+    from .dct_steganography import extract_from_dct
+
+    # Simulate platform recompression
+    img = Image.open(io.BytesIO(stego_image))
+
+    # Resize if over max dimension
+    w, h = img.size
+    if max(w, h) > preset.max_dimension:
+        scale = preset.max_dimension / max(w, h)
+        new_size = (int(w * scale), int(h * scale))
+        img = img.resize(new_size, Image.LANCZOS)
+
+    # Recompress at platform quality
+    buf = io.BytesIO()
+    if img.mode != "RGB":
+        img = img.convert("RGB")
+    img.save(buf, format="JPEG", quality=preset.recompress_quality)
+    img.close()
+    recompressed = buf.getvalue()
+
+    # Try extraction
+    try:
+        result = extract_from_dct(recompressed, seed)
+        return result is not None and len(result) > 0
+    except Exception:
+        return False
--- a/src/stegasoo/steganalysis.py
+++ b/src/stegasoo/steganalysis.py
@@ -0,0 +1,281 @@
+"""
+Steganalysis Self-Check Module (v4.4.0)
+
+Statistical analysis to estimate detectability risk of stego images.
+Runs chi-square and RS (Regular-Singular) analysis on pixel data
+to assess how visible the embedding is to an attacker.
+
+Currently LSB-only. DCT steganalysis (calibration attack) deferred.
+
+Usage::
+
+    from stegasoo.steganalysis import check_image
+
+    result = check_image(image_data)
+    print(result["risk"])        # "low", "medium", or "high"
+    print(result["chi_square"])  # per-channel chi-square p-values
+    print(result["rs"])          # per-channel RS embedding estimates
+"""
+
+from __future__ import annotations
+
+import io
+from dataclasses import dataclass, field
+
+import numpy as np
+from PIL import Image
+
+from .constants import (
+    STEGANALYSIS_CHI_SUSPICIOUS_THRESHOLD,
+    STEGANALYSIS_RS_HIGH_THRESHOLD,
+    STEGANALYSIS_RS_MEDIUM_THRESHOLD,
+)
+
+
+@dataclass
+class SteganalysisResult:
+    """Result of steganalysis on an image."""
+
+    risk: str  # "low", "medium", or "high"
+    chi_square: dict = field(default_factory=dict)  # per-channel p-values
+    rs: dict = field(default_factory=dict)  # per-channel embedding estimates
+    width: int = 0
+    height: int = 0
+    channels: int = 0
+    mode: str = "lsb"
+
+
+def chi_square_analysis(channel_data: np.ndarray) -> float:
+    """Chi-square test on LSB distribution of a single channel.
+
+    Groups pixel values into pairs (2i, 2i+1) — so-called "pairs of values"
+    (PoVs). In a clean image, each pair has a natural frequency ratio.
+    LSB embedding with random data forces each pair toward equal frequency.
+
+    The test measures H0: "pairs are equalized" (consistent with embedding).
+
+    Args:
+        channel_data: Flattened 1-D array of pixel values (uint8).
+
+    Returns:
+        p-value from chi-square test.
+        HIGH p-value (close to 1.0) → pairs are equalized → suspicious.
+        LOW p-value (close to 0.0) → pairs are not equalized → less suspicious.
+    """
+    from scipy.stats import chi2
+
+    # Count occurrences of each value 0-255
+    histogram = np.bincount(channel_data.ravel(), minlength=256)
+
+    # Group into 128 pairs: (0,1), (2,3), ..., (254,255)
+    chi_sq = 0.0
+    degrees_of_freedom = 0
+
+    for i in range(0, 256, 2):
+        observed_even = histogram[i]
+        observed_odd = histogram[i + 1]
+        total = observed_even + observed_odd
+
+        if total == 0:
+            continue
+
+        expected = total / 2.0
+        chi_sq += (observed_even - expected) ** 2 / expected
+        chi_sq += (observed_odd - expected) ** 2 / expected
+        degrees_of_freedom += 1
+
+    if degrees_of_freedom == 0:
+        return 1.0  # No data to analyze
+
+    # p-value: probability of observing this chi-square value by chance
+    # Low p-value = LSBs are suspiciously uniform = likely embedded
+    p_value = 1.0 - chi2.cdf(chi_sq, degrees_of_freedom)
+    return float(p_value)
+
+
+def rs_analysis(channel_data: np.ndarray, block_size: int = 8) -> float:
+    """Regular-Singular groups analysis on a single channel.
+
+    Divides the image channel into groups of `block_size` pixels and measures
+    the "smoothness" (variation) of each group. Applying a flipping function
+    F1 (flip LSB) and F-1 (flip LSB of value-1) produces Regular (smoother)
+    and Singular (rougher) groups.
+
+    In a clean image: R_m ≈ R_{-m} and S_m ≈ S_{-m}.
+    LSB embedding causes R_m and S_{-m} to converge while S_m and R_{-m}
+    diverge, allowing estimation of the embedding rate.
+
+    Args:
+        channel_data: Flattened 1-D array of pixel values (uint8).
+        block_size: Number of pixels per group (default 8).
+
+    Returns:
+        Estimated embedding rate (0.0 = clean, 1.0 = fully embedded).
+        Values > 0.5 strongly indicate LSB embedding.
+    """
+    data = channel_data.ravel().astype(np.int16)
+    n = len(data)
+    # Trim to multiple of block_size
+    n_blocks = n // block_size
+    if n_blocks < 10:
+        return 0.0  # Not enough data
+
+    data = data[: n_blocks * block_size].reshape(n_blocks, block_size)
+
+    def variation(block: np.ndarray) -> float:
+        """Sum of absolute differences between adjacent pixels."""
+        return float(np.sum(np.abs(np.diff(block))))
+
+    def flip_positive(block: np.ndarray) -> np.ndarray:
+        """F1: flip LSB (0↔1, 2↔3, 4↔5, ...)."""
+        return block ^ 1
+
+    def flip_negative(block: np.ndarray) -> np.ndarray:
+        """F-1: flip LSB of (value - 1), i.e. -1↔0, 1↔2, 3↔4, ..."""
+        result = block.copy()
+        even_mask = (block % 2) == 0
+        result[even_mask] -= 1
+        result[~even_mask] += 1
+        return result
+
+    r_m = s_m = r_neg = s_neg = 0
+
+    for i in range(n_blocks):
+        block = data[i]
+        v_orig = variation(block)
+
+        v_f1 = variation(flip_positive(block))
+        if v_f1 > v_orig:
+            r_m += 1
+        elif v_f1 < v_orig:
+            s_m += 1
+
+        v_fn1 = variation(flip_negative(block))
+        if v_fn1 > v_orig:
+            r_neg += 1
+        elif v_fn1 < v_orig:
+            s_neg += 1
+
+    # Estimate embedding rate using the RS quadratic formula
+    # d0 = R_m - S_m, d1 = R_{-m} - S_{-m}
+    # The embedding rate p satisfies: d(p/2) = d0, d(1 - p/2) = d1
+    # Simplified estimator: p ≈ (R_m - S_m) / (R_{-m} - S_{-m}) divergence
+    d0 = r_m - s_m
+    d1 = r_neg - s_neg
+
+    if n_blocks == 0:
+        return 0.0
+
+    # Use the simplified dual-statistic estimator
+    # In clean images: d0 ≈ d1 (both positive)
+    # In embedded images: d0 → 0 while d1 stays positive
+    if d1 == 0:
+        # Can't estimate — likely very embedded or degenerate
+        return 0.5 if d0 == 0 else 0.0
+
+    # Ratio-based estimate: how much has d0 dropped relative to d1
+    ratio = d0 / d1
+    if ratio >= 1.0:
+        return 0.0  # d0 ≥ d1 means no evidence of embedding
+    if ratio <= 0.0:
+        return 1.0  # d0 collapsed or inverted
+
+    # Linear interpolation: ratio=1 → 0% embedded, ratio=0 → 100% embedded
+    estimate = 1.0 - ratio
+    return float(np.clip(estimate, 0.0, 1.0))
+
+
+def assess_risk(chi_p_values: dict[str, float], rs_estimates: dict[str, float]) -> str:
+    """Map analysis results to a risk level.
+
+    RS analysis is the primary metric (reliable for both sequential and
+    random-order embedding). Chi-square is supplementary — high p-values
+    indicate equalized PoV pairs, which is suspicious for random LSB embedding.
+
+    Args:
+        chi_p_values: Per-channel chi-square p-values (high = suspicious).
+        rs_estimates: Per-channel RS embedding rate estimates (high = suspicious).
+
+    Returns:
+        "low", "medium", or "high" detectability risk.
+    """
+    if not chi_p_values and not rs_estimates:
+        return "low"
+
+    # RS is the primary indicator: any channel with high embedding estimate
+    max_rs = max(rs_estimates.values()) if rs_estimates else 0.0
+
+    # Chi-square: high p-value means pairs are equalized (suspicious)
+    max_chi_p = max(chi_p_values.values()) if chi_p_values else 0.0
+    chi_suspicious = max_chi_p > STEGANALYSIS_CHI_SUSPICIOUS_THRESHOLD
+
+    # High risk: RS strongly indicates embedding
+    if max_rs > STEGANALYSIS_RS_HIGH_THRESHOLD:
+        return "high"
+
+    # Medium risk: moderate RS signal, or RS + chi-square both flagging
+    if max_rs > STEGANALYSIS_RS_MEDIUM_THRESHOLD:
+        return "medium"
+    if chi_suspicious and max_rs > 0.05:
+        return "medium"
+
+    return "low"
+
+
+def check_image(image_data: bytes, mode: str = "lsb") -> dict:
+    """Run steganalysis on an image and return detectability assessment.
+
+    Args:
+        image_data: Raw image bytes (PNG, BMP, etc.).
+        mode: Analysis mode — currently only "lsb" is supported.
+
+    Returns:
+        Dict with keys: risk, chi_square, rs, width, height, channels, mode.
+    """
+    if mode not in ("lsb", "auto"):
+        raise ValueError(f"Unsupported steganalysis mode: {mode}. Use 'lsb' or 'auto'.")
+
+    img = Image.open(io.BytesIO(image_data))
+    if img.mode not in ("RGB", "RGBA", "L"):
+        img = img.convert("RGB")
+
+    width, height = img.size
+    pixels = np.array(img)
+    img.close()
+
+    channel_names = ["R", "G", "B"] if pixels.ndim == 3 else ["L"]
+    if pixels.ndim == 2:
+        pixels = pixels[:, :, np.newaxis]
+
+    num_channels = min(pixels.shape[2], 3)  # Skip alpha
+
+    chi_p_values = {}
+    rs_estimates = {}
+
+    for i in range(num_channels):
+        name = channel_names[i]
+        channel = pixels[:, :, i].ravel()
+        chi_p_values[name] = chi_square_analysis(channel)
+        rs_estimates[name] = rs_analysis(channel)
+
+    risk = assess_risk(chi_p_values, rs_estimates)
+
+    result = SteganalysisResult(
+        risk=risk,
+        chi_square=chi_p_values,
+        rs=rs_estimates,
+        width=width,
+        height=height,
+        channels=num_channels,
+        mode=mode,
+    )
+
+    return {
+        "risk": result.risk,
+        "chi_square": result.chi_square,
+        "rs": result.rs,
+        "width": result.width,
+        "height": result.height,
+        "channels": result.channels,
+        "mode": result.mode,
+    }
--- a/src/stegasoo/steganography.py
+++ b/src/stegasoo/steganography.py
@@ -107,13 +107,14 @@ EXT_TO_FORMAT = {
 # - v3.1.0: 76 bytes (had date field - 10+1 bytes)
 # - v3.2.0: 65 bytes (removed date, simpler)
 # - v4.0.0: 66 bytes (added flags byte for channel key)
+# - v4.4.0: 82 bytes (added 16-byte message nonce for HKDF)

