Add extract-then-strip EXIF pipeline for attestation intake

Resolves the tension between steganography (strip everything to
protect sources) and attestation (preserve evidence of provenance):

- New soosef.metadata module with extract_and_classify() and
  extract_strip_pipeline() — classifies EXIF fields as evidentiary
  (GPS, timestamp — valuable for proving provenance) vs dangerous
  (device serial, firmware — could identify the source)
- Drop box now uses extract-then-strip: attests ORIGINAL bytes (hash
  matches what source submitted), extracts evidentiary EXIF into
  attestation metadata, strips dangerous fields, stores clean copy
- Attest route gains strip_device option: when enabled, includes
  GPS/timestamp in attestation but excludes device serial/firmware
- Stego encode unchanged: still strips all metadata from carriers
  (correct for steganography threat model)

The key insight: for stego, the carrier is a vessel (strip everything).
For attestation, EXIF is the evidence (extract, classify, preserve
selectively). Both hashes (original + stripped) are recorded so the
relationship between raw submission and stored copy is provable.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontends/web/blueprints/attest.py

@@ -141,6 +141,24 @@ def attest():
                 metadata["investigation"] = investigation
 
             auto_exif = request.form.get("auto_exif", "on") == "on"
+            strip_device = request.form.get("strip_device", "on") == "on"
+
+            # Extract-then-classify: get evidentiary metadata before attestation
+            # so user can control what's included
+            if auto_exif and strip_device:
+                from soosef.metadata import extract_and_classify
+
+                extraction = extract_and_classify(image_data)
+                # Merge evidentiary fields (GPS, timestamp) but exclude
+                # dangerous device fields (serial, firmware version)
+                for key, value in extraction.evidentiary.items():
+                    if key not in metadata:  # User metadata takes precedence
+                        if hasattr(value, "isoformat"):
+                            metadata[f"exif_{key}"] = value.isoformat()
+                        elif isinstance(value, dict):
+                            metadata[f"exif_{key}"] = value
+                        else:
+                            metadata[f"exif_{key}"] = str(value)
 
             # Create the attestation
             from soosef.verisoo.attestation import create_attestation
@@ -149,7 +167,7 @@ def attest():
                 image_data=image_data,
                 private_key=private_key,
                 metadata=metadata if metadata else None,
-                auto_exif=auto_exif,
+                auto_exif=auto_exif and not strip_device,  # Full EXIF only if not stripping device
             )
 
             # Store in the append-only log

frontends/web/blueprints/dropbox.py

@@ -120,42 +120,55 @@ def upload(token):
             if token_data["used"] >= token_data["max_files"]:
                 break
 
-            file_data = f.read()
-            if not file_data:
+            raw_data = f.read()
+            if not raw_data:
                 continue
 
-            # Strip EXIF metadata
-            try:
-                import io
+            # Extract-then-strip pipeline:
+            # 1. Extract EXIF into attestation metadata (evidentiary fields)
+            # 2. Attest the ORIGINAL bytes (hash matches what source submitted)
+            # 3. Strip metadata from the stored copy (protect source device info)
+            from soosef.metadata import extract_strip_pipeline
 
-                from PIL import Image
+            extraction, stripped_data = extract_strip_pipeline(raw_data)
 
-                img = Image.open(io.BytesIO(file_data))
-                clean = io.BytesIO()
-                img.save(clean, format=img.format or "PNG")
-                file_data = clean.getvalue()
-            except Exception:
-                pass  # Not an image, or Pillow can't handle it — keep as-is
+            # SHA-256 of what the source actually submitted
+            sha256 = extraction.original_sha256
 
-            # Compute SHA-256
-            sha256 = hashlib.sha256(file_data).hexdigest()
-
-            # Save file
+            # Save the stripped copy for display/storage (no device fingerprint on disk)
             dest = _TOKEN_DIR / f"{sha256[:16]}_{f.filename}"
-            dest.write_bytes(file_data)
+            dest.write_bytes(stripped_data)
 
-            # Auto-attest
-            chain_index = None
+            # Auto-attest the ORIGINAL bytes so the attestation hash matches
+            # what the source submitted. Evidentiary EXIF (GPS, timestamp)
+            # is preserved in the attestation metadata; dangerous fields
+            # (device serial) are excluded.
             try:
                 from soosef.verisoo.attestation import create_attestation
-                from soosef.verisoo.storage import LocalStorage
 
                 from blueprints.attest import _get_private_key, _get_storage
 
+                attest_metadata = {
+                    "source": "dropbox",
+                    "label": token_data["label"],
+                    "stripped_sha256": extraction.stripped_sha256,
+                }
+                # Include evidentiary EXIF in attestation (GPS, timestamp)
+                for key, value in extraction.evidentiary.items():
+                    if hasattr(value, "isoformat"):
+                        attest_metadata[key] = value.isoformat()
+                    elif hasattr(value, "__dataclass_fields__"):
+                        from dataclasses import asdict
+                        attest_metadata[key] = asdict(value)
+                    elif isinstance(value, dict):
+                        attest_metadata[key] = value
+                    else:
+                        attest_metadata[key] = str(value)
+
                 private_key = _get_private_key()
                 if private_key:
                     attestation = create_attestation(
-                        file_data, private_key, metadata={"source": "dropbox", "label": token_data["label"]}
+                        raw_data, private_key, metadata=attest_metadata
                     )
                     storage = _get_storage()
                     storage.append_record(attestation.record)