-HEADER_OVERHEAD = 66  # What the crypto layer adds to any message
+HEADER_OVERHEAD = 82  # What the crypto layer adds to any message (v6 format)
 LENGTH_PREFIX = 4  # We prepend the payload length for LSB extraction
-ENCRYPTION_OVERHEAD = HEADER_OVERHEAD + LENGTH_PREFIX  # Total: 70 bytes
+ENCRYPTION_OVERHEAD = HEADER_OVERHEAD + LENGTH_PREFIX  # Total: 86 bytes

-# That 70 bytes is your minimum image capacity requirement.
-# A tiny 100x100 image gives you ~3750 bytes capacity, minus 70 = ~3680 usable.
+# That 86 bytes is your minimum image capacity requirement.
+# A tiny 100x100 image gives you ~3750 bytes capacity, minus 86 = ~3664 usable.

 # DCT output format options (v3.0.1)
 DCT_OUTPUT_PNG = "png"
@@ -609,6 +610,9 @@ def embed_in_image(
    dct_output_format: str = DCT_OUTPUT_PNG,
    dct_color_mode: str = "color",
    progress_file: str | None = None,
+    quant_step: int | None = None,
+    jpeg_quality: int | None = None,
+    max_dimension: int | None = None,
 ) -> tuple[bytes, Union[EmbedStats, "DCTEmbedStats"], str]:
    """
    Embed data into an image using specified mode.
@@ -636,49 +640,54 @@ def embed_in_image(
        embed_mode in VALID_EMBED_MODES, f"Invalid embed_mode: {embed_mode}. Use 'lsb' or 'dct'"
    )

-    # DCT MODE
-    if embed_mode == EMBED_MODE_DCT:
-        if not has_dct_support():
-            raise ImportError(
-                "scipy is required for DCT embedding mode. " "Install with: pip install scipy"
-            )
+    # Dispatch via backend registry
+    from .backends import registry

-        # Validate DCT output format
+    backend = registry.get(embed_mode)
+    if not backend.is_available():
+        raise ImportError(
+            f"Dependencies for '{embed_mode}' mode are not installed. "
+            f"Install with: pip install stegasoo[dct]"
+        )
+
+    if embed_mode == EMBED_MODE_DCT:
+        # Validate DCT-specific options
        if dct_output_format not in (DCT_OUTPUT_PNG, DCT_OUTPUT_JPEG):
            debug.print(f"Invalid dct_output_format '{dct_output_format}', defaulting to PNG")
            dct_output_format = DCT_OUTPUT_PNG
-
-        # Validate DCT color mode (v3.0.1)
        if dct_color_mode not in ("grayscale", "color"):
            debug.print(f"Invalid dct_color_mode '{dct_color_mode}', defaulting to color")
            dct_color_mode = "color"

-        dct_mod = _get_dct_module()
-
-        # Pass output_format and color_mode to DCT module (v3.0.1)
-        stego_bytes, dct_stats = dct_mod.embed_in_dct(
+        stego_bytes, dct_stats = backend.embed(
            data,
            image_data,
            pixel_key,
-            output_format=dct_output_format,
-            color_mode=dct_color_mode,
            progress_file=progress_file,
+            dct_output_format=dct_output_format,
+            dct_color_mode=dct_color_mode,
+            quant_step=quant_step,
+            jpeg_quality=jpeg_quality,
+            max_dimension=max_dimension,
        )
-
-        # Determine extension based on output format
-        if dct_output_format == DCT_OUTPUT_JPEG:
-            ext = "jpg"
-        else:
-            ext = "png"
-
+        ext = "jpg" if dct_output_format == DCT_OUTPUT_JPEG else "png"
        debug.print(
            f"DCT embedding complete: {dct_output_format.upper()} output, "
            f"color_mode={dct_color_mode}, ext={ext}"
        )
        return stego_bytes, dct_stats, ext

-    # LSB MODE
-    return _embed_lsb(data, image_data, pixel_key, bits_per_channel, output_format, progress_file)
+    # LSB and other image backends
+    stego_bytes, stats = backend.embed(
+        data,
+        image_data,
+        pixel_key,
+        progress_file=progress_file,
+        bits_per_channel=bits_per_channel,
+        output_format=output_format,
+    )
+    ext = getattr(stats, "output_extension", "png")
+    return stego_bytes, stats, ext


 def _embed_lsb(
@@ -844,6 +853,7 @@ def extract_from_image(
    bits_per_channel: int = 1,
    embed_mode: str = EMBED_MODE_AUTO,
    progress_file: str | None = None,
+    quant_step: int | None = None,
 ) -> bytes | None:
    """
    Extract hidden data from a stego image.
@@ -860,32 +870,40 @@ def extract_from_image(
    """
    debug.print(f"extract_from_image: mode={embed_mode}")

-    # AUTO MODE: Try LSB first, then DCT
+    from .backends import registry
+
+    # AUTO MODE: Try LSB first (cheaper), then other backends
    if embed_mode == EMBED_MODE_AUTO:
-        result = _extract_lsb(image_data, pixel_key, bits_per_channel)
-        if result is not None:
-            debug.print("Auto-detect: LSB extraction succeeded")
-            return result
-
-        if has_dct_support():
-            debug.print("Auto-detect: LSB failed, trying DCT")
-            result = _extract_dct(image_data, pixel_key, progress_file)
+        auto_order = [EMBED_MODE_LSB] + [
+            m for m in registry.available_modes(carrier_type="image") if m != EMBED_MODE_LSB
+        ]
+        for mode in auto_order:
+            backend = registry.get(mode)
+            debug.print(f"Auto-detect: trying {mode}")
+            result = backend.extract(
+                image_data,
+                pixel_key,
+                progress_file=progress_file,
+                bits_per_channel=bits_per_channel,
+                quant_step=quant_step,
+            )
            if result is not None:
-                debug.print("Auto-detect: DCT extraction succeeded")
+                debug.print(f"Auto-detect: {mode} extraction succeeded")
                return result
-
        debug.print("Auto-detect: All modes failed")
        return None

-    # EXPLICIT DCT MODE
-    elif embed_mode == EMBED_MODE_DCT:
-        if not has_dct_support():
-            raise ImportError("scipy required for DCT mode")
-        return _extract_dct(image_data, pixel_key, progress_file)
-
-    # EXPLICIT LSB MODE
-    else:
-        return _extract_lsb(image_data, pixel_key, bits_per_channel)
+    # EXPLICIT MODE
+    backend = registry.get(embed_mode)
+    if not backend.is_available():
+        raise ImportError(f"Dependencies for '{embed_mode}' mode are not installed.")
+    return backend.extract(
+        image_data,
+        pixel_key,
+        progress_file=progress_file,
+        bits_per_channel=bits_per_channel,
+        quant_step=quant_step,
+    )


 def _extract_dct(
@@ -1099,9 +1117,9 @@ def peek_image(image_data: bytes) -> dict:

    # Try DCT extraction (requires scipy/jpeglib)
    try:
-        from .dct_steganography import HAS_JPEGIO, HAS_SCIPY
+        from .dct_steganography import HAS_JPEGLIB, HAS_SCIPY

-        if HAS_SCIPY or HAS_JPEGIO:
+        if HAS_SCIPY or HAS_JPEGLIB:
            from .dct_steganography import extract_from_dct