src/soosef/metadata.py

@@ -0,0 +1,149 @@
+"""
+Metadata handling pipeline: extract, classify, strip.
+
+Implements the extract-then-strip model for image metadata:
+1. Extract all EXIF/metadata from raw bytes
+2. Classify fields as evidentiary (GPS, timestamp) or dangerous (device serial)
+3. Preserve evidentiary fields in a sidecar record
+4. Strip all metadata from the stored/display copy
+
+This resolves the tension between steganography (strip everything to protect
+sources) and attestation (preserve everything to prove provenance).
+"""
+
+from __future__ import annotations
+
+import io
+from dataclasses import dataclass, field
+
+
+# Fields classified by risk level:
+# - EVIDENTIARY: valuable for proving provenance (location, time)
+# - DANGEROUS: could identify the source's device or person
+# - NEUTRAL: neither helpful nor harmful
+
+EVIDENTIARY_FIELDS = {
+    "captured_at",      # When the photo was taken
+    "location",         # GPS coordinates (proves location)
+    "width",            # Image dimensions
+    "height",
+}
+
+DANGEROUS_FIELDS = {
+    "device",           # Camera make/model/software/serial — fingerprints the source
+}
+
+# Granular device sub-fields
+DANGEROUS_DEVICE_FIELDS = {
+    "serial_hash",      # Device serial number hash
+    "software",         # Firmware version (narrows device population)
+}
+
+SAFE_DEVICE_FIELDS = {
+    "make",             # Broad manufacturer (e.g., "Apple") — low risk
+    "model",            # Device model — moderate risk, useful for sensor analysis
+}
+
+
+@dataclass
+class MetadataExtraction:
+    """Result of extracting and classifying image metadata."""
+
+    # Raw EXIF data as extracted by verisoo
+    raw_exif: dict = field(default_factory=dict)
+
+    # Classified subsets
+    evidentiary: dict = field(default_factory=dict)  # Safe to include in attestation
+    dangerous: dict = field(default_factory=dict)     # Should be redacted or stored separately
+    all_fields: dict = field(default_factory=dict)    # Everything (for secure sidecar)
+
+    # Hashes
+    original_sha256: str = ""   # Hash of original bytes (with EXIF)
+    stripped_sha256: str = ""   # Hash of stripped bytes (without EXIF)
+
+
+def extract_and_classify(image_data: bytes) -> MetadataExtraction:
+    """Extract EXIF metadata from image bytes and classify by risk level.
+
+    Returns a MetadataExtraction with evidentiary (safe for attestation)
+    and dangerous (should be redacted) fields separated.
+    """
+    import hashlib
+
+    result = MetadataExtraction()
+    result.original_sha256 = hashlib.sha256(image_data).hexdigest()
+
+    # Extract raw EXIF via verisoo's extractor
+    try:
+        from soosef.verisoo.attestation import extract_exif_metadata
+
+        raw = extract_exif_metadata(image_data)
+        result.raw_exif = raw
+        result.all_fields = dict(raw)
+    except ImportError:
+        return result
+
+    # Classify fields
+    for key in EVIDENTIARY_FIELDS:
+        if key in raw:
+            result.evidentiary[key] = raw[key]
+
+    # Handle device fields granularly
+    device = raw.get("device")
+    if device:
+        # Keep safe device fields in evidentiary
+        safe_device = {}
+        dangerous_device = {}
+        device_dict = device if isinstance(device, dict) else {}
+        if hasattr(device, "__dataclass_fields__"):
+            from dataclasses import asdict
+            device_dict = asdict(device)
+
+        for k, v in device_dict.items():
+            if v is None:
+                continue
+            if k in DANGEROUS_DEVICE_FIELDS:
+                dangerous_device[k] = v
+            elif k in SAFE_DEVICE_FIELDS:
+                safe_device[k] = v
+            else:
+                safe_device[k] = v  # Default to safe for unknown fields
+
+        if safe_device:
+            result.evidentiary["device"] = safe_device
+        if dangerous_device:
+            result.dangerous["device"] = dangerous_device
+
+    return result
+
+
+def strip_metadata(image_data: bytes) -> bytes:
+    """Strip all metadata from image bytes. Returns clean image bytes."""
+    import hashlib
+
+    try:
+        from PIL import Image
+
+        img = Image.open(io.BytesIO(image_data))
+        clean = io.BytesIO()
+        # Re-save without copying info/exif — strips all metadata
+        img.save(clean, format=img.format or "PNG")
+        return clean.getvalue()
+    except Exception:
+        # Not an image or Pillow can't handle it — return as-is
+        return image_data
+
+
+def extract_strip_pipeline(image_data: bytes) -> tuple[MetadataExtraction, bytes]:
+    """Full extract-then-strip pipeline.
+
+    Returns:
+        (extraction, stripped_bytes) — extraction contains classified metadata,
+        stripped_bytes is the clean image for storage/display.
+    """
+    import hashlib
+
+    extraction = extract_and_classify(image_data)
+    stripped = strip_metadata(image_data)
+    extraction.stripped_sha256 = hashlib.sha256(stripped).hexdigest()
+    return extraction, stripped