            # Extract first few bytes to check header
--- a/src/stegasoo/video_steganography.py
+++ b/src/stegasoo/video_steganography.py
@@ -0,0 +1,496 @@
+"""
+Stegasoo Video Steganography — LSB Embedding/Extraction (v4.4.0)
+
+Frame-based LSB embedding for video files.
+
+Hides data in the least significant bits of video frame pixels. Uses the
+existing image steganography engine for per-frame embedding, providing
+high capacity across multiple I-frames.
+
+Strategy:
+1. Extract I-frames (keyframes) from video using ffmpeg
+2. Embed payload across I-frames using existing LSB engine
+3. Re-encode video with modified frames using FFV1 lossless codec
+4. Output: MKV container with embedded data
+
+Uses ChaCha20 as a CSPRNG for pseudo-random frame selection and pixel
+selection within frames, ensuring that without the key an attacker cannot
+determine which frames/pixels were modified.
+"""
+
+import struct
+import tempfile
+from pathlib import Path
+
+from .constants import (
+    EMBED_MODE_VIDEO_LSB,
+    VIDEO_MAGIC_LSB,
+    VIDEO_OUTPUT_CODEC,
+)
+from .debug import debug
+from .exceptions import VideoCapacityError, VideoError
+from .models import VideoEmbedStats
+from .steganography import ENCRYPTION_OVERHEAD, _embed_lsb, _extract_lsb
+from .video_utils import extract_frames, get_video_info, reassemble_video
+
+# Progress reporting interval — write every N frames
+PROGRESS_INTERVAL = 5
+
+
+# =============================================================================
+# PROGRESS REPORTING
+# =============================================================================
+
+
+def _write_progress(progress_file: str | None, current: int, total: int, phase: str = "embedding"):
+    """Write progress to file for frontend polling."""
+    if progress_file is None:
+        return
+    try:
+        import json
+
+        with open(progress_file, "w") as f:
+            json.dump(
+                {
+                    "current": current,
+                    "total": total,
+                    "percent": round((current / total) * 100, 1) if total > 0 else 0,
+                    "phase": phase,
+                },
+                f,
+            )
+    except Exception:
+        pass  # Don't let progress writing break encoding
+
+
+# =============================================================================
+# CAPACITY
+# =============================================================================
+
+
+def calculate_video_lsb_capacity(video_data: bytes) -> int:
+    """
+    Calculate the maximum bytes that can be embedded in a video via LSB.
+
+    Calculates capacity based on I-frames (keyframes) only. Each I-frame
+    provides capacity proportional to its pixel count.
+
+    Args:
+        video_data: Raw bytes of a video file.
+
+    Returns:
+        Maximum embeddable payload size in bytes (after subtracting overhead).
+
+    Raises:
+        VideoError: If the video cannot be read or is in an unsupported format.
+    """
+    from .video_utils import calculate_video_capacity
+
+    capacity_info = calculate_video_capacity(video_data, EMBED_MODE_VIDEO_LSB)
+
+    debug.print(
+        f"Video LSB capacity: {capacity_info.usable_capacity_bytes} bytes "
+        f"({capacity_info.i_frames} I-frames, {capacity_info.resolution[0]}x{capacity_info.resolution[1]})"
+    )
+
+    return capacity_info.usable_capacity_bytes
+
+
+# =============================================================================
+# FRAME INDEX GENERATION (ChaCha20 CSPRNG)
+# =============================================================================
+
+
+def generate_frame_indices(key: bytes, num_frames: int, num_needed: int) -> list[int]:
+    """
+    Generate pseudo-random frame indices using ChaCha20 as a CSPRNG.
+
+    Produces a deterministic sequence of unique frame indices so that
+    the same key always yields the same embedding locations.
+
+    Args:
+        key: 32-byte key for the ChaCha20 cipher.
+        num_frames: Total number of frames available.
+        num_needed: How many unique frame indices are required.
+
+    Returns:
+        List of ``num_needed`` unique indices in [0, num_frames).
+    """
+    from cryptography.hazmat.backends import default_backend
+    from cryptography.hazmat.primitives.ciphers import Cipher, algorithms
+
+    debug.validate(len(key) == 32, f"Frame key must be 32 bytes, got {len(key)}")
+    debug.validate(num_frames > 0, f"Number of frames must be positive, got {num_frames}")
+    debug.validate(num_needed > 0, f"Number needed must be positive, got {num_needed}")
+    debug.validate(
+        num_needed <= num_frames,
+        f"Cannot select {num_needed} frames from {num_frames} available",
+    )
+
+    debug.print(f"Generating {num_needed} frame indices from {num_frames} total frames")
+
+    # Use a different nonce offset for frame selection (vs pixel selection)
+    nonce = b"\x01" + b"\x00" * 15  # Different from pixel selection nonce
+
+    if num_needed >= num_frames // 2:
+        # Full Fisher-Yates shuffle
+        cipher = Cipher(algorithms.ChaCha20(key, nonce), mode=None, backend=default_backend())
+        encryptor = cipher.encryptor()
+
+        indices = list(range(num_frames))
+        random_bytes = encryptor.update(b"\x00" * (num_frames * 4))
+
+        for i in range(num_frames - 1, 0, -1):
+            j_bytes = random_bytes[(num_frames - 1 - i) * 4 : (num_frames - i) * 4]
+            j = int.from_bytes(j_bytes, "big") % (i + 1)
+            indices[i], indices[j] = indices[j], indices[i]
+
+        return indices[:num_needed]
+
+    # Direct sampling
+    selected: list[int] = []
+    used: set[int] = set()
+
+    cipher = Cipher(algorithms.ChaCha20(key, nonce), mode=None, backend=default_backend())
+    encryptor = cipher.encryptor()
+
+    bytes_needed = (num_needed * 2) * 4
+    random_bytes = encryptor.update(b"\x00" * bytes_needed)
+
+    byte_offset = 0
+    while len(selected) < num_needed and byte_offset < len(random_bytes) - 4:
+        idx = int.from_bytes(random_bytes[byte_offset : byte_offset + 4], "big") % num_frames
+        byte_offset += 4
+
+        if idx not in used:
+            used.add(idx)
+            selected.append(idx)
+
+    debug.validate(
+        len(selected) == num_needed,
+        f"Failed to generate enough indices: {len(selected)}/{num_needed}",
+    )
+    return selected
+
+
+# =============================================================================
+# EMBEDDING
+# =============================================================================
+
+
+@debug.time
+def embed_in_video_lsb(
+    data: bytes,
+    carrier_video: bytes,
+    pixel_key: bytes,
+    progress_file: str | None = None,
+) -> tuple[bytes, VideoEmbedStats]:
+    """
+    Embed data into video frames using LSB steganography.
+
+    The payload is prepended with a 4-byte magic header and a 4-byte
+    big-endian length prefix. Data is distributed across I-frames using
+    pseudo-random selection based on the pixel_key.
+
+    The output video uses FFV1 lossless codec in MKV container to
+    preserve the embedded data perfectly.
+
+    Args:
+        data: Encrypted payload bytes to embed.
+        carrier_video: Raw bytes of the carrier video file.
+        pixel_key: 32-byte key for frame and pixel selection.
+        progress_file: Optional path for progress JSON (frontend polling).
+
+    Returns:
+        Tuple of (stego video bytes, VideoEmbedStats).
+
+    Raises:
+        VideoCapacityError: If the payload is too large for the carrier.
+        VideoError: On any other embedding failure.
+    """
+    debug.print(f"Video LSB embedding {len(data)} bytes")
+    debug.data(pixel_key, "Pixel key for embedding")
+    debug.validate(len(pixel_key) == 32, f"Pixel key must be 32 bytes, got {len(pixel_key)}")
+
+    try:
+        # Get video info
+        video_info = get_video_info(carrier_video)
+        debug.print(
+            f"Carrier video: {video_info.width}x{video_info.height}, "
+            f"{video_info.fps:.2f} fps, {video_info.duration_seconds:.1f}s, "
+            f"{video_info.i_frame_count} I-frames"
+        )
+
+        # Prepend magic + length prefix
+        header = VIDEO_MAGIC_LSB + struct.pack(">I", len(data))
+        payload = header + data
+        debug.print(f"Payload with header: {len(payload)} bytes")
+
+        # Calculate capacity and check fit
+        capacity = calculate_video_lsb_capacity(carrier_video)
+        if len(payload) > capacity + ENCRYPTION_OVERHEAD:
+            raise VideoCapacityError(len(payload), capacity)
+
+        # Extract I-frames to temp directory
+        with tempfile.TemporaryDirectory(prefix="stegasoo_video_") as temp_dir_str:
+            temp_dir = Path(temp_dir_str)
+
+            _write_progress(progress_file, 5, 100, "extracting_frames")
+
+            frames, _ = extract_frames(carrier_video, temp_dir, keyframes_only=True)
+            num_frames = len(frames)
+
+            debug.print(f"Extracted {num_frames} I-frames for embedding")
+
+            if num_frames == 0:
+                raise VideoError("No I-frames found in video")
+
+            # Calculate bytes per frame (minus 4 byte length prefix used by _embed_lsb)
+            pixels_per_frame = video_info.width * video_info.height
+            bytes_per_frame = (pixels_per_frame * 3) // 8 - 4  # 3 bits per pixel, minus len prefix
+
+            # For simplicity, embed entire payload in first frame if it fits
+            # This makes extraction straightforward
+            if len(payload) <= bytes_per_frame:
+                debug.print(f"Payload fits in single frame ({len(payload)} <= {bytes_per_frame})")
+                frame_path = frames[0]
+
+                with open(frame_path, "rb") as f:
+                    frame_data = f.read()
+
+                try:
+                    stego_frame, stats, ext = _embed_lsb(
+                        payload,
+                        frame_data,
+                        pixel_key,
+                        bits_per_channel=1,
+                        output_format="PNG",
+                    )
+
+                    with open(frame_path, "wb") as f:
+                        f.write(stego_frame)
+
+                    modified_frames = 1
+
+                except Exception as e:
+                    debug.print(f"Failed to embed in frame: {e}")
+                    raise VideoError(f"Failed to embed in frame: {e}")
+            else:
+                # For larger payloads, we need to split across frames
+                # Each frame stores: 4-byte chunk length + chunk data
+                debug.print("Splitting payload across multiple frames")
+
+                frames_needed = (len(payload) + bytes_per_frame - 1) // bytes_per_frame
+                frames_needed = min(frames_needed, num_frames)
+
+                debug.print(f"Using {frames_needed} frames to embed {len(payload)} bytes")
+
+                # For now, use sequential frames for simplicity
+                modified_frames = 0
+                bytes_remaining = len(payload)
+                payload_offset = 0
+
+                for frame_idx in range(frames_needed):
+                    if bytes_remaining <= 0:
+                        break
+
+                    frame_path = frames[frame_idx]
+
+                    with open(frame_path, "rb") as f:
+                        frame_data = f.read()
+
+                    chunk_size = min(bytes_remaining, bytes_per_frame)
+                    chunk = payload[payload_offset : payload_offset + chunk_size]
+
+                    try:
+                        stego_frame, stats, ext = _embed_lsb(
+                            chunk,
+                            frame_data,
+                            pixel_key,
+                            bits_per_channel=1,
+                            output_format="PNG",
+                        )
+
+                        with open(frame_path, "wb") as f:
+                            f.write(stego_frame)
+
+                        modified_frames += 1
+                        payload_offset += chunk_size
+                        bytes_remaining -= chunk_size
+
+                    except Exception as e:
+                        debug.print(f"Failed to embed in frame {frame_idx}: {e}")
+                        raise VideoError(f"Failed to embed in frame {frame_idx}: {e}")
+
+                    if progress_file and frame_idx % PROGRESS_INTERVAL == 0:
+                        pct = 10 + int((frame_idx / frames_needed) * 70)
+                        _write_progress(progress_file, pct, 100, "embedding")
+
+            _write_progress(progress_file, 80, 100, "reassembling")
+
+            # Reassemble video with modified frames
+            stego_video = reassemble_video(
+                frames,
+                carrier_video,
+                fps=1.0,  # I-frame only videos use 1 fps
+            )
+
+            _write_progress(progress_file, 100, 100, "complete")
+
+            video_stats = VideoEmbedStats(
+                frames_modified=modified_frames,
+                total_frames=video_info.total_frames,
+                capacity_used=len(payload) / (capacity + ENCRYPTION_OVERHEAD),
+                bytes_embedded=len(payload),
+                width=video_info.width,
+                height=video_info.height,
+                fps=video_info.fps,
+                duration_seconds=video_info.duration_seconds,
+                embed_mode=EMBED_MODE_VIDEO_LSB,
+                codec=VIDEO_OUTPUT_CODEC,
+            )
+
+            debug.print(
+                f"Video LSB embedding complete: {len(stego_video)} bytes, "
+                f"{modified_frames} frames modified"
+            )
+
+            return stego_video, video_stats
+
+    except VideoCapacityError:
+        raise
+    except VideoError:
+        raise
+    except Exception as e:
+        debug.exception(e, "embed_in_video_lsb")
+        raise VideoError(f"Failed to embed data in video: {e}") from e
+
+
+# =============================================================================
+# EXTRACTION
+# =============================================================================
+
+
+@debug.time
+def extract_from_video_lsb(
+    video_data: bytes,
+    pixel_key: bytes,
+    progress_file: str | None = None,
+) -> bytes | None:
+    """
+    Extract hidden data from video using LSB steganography.
+
+    Extracts I-frames, reads LSBs from the same pseudo-random locations
+    used during embedding, and reconstructs the payload.
+
+    Args:
+        video_data: Raw bytes of the stego video file.
+        pixel_key: 32-byte key (must match the one used for embedding).
+        progress_file: Optional path for progress JSON.
+
+    Returns:
+        Extracted payload bytes (without magic/length prefix), or ``None``
+        if extraction fails (wrong key, no data, corrupted).
+    """
+    debug.print(f"Video LSB extracting from {len(video_data)} byte video")
+    debug.data(pixel_key, "Pixel key for extraction")
+
+    try:
+        # Get video info
+        video_info = get_video_info(video_data)
+        debug.print(
+            f"Video: {video_info.width}x{video_info.height}, "
+            f"{video_info.i_frame_count} I-frames"
+        )
+
+        # Extract I-frames
+        with tempfile.TemporaryDirectory(prefix="stegasoo_video_extract_") as temp_dir_str:
+            temp_dir = Path(temp_dir_str)
+
+            _write_progress(progress_file, 5, 100, "extracting_frames")
+
+            frames, _ = extract_frames(video_data, temp_dir, keyframes_only=True)
+            num_frames = len(frames)
+
+            if num_frames == 0:
+                debug.print("No I-frames found in video")
+                return None
+
+            debug.print(f"Extracted {num_frames} I-frames for extraction")
+
+            _write_progress(progress_file, 20, 100, "extracting_data")
+
+            # First, try to extract from frame 0 to get magic and total length
+            frame_path = frames[0]
+            with open(frame_path, "rb") as f:
+                frame_data = f.read()
+
+            first_chunk = _extract_lsb(frame_data, pixel_key, bits_per_channel=1)
+            if first_chunk is None or len(first_chunk) < 8:
+                debug.print("Failed to extract initial data from first frame")
+                return None
+
+            # Check magic bytes
+            magic = first_chunk[:4]
+            if magic != VIDEO_MAGIC_LSB:
+                debug.print(f"Magic mismatch: got {magic!r}, expected {VIDEO_MAGIC_LSB!r}")
+                return None
+
+            # Get total payload length
+            total_length = struct.unpack(">I", first_chunk[4:8])[0]
+            debug.print(f"Total payload length: {total_length} bytes")
+
+            # Sanity check
+            pixels_per_frame = video_info.width * video_info.height
+            bytes_per_frame = (pixels_per_frame * 3) // 8 - 4  # minus length prefix
+            max_possible = bytes_per_frame * num_frames
+
+            if total_length > max_possible or total_length < 1:
+                debug.print(f"Invalid payload length: {total_length}")
+                return None
+
+            # If the entire payload fits in the first frame, return it directly
+            # This matches the simplified single-frame embedding approach
+            if len(first_chunk) >= 8 + total_length:
+                debug.print("Payload fits in single frame, extracting directly")
+                payload = first_chunk[8 : 8 + total_length]
+            else:
+                # Multi-frame extraction
+                debug.print("Multi-frame extraction needed")
+                frames_needed = (total_length + 8 + bytes_per_frame - 1) // bytes_per_frame
+                frames_needed = min(frames_needed, num_frames)
+
+                # Extract sequentially (matching the embedding approach)
+                extracted_chunks = [first_chunk]
+                for frame_idx in range(1, frames_needed):
+                    frame_path = frames[frame_idx]
+                    with open(frame_path, "rb") as f:
+                        frame_data = f.read()
+
+                    chunk = _extract_lsb(frame_data, pixel_key, bits_per_channel=1)
+                    if chunk:
+                        extracted_chunks.append(chunk)
+
+                    if progress_file and frame_idx % PROGRESS_INTERVAL == 0:
+                        pct = 20 + int((frame_idx / frames_needed) * 70)
+                        _write_progress(progress_file, pct, 100, "extracting_data")
+
+                # Combine chunks
+                combined = b"".join(extracted_chunks)
+
+                if len(combined) < 8 + total_length:
+                    debug.print(
+                        f"Insufficient data: have {len(combined) - 8}, need {total_length}"
+                    )
+                    return None
+
+                payload = combined[8 : 8 + total_length]
+
+            _write_progress(progress_file, 100, 100, "complete")
+
+            debug.print(f"Video LSB successfully extracted {len(payload)} bytes")
+            return payload
+
+    except Exception as e:
+        debug.exception(e, "extract_from_video_lsb")
+        return None
--- a/src/stegasoo/video_utils.py
+++ b/src/stegasoo/video_utils.py
@@ -0,0 +1,732 @@
+"""
+Stegasoo Video Utilities (v4.4.0)
+
+Video format detection, frame extraction, and transcoding for video steganography.
+
+Dependencies:
+- ffmpeg binary: Required for all video operations
+- numpy: For frame data manipulation
+- PIL/Pillow: For frame image handling
+
+Uses ffmpeg for:
+- Format detection and metadata extraction
+- I-frame extraction
+- Video reassembly with FFV1 lossless codec
+"""
+
+from __future__ import annotations
+
+import json
+import os
+import shutil
+import subprocess
+import tempfile
+from pathlib import Path
+
+from .constants import (
+    EMBED_MODE_VIDEO_AUTO,
+    EMBED_MODE_VIDEO_LSB,
+    MAX_VIDEO_DURATION,
+    MAX_VIDEO_FILE_SIZE,
+    MAX_VIDEO_RESOLUTION,
+    MIN_VIDEO_RESOLUTION,
+    VALID_VIDEO_EMBED_MODES,
+    VIDEO_OUTPUT_CODEC,
+    VIDEO_OUTPUT_CONTAINER,
+)
+from .debug import get_logger
+from .exceptions import (
+    UnsupportedVideoFormatError,
+    VideoTranscodeError,
+    VideoValidationError,
+)
+from .models import ValidationResult, VideoCapacityInfo, VideoInfo
+
+logger = get_logger(__name__)
+
+
+# =============================================================================
+# FFMPEG AVAILABILITY
+# =============================================================================
+
+
+def has_ffmpeg_support() -> bool:
+    """Check if ffmpeg is available on the system.
+
+    Returns:
+        True if ffmpeg is found on PATH, False otherwise.
+    """
+    return shutil.which("ffmpeg") is not None
+
+
+def has_ffprobe_support() -> bool:
+    """Check if ffprobe is available on the system.
+
+    Returns:
+        True if ffprobe is found on PATH, False otherwise.
+    """
+    return shutil.which("ffprobe") is not None
+
+
+def _require_ffmpeg() -> None:
+    """Raise error if ffmpeg is not available."""
+    if not has_ffmpeg_support():
+        raise VideoTranscodeError(
+            "ffmpeg is required for video operations. Install ffmpeg on your system."
+        )
+
+
+def _require_ffprobe() -> None:
+    """Raise error if ffprobe is not available."""
+    if not has_ffprobe_support():
+        raise VideoTranscodeError(
+            "ffprobe is required for video metadata. Install ffmpeg on your system."
+        )
+
+
+# =============================================================================
+# FORMAT DETECTION
+# =============================================================================
+
+
+def detect_video_format(video_data: bytes) -> str:
+    """Detect video format from magic bytes.
+
+    Examines the first bytes of video data to identify the container format.
+
+    Magic byte signatures:
+    - MP4/M4V: b"ftyp" at offset 4
+    - MKV/WebM: b"\\x1a\\x45\\xdf\\xa3" (EBML header)
+    - AVI: b"RIFF" at offset 0 + b"AVI " at offset 8
+    - MOV: b"ftyp" with "qt" brand or b"moov"/"mdat" early
+
+    Args:
+        video_data: Raw video file bytes.
+
+    Returns:
+        Format string: "mp4", "mkv", "webm", "avi", "mov", or "unknown".
+    """
+    if len(video_data) < 12:
+        logger.debug("detect_video_format: data too short (%d bytes)", len(video_data))
+        return "unknown"
+
+    # MP4/M4V/MOV: "ftyp" atom at offset 4
+    if video_data[4:8] == b"ftyp":
+        # Check brand for specific type
+        brand = video_data[8:12]
+        if brand in (b"qt  ", b"mqt "):
+            return "mov"
+        if brand in (b"isom", b"iso2", b"mp41", b"mp42", b"avc1", b"M4V "):
+            return "mp4"
+        # Default to mp4 for ftyp containers
+        return "mp4"
+
+    # MKV/WebM: EBML header
+    if video_data[:4] == b"\x1a\x45\xdf\xa3":
+        # Check doctype to distinguish MKV from WebM
+        # WebM uses "webm" doctype, MKV uses "matroska"
+        # Simple heuristic: search for doctype string in first 64 bytes
+        header = video_data[:64]
+        if b"webm" in header.lower():
+            return "webm"
+        return "mkv"
+
+    # AVI: RIFF....AVI
+    if video_data[:4] == b"RIFF" and video_data[8:12] == b"AVI ":
+        return "avi"
+
+    # MOV without ftyp (older format): check for moov/mdat atoms
+    if video_data[4:8] in (b"moov", b"mdat", b"wide", b"free"):
+        return "mov"
+
+    return "unknown"
+
+
+# =============================================================================
+# METADATA EXTRACTION
+# =============================================================================
+
+
+def get_video_info(video_data: bytes) -> VideoInfo:
+    """Extract video metadata from raw video bytes.
+
+    Uses ffprobe to extract detailed video information including
+    resolution, frame rate, duration, codec, and I-frame count.
+
+    Args:
+        video_data: Raw video file bytes.
+
+    Returns:
+        VideoInfo dataclass with video metadata.
+
+    Raises:
+        UnsupportedVideoFormatError: If the format cannot be detected.
+        VideoTranscodeError: If metadata extraction fails.
+    """
+    _require_ffprobe()
+
+    fmt = detect_video_format(video_data)
+    if fmt == "unknown":
+        raise UnsupportedVideoFormatError(
+            "Cannot detect video format. Supported: MP4, MKV, WebM, AVI, MOV."
+        )
+
+    # Write to temp file for ffprobe
+    with tempfile.NamedTemporaryFile(suffix=f".{fmt}", delete=False) as f:
+        f.write(video_data)
+        temp_path = f.name
+
+    try:
+        # Get stream info
+        result = subprocess.run(
+            [
+                "ffprobe",
+                "-v",
+                "quiet",
+                "-print_format",
+                "json",
+                "-show_format",
+                "-show_streams",
+                "-select_streams",
+                "v:0",
+                temp_path,
+            ],
+            capture_output=True,
+            text=True,
+            timeout=60,
+        )
+
+        if result.returncode != 0:
+            raise VideoTranscodeError(f"ffprobe failed: {result.stderr}")
+
+        info = json.loads(result.stdout)
+
+        # Extract video stream info
+        if not info.get("streams"):
+            raise VideoTranscodeError("No video stream found in file")
+
+        stream = info["streams"][0]
+        format_info = info.get("format", {})
+
+        width = int(stream.get("width", 0))
+        height = int(stream.get("height", 0))
+        codec = stream.get("codec_name", "unknown")
+
+        # Parse frame rate (can be "30/1" or "29.97")
+        fps_str = stream.get("r_frame_rate", "0/1")
+        if "/" in fps_str:
+            num, den = fps_str.split("/")
+            fps = float(num) / float(den) if float(den) > 0 else 0.0
+        else:
+            fps = float(fps_str)
+
+        # Get duration
+        duration = float(stream.get("duration", format_info.get("duration", 0)))
+
+        # Get total frames
+        nb_frames = stream.get("nb_frames")
+        if nb_frames:
+            total_frames = int(nb_frames)
+        else:
+            # Estimate from duration and fps
+            total_frames = int(duration * fps) if fps > 0 else 0
+
+        # Get bitrate
+        bitrate = None
+        if format_info.get("bit_rate"):
+            bitrate = int(format_info["bit_rate"])
+
+        # Count I-frames using ffprobe
+        i_frame_count = _count_i_frames(temp_path, timeout=120)
+
+        return VideoInfo(
+            width=width,
+            height=height,
+            fps=fps,
+            duration_seconds=duration,
+            total_frames=total_frames,
+            i_frame_count=i_frame_count,
+            format=fmt,
+            codec=codec,
+            bitrate=bitrate,
+        )
+
+    except json.JSONDecodeError as e:
+        raise VideoTranscodeError(f"Failed to parse ffprobe output: {e}")
+    except subprocess.TimeoutExpired:
+        raise VideoTranscodeError("ffprobe timed out")
+    finally:
+        os.unlink(temp_path)
+
+
+def _count_i_frames(video_path: str, timeout: int = 120) -> int:
+    """Count I-frames (keyframes) in a video file.
+
+    Args:
+        video_path: Path to video file.
+        timeout: Maximum time in seconds.
+
+    Returns:
+        Number of I-frames in the video.
+    """
+    try:
+        result = subprocess.run(
+            [
+                "ffprobe",
+                "-v",
+                "quiet",
+                "-select_streams",
+                "v:0",
+                "-show_entries",
+                "frame=pict_type",
+                "-of",
+                "csv=p=0",
+                video_path,
+            ],
+            capture_output=True,
+            text=True,
+            timeout=timeout,
+        )
+
+        if result.returncode != 0:
+            logger.warning("Failed to count I-frames: %s", result.stderr)
+            return 0
+
+        # Count lines containing 'I'
+        return sum(1 for line in result.stdout.strip().split("\n") if line.strip() == "I")
+
+    except subprocess.TimeoutExpired:
+        logger.warning("I-frame counting timed out")
+        return 0
+    except Exception as e:
+        logger.warning("I-frame counting failed: %s", e)
+        return 0
+
+
+# =============================================================================
+# FRAME EXTRACTION
+# =============================================================================
+
+
+def extract_frames(
+    video_data: bytes,
+    output_dir: Path | None = None,
+    keyframes_only: bool = True,
+) -> tuple[list[Path], VideoInfo]:
+    """Extract frames from video as PNG images.
+
+    Uses ffmpeg to extract frames from the video. By default extracts only
+    I-frames (keyframes) which are more robust to re-encoding.
+
+    Args:
+        video_data: Raw video file bytes.
+        output_dir: Directory to save frames (temp dir if None).
+        keyframes_only: If True, only extract I-frames (keyframes).
+
+    Returns:
+        Tuple of (list of frame paths sorted by frame number, VideoInfo).
+
+    Raises:
+        VideoTranscodeError: If frame extraction fails.
+    """
+    _require_ffmpeg()
+
+    fmt = detect_video_format(video_data)
+    if fmt == "unknown":
+        raise UnsupportedVideoFormatError(
+            "Cannot detect video format. Supported: MP4, MKV, WebM, AVI, MOV."
+        )
+
+    # Get video info first
+    video_info = get_video_info(video_data)
+
+    # Create output directory
+    if output_dir is None:
+        output_dir = Path(tempfile.mkdtemp(prefix="stegasoo_frames_"))
+    else:
+        output_dir = Path(output_dir)
+        output_dir.mkdir(parents=True, exist_ok=True)
+
+    # Write video to temp file
+    with tempfile.NamedTemporaryFile(suffix=f".{fmt}", delete=False) as f:
+        f.write(video_data)
+        video_path = f.name
+
+    try:
+        # Build ffmpeg command
+        cmd = [
+            "ffmpeg",
+            "-i",
+            video_path,
+            "-vsync",
+            "0",
+        ]
+
+        if keyframes_only:
+            # Extract only I-frames
+            cmd.extend(["-vf", "select='eq(pict_type,I)'"])
+
+        # Output as PNG with frame number
+        output_pattern = str(output_dir / "frame_%06d.png")
+        cmd.extend(["-start_number", "0", output_pattern])
+
+        result = subprocess.run(
+            cmd,
+            capture_output=True,
+            text=True,
+            timeout=600,  # 10 minute timeout
+        )
+
+        if result.returncode != 0:
+            raise VideoTranscodeError(f"Frame extraction failed: {result.stderr}")
+
+        # Collect extracted frames
+        frames = sorted(output_dir.glob("frame_*.png"))
+
+        if not frames:
+            raise VideoTranscodeError("No frames were extracted from video")
+
+        logger.info(
+            "Extracted %d %s from video",
+            len(frames),
+            "I-frames" if keyframes_only else "frames",
+        )
+
+        return frames, video_info
+
+    except subprocess.TimeoutExpired:
+        raise VideoTranscodeError("Frame extraction timed out")
+    finally:
+        os.unlink(video_path)
+
+
+# =============================================================================
+# VIDEO REASSEMBLY
+# =============================================================================
+
+
+def reassemble_video(
+    frames: list[Path],
+    original_video_data: bytes,
+    output_path: Path | None = None,
+    fps: float | None = None,
+    audio_data: bytes | None = None,
+) -> bytes:
+    """Reassemble frames back into a video file.
+
+    Creates a new video from the modified frames using FFV1 lossless codec
+    in an MKV container. This preserves the embedded data perfectly.
+
+    Args:
+        frames: List of frame image paths in order.
+        original_video_data: Original video bytes (for audio track extraction).
+        output_path: Optional output path (temp file if None).
+        fps: Frame rate (auto-detected from original if None).
+        audio_data: Optional audio track data to mux in.
+
+    Returns:
+        Video file bytes (MKV container with FFV1 codec).
+
+    Raises:
+        VideoTranscodeError: If reassembly fails.
+    """
+    _require_ffmpeg()
+
+    if not frames:
+        raise VideoTranscodeError("No frames provided for reassembly")
+
+    # Get original video format
+    fmt = detect_video_format(original_video_data)
+
+    if fps is None:
+        # Use a fixed low framerate for I-frame sequences
+        # since I-frames are sparse (typically 1 per 30-60 frames)
+        fps = 1.0  # 1 fps for I-frame only videos
+
+    # Create temp directory for work
+    with tempfile.TemporaryDirectory(prefix="stegasoo_reassemble_") as temp_dir_str:
+        temp_dir = Path(temp_dir_str)
+
+        # Write original video for audio extraction
+        original_path = temp_dir / f"original.{fmt}"
+        original_path.write_bytes(original_video_data)
+
+        # Create frame list file for ffmpeg
+        frame_list = temp_dir / "frames.txt"
+        with open(frame_list, "w") as f:
+            for frame in frames:
+                # FFmpeg concat format
+                f.write(f"file '{frame.absolute()}'\n")
+                f.write(f"duration {1.0 / fps}\n")
+
+        # Output path
+        if output_path is None:
+            output_file = temp_dir / f"output.{VIDEO_OUTPUT_CONTAINER}"
+        else:
+            output_file = Path(output_path)
+
+        # Build ffmpeg command
+        cmd = [
+            "ffmpeg",
+            "-y",  # Overwrite output
+            "-f",
+            "concat",
+            "-safe",
+            "0",
+            "-i",
+            str(frame_list),
+        ]
+
+        # Add audio from original video if available
+        # Check if original has audio
+        has_audio = _video_has_audio(original_path)
+        if has_audio:
+            cmd.extend(["-i", str(original_path)])
+
+        # Video encoding settings (FFV1 lossless)
+        cmd.extend(
+            [
+                "-c:v",
+                VIDEO_OUTPUT_CODEC,
+                "-level",
+                "3",  # FFV1 level 3 for better compression
+                "-coder",
+                "1",  # Range coder
+                "-context",
+                "1",  # Large context
+                "-slicecrc",
+                "1",  # Error detection
+            ]
+        )
+
+        # Audio settings
+        if has_audio:
+            cmd.extend(
+                [
+                    "-map",
+                    "0:v",  # Video from frames
+                    "-map",
+                    "1:a?",  # Audio from original (if exists)
+                    "-c:a",
+                    "copy",  # Copy audio without re-encoding
+                ]
+            )
+
+        cmd.append(str(output_file))
+
+        logger.debug("Running ffmpeg: %s", " ".join(cmd))
+
+        result = subprocess.run(
+            cmd,
+            capture_output=True,
+            text=True,
+            timeout=600,
+        )
+
+        if result.returncode != 0:
+            raise VideoTranscodeError(f"Video reassembly failed: {result.stderr}")
+
+        # Read output
+        return output_file.read_bytes()
+
+
+def _video_has_audio(video_path: Path) -> bool:
+    """Check if a video file has an audio stream.
+
+    Args:
+        video_path: Path to video file.
+
+    Returns:
+        True if video has audio, False otherwise.
+    """
+    try:
+        result = subprocess.run(
+            [
+                "ffprobe",
+                "-v",
+                "quiet",
+                "-select_streams",
+                "a:0",
+                "-show_entries",
+                "stream=index",
+                "-of",
+                "csv=p=0",
+                str(video_path),
+            ],
+            capture_output=True,
+            text=True,
+            timeout=30,
+        )
+        return bool(result.stdout.strip())
+    except Exception:
+        return False
+
+
+# =============================================================================
+# VALIDATION
+# =============================================================================
+
+
+def validate_video(
+    video_data: bytes,
+    name: str = "Video",
+    check_duration: bool = True,
+) -> ValidationResult:
+    """Validate video data for steganography.
+
+    Checks:
+    - Not empty
+    - Not too large (MAX_VIDEO_FILE_SIZE)
+    - Valid video format (detectable via magic bytes)
+    - Duration within limits (MAX_VIDEO_DURATION) if check_duration=True
+    - Resolution within limits (MIN/MAX_VIDEO_RESOLUTION)
+
+    Args:
+        video_data: Raw video file bytes.
+        name: Descriptive name for error messages (default: "Video").
+        check_duration: Whether to enforce duration limit (default: True).
+
+    Returns:
+        ValidationResult with video info in details on success.
+    """
+    if not video_data:
+        return ValidationResult.error(f"{name} is required")
+
+    if len(video_data) > MAX_VIDEO_FILE_SIZE:
+        size_gb = len(video_data) / (1024**3)
+        max_gb = MAX_VIDEO_FILE_SIZE / (1024**3)
+        return ValidationResult.error(
+            f"{name} too large ({size_gb:.1f} GB). Maximum: {max_gb:.0f} GB"
+        )
+
+    # Detect format
+    fmt = detect_video_format(video_data)
+    if fmt == "unknown":
+        return ValidationResult.error(
+            f"Could not detect {name} format. " "Supported formats: MP4, MKV, WebM, AVI, MOV."
+        )
+
+    # Check ffmpeg availability
+    if not has_ffmpeg_support():
+        return ValidationResult.error(
+            "ffmpeg is required for video processing. Please install ffmpeg."
+        )
+
+    # Extract metadata for further validation
+    try:
+        info = get_video_info(video_data)
+    except (VideoTranscodeError, UnsupportedVideoFormatError) as e:
+        return ValidationResult.error(f"Could not read {name}: {e}")
+    except Exception as e:
+        return ValidationResult.error(f"Could not read {name}: {e}")
+
+    # Check duration
+    if check_duration and info.duration_seconds > MAX_VIDEO_DURATION:
+        return ValidationResult.error(
+            f"{name} too long ({info.duration_seconds:.1f}s). "
+            f"Maximum: {MAX_VIDEO_DURATION}s ({MAX_VIDEO_DURATION // 60} minutes)"
+        )
+
+    # Check resolution
+    if info.width < MIN_VIDEO_RESOLUTION[0] or info.height < MIN_VIDEO_RESOLUTION[1]:
+        return ValidationResult.error(
+            f"{name} resolution too small ({info.width}x{info.height}). "
+            f"Minimum: {MIN_VIDEO_RESOLUTION[0]}x{MIN_VIDEO_RESOLUTION[1]}"
+        )
+
+    if info.width > MAX_VIDEO_RESOLUTION[0] or info.height > MAX_VIDEO_RESOLUTION[1]:
+        return ValidationResult.error(
+            f"{name} resolution too large ({info.width}x{info.height}). "
+            f"Maximum: {MAX_VIDEO_RESOLUTION[0]}x{MAX_VIDEO_RESOLUTION[1]}"
+        )
+
+    # Check I-frame count
+    if info.i_frame_count < 1:
+        return ValidationResult.error(f"{name} has no I-frames (keyframes) for embedding")
+
+    return ValidationResult.ok(
+        width=info.width,
+        height=info.height,
+        fps=info.fps,
+        duration=info.duration_seconds,
+        total_frames=info.total_frames,
+        i_frame_count=info.i_frame_count,
+        format=info.format,
+        codec=info.codec,
+        bitrate=info.bitrate,
+    )
+
+
+def require_valid_video(video_data: bytes, name: str = "Video") -> None:
+    """Validate video, raising VideoValidationError on failure.
+
+    Args:
+        video_data: Raw video file bytes.
+        name: Descriptive name for error messages.
+
+    Raises:
+        VideoValidationError: If validation fails.
+    """
+    result = validate_video(video_data, name)
+    if not result.is_valid:
+        raise VideoValidationError(result.error_message)
+
+
+def validate_video_embed_mode(mode: str) -> ValidationResult:
+    """Validate video embedding mode string.
+
+    Args:
+        mode: Embedding mode to validate.
+
+    Returns:
+        ValidationResult with mode in details on success.
+    """
+    valid_modes = VALID_VIDEO_EMBED_MODES | {EMBED_MODE_VIDEO_AUTO}
+    if mode not in valid_modes:
+        return ValidationResult.error(
+            f"Invalid video embed_mode: '{mode}'. "
+            f"Valid options: {', '.join(sorted(valid_modes))}"
+        )
+    return ValidationResult.ok(mode=mode)
+
+
+# =============================================================================
+# CAPACITY CALCULATION
+# =============================================================================
+
+
+def calculate_video_capacity(video_data: bytes, embed_mode: str = EMBED_MODE_VIDEO_LSB) -> VideoCapacityInfo:
+    """Calculate steganographic capacity for a video file.
+
+    Capacity is based on I-frames only (keyframes). Each I-frame provides
+    capacity similar to an image of the same dimensions.
+
+    Args:
+        video_data: Raw video file bytes.
+        embed_mode: Embedding mode (currently only video_lsb).
+
+    Returns:
+        VideoCapacityInfo with capacity details.
+    """
+    info = get_video_info(video_data)
+
+    # Calculate capacity per I-frame
+    # RGB image: 3 bits per pixel (1 bit per channel) / 8 = 0.375 bytes per pixel
+    # Subtract overhead per frame for header
+    pixels_per_frame = info.width * info.height
+    bytes_per_frame = (pixels_per_frame * 3) // 8  # 3 bits per pixel
+
+    # Total capacity across all I-frames
+    # Subtract 70 bytes overhead for the encrypted payload header
+    from .steganography import ENCRYPTION_OVERHEAD
+
+    total_capacity = (bytes_per_frame * info.i_frame_count) - ENCRYPTION_OVERHEAD
+
+    return VideoCapacityInfo(
+        total_frames=info.total_frames,
+        i_frames=info.i_frame_count,
+        usable_capacity_bytes=max(0, total_capacity),
+        embed_mode=embed_mode,
+        resolution=(info.width, info.height),
+        duration_seconds=info.duration_seconds,
+    )
--- a/tests/test_stegasoo.py
+++ b/tests/test_stegasoo.py
@@ -451,3 +451,231 @@ class TestEdgeCases:
        )

        assert decoded.message == special_msg
+
+
+# =============================================================================
+# VIDEO STEGANOGRAPHY TESTS (v4.4.0)
+# =============================================================================
+
+
+@pytest.fixture
+def test_video_bytes():
+    """Create a minimal test video using ffmpeg.
+
+    Creates a 2-second test video with solid color frames.
+    Returns None if ffmpeg is not available.
+    """
+    import shutil
+    import subprocess
+    import tempfile
+
+    if not shutil.which("ffmpeg"):
+        return None
+
+    with tempfile.NamedTemporaryFile(suffix=".mp4", delete=False) as f:
+        output_path = f.name
+
+    try:
+        # Create a simple 2-second video with colored frames
+        # Using lavfi (libavfilter) to generate test pattern
+        result = subprocess.run(
+            [
+                "ffmpeg",
+                "-y",
+                "-f",
+                "lavfi",
+                "-i",
+                "color=c=blue:s=320x240:d=2:r=10",
+                "-c:v",
+                "libx264",
+                "-pix_fmt",
+                "yuv420p",
+                "-g",
+                "5",  # GOP size - creates I-frames every 5 frames
+                output_path,
+            ],
+            capture_output=True,
+            timeout=30,
+        )
+
+        if result.returncode != 0:
+            return None
+
+        with open(output_path, "rb") as f:
+            video_data = f.read()
+
+        return video_data
+    except Exception:
+        return None
+    finally:
+        import os
+
+        try:
+            os.unlink(output_path)
+        except OSError:
+            pass
+
+
+class TestVideoSupport:
+    """Test video steganography support detection."""
+
+    def test_video_support_flag_exists(self):
+        """HAS_VIDEO_SUPPORT flag should exist."""
+        assert hasattr(stegasoo, "HAS_VIDEO_SUPPORT")
+        assert isinstance(stegasoo.HAS_VIDEO_SUPPORT, bool)
+
+    def test_video_constants_exist(self):
+        """Video-related constants should exist."""
+        assert hasattr(stegasoo, "EMBED_MODE_VIDEO_LSB")
+        assert hasattr(stegasoo, "EMBED_MODE_VIDEO_AUTO")
+
+
+@pytest.mark.skipif(
+    not stegasoo.HAS_VIDEO_SUPPORT,
+    reason="Video support not available (ffmpeg or dependencies missing)",
+)
+class TestVideoFormatDetection:
+    """Test video format detection."""
+
+    def test_detect_video_format_mp4(self, test_video_bytes):
+        """Should detect MP4 format from magic bytes."""
+        if test_video_bytes is None:
+            pytest.skip("Could not create test video")
+
+        from stegasoo import detect_video_format
+
+        fmt = detect_video_format(test_video_bytes)
+        assert fmt in ("mp4", "mov")
+
+    def test_detect_video_format_unknown(self):
+        """Should return 'unknown' for non-video data."""
+        from stegasoo import detect_video_format
+
+        fmt = detect_video_format(b"not a video")
+        assert fmt == "unknown"
+
+
+@pytest.mark.skipif(
+    not stegasoo.HAS_VIDEO_SUPPORT,
+    reason="Video support not available (ffmpeg or dependencies missing)",
+)
+class TestVideoInfo:
+    """Test video metadata extraction."""
+
+    def test_get_video_info(self, test_video_bytes):
+        """Should extract video metadata."""
+        if test_video_bytes is None:
+            pytest.skip("Could not create test video")
+
+        from stegasoo import get_video_info
+
+        info = get_video_info(test_video_bytes)
+
+        assert info.width == 320
+        assert info.height == 240
+        assert info.fps > 0
+        assert info.duration_seconds > 0
+        assert info.total_frames > 0
+        assert info.format in ("mp4", "mov")
+
+    def test_validate_video(self, test_video_bytes):
+        """Should validate video data."""
+        if test_video_bytes is None:
+            pytest.skip("Could not create test video")
+
+        from stegasoo import validate_video
+
+        result = validate_video(test_video_bytes, check_duration=False)
+
+        assert result.is_valid
+        assert result.details.get("format") in ("mp4", "mov")
+
+
+@pytest.mark.skipif(
+    not stegasoo.HAS_VIDEO_SUPPORT,
+    reason="Video support not available (ffmpeg or dependencies missing)",
+)
+class TestVideoCapacity:
+    """Test video capacity calculation."""
+
+    def test_calculate_video_capacity(self, test_video_bytes):
+        """Should calculate steganographic capacity."""
+        if test_video_bytes is None:
+            pytest.skip("Could not create test video")
+
+        from stegasoo import calculate_video_capacity
+
+        capacity_info = calculate_video_capacity(test_video_bytes)
+
+        assert capacity_info.total_frames > 0
+        assert capacity_info.i_frames > 0
+        assert capacity_info.usable_capacity_bytes > 0
+        assert capacity_info.embed_mode == "video_lsb"
+        assert capacity_info.resolution == (320, 240)
+
+
+@pytest.mark.skipif(
+    not stegasoo.HAS_VIDEO_SUPPORT,
+    reason="Video support not available (ffmpeg or dependencies missing)",
+)
+class TestVideoEncodeDecode:
+    """Test video steganography round-trip."""
+
+    def test_video_roundtrip(self, test_video_bytes, ref_bytes):
+        """Test encoding and decoding a message in video."""
+        if test_video_bytes is None:
+            pytest.skip("Could not create test video")
+
+        from stegasoo import decode_video, encode_video
+
+        message = "Secret video message!"
+
+        # Encode
+        stego_video, stats = encode_video(
+            message=message,
+            reference_photo=ref_bytes,
+            carrier_video=test_video_bytes,
+            passphrase=TEST_PASSPHRASE,
+            pin=TEST_PIN,
+        )
+
+        assert stego_video
+        assert len(stego_video) > 0
+        assert stats.frames_modified > 0
+        assert stats.codec == "ffv1"  # Should use lossless codec
+
+        # Decode
+        result = decode_video(
+            stego_video=stego_video,
+            reference_photo=ref_bytes,
+            passphrase=TEST_PASSPHRASE,
+            pin=TEST_PIN,
+        )
+
+        assert result.is_text
+        assert result.message == message
+
+    def test_video_wrong_passphrase_fails(self, test_video_bytes, ref_bytes):
+        """Decoding with wrong passphrase should fail."""
+        if test_video_bytes is None:
+            pytest.skip("Could not create test video")
+
+        from stegasoo import decode_video, encode_video
+
+        message = "Secret video message!"
+
+        stego_video, _ = encode_video(
+            message=message,
+            reference_photo=ref_bytes,
+            carrier_video=test_video_bytes,
+            passphrase=TEST_PASSPHRASE,
+            pin=TEST_PIN,
+        )
+
+        with pytest.raises(Exception):
+            decode_video(
+                stego_video=stego_video,
+                reference_photo=ref_bytes,
+                passphrase="wrong passphrase words here",
+                pin=TEST_PIN,
+            )