Match the vendor registries against metadata, not coded pixels

The registries are raw substrings and the shortest tokens are four and five bytes
(`Bria`, `Adobe`, `Canva`). Over a megabyte of compressed pixel data such a sequence
turns up by chance: `Bria` matched inside the entropy-coded scan of 4 of 14,707
corpus JPEGs, in none of which the manifest names Bria. The rate is what a four-byte
pattern predicts on that corpus, and the Bria entry asserts AI, so a chance match can
declare an image AI-generated rather than merely mislabel its signer.

`_metadata_region` gives the registry scans the container's metadata: JPEG marker
segments before the coded scan, PNG chunks other than IDAT, both trailers, and
whatever `scan_head` appended past the window. Every other check keeps the full
buffer -- their markers are long and distinctive. A container that does not parse is
returned whole, since dropping real evidence to avoid a chance match is the wrong
trade. `c2pa_marker_in` already refuses a bare `c2pa` substring for this reason;
this is the same defence for the registries.

Verified the way the rules require for a change that MOVES a verdict: over all 48,905
corpus images, exactly one file changed, the one named in advance, from
"C2PA Content Credentials (Bria Artificial Intelligence)" to "(unknown signer)".
Record-path parity is 0 disagreements, down from 75 when this work started.

The audit's own baseline comparison is fixed here too. It compared confidence and
signals only, and so reported "0 changed" for the run whose single intended
correction was a watermark line -- the change it exists to show.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/remove_ai_watermarks/identify.py

@@ -22,6 +22,7 @@ from __future__ import annotations
 import base64
 import itertools
 import logging
+import struct
 from dataclasses import dataclass, field
 from typing import TYPE_CHECKING, Any, cast
 
@@ -479,6 +480,72 @@ _DEVICE_C2PA_PLATFORM: tuple[tuple[bytes, str], ...] = (
 )
 
 
+def _metadata_region(head: bytes) -> bytes:
+    """The part of the scan buffer that can hold metadata, with the coded pixels cut out.
+
+    The vendor registries are matched as raw substrings, and the shortest tokens are
+    four and five bytes (``Bria``, ``Adobe``, ``Canva``). Over a megabyte of compressed
+    pixel data a four-byte sequence appears by chance about once in three thousand
+    images -- measured: ``Bria`` matched inside the entropy-coded scan of 4 of 14,707
+    corpus JPEGs, in none of which the manifest names Bria. That is not a cosmetic
+    mislabel, because the Bria entry carries ``asserts_ai``: a chance match can declare
+    an image AI-generated.
+
+    ``c2pa_marker_in`` already refuses a bare ``c2pa`` substring for the same reason.
+    This is the same defence for the registries: they see the container's metadata and
+    not its pixels.
+
+    JPEG keeps the marker segments before the entropy-coded scan, PNG every chunk but
+    ``IDAT``, and both keep the trailer past the end marker. Anything ``scan_head``
+    APPENDED past the window is metadata by construction (late chunks, boxes, decoder
+    text), so it is always kept and never walked -- walking it is what produced 11 MB
+    records and a phantom AIGC signal in the record collector.
+
+    Trimming happens only when the container actually parses: a JPEG whose marker walk
+    reaches the coded scan, a PNG whose chunk walk reaches ``IDAT``. Anything else --
+    a malformed container, a synthetic blob, a format with no walker here -- is
+    returned whole. Cutting a buffer this function did not understand would drop real
+    evidence to avoid a chance match, which is the wrong way round.
+    """
+    raw, appended = head[:_SCAN_BYTES], head[_SCAN_BYTES:]
+    if raw[:2] == b"\xff\xd8":
+        index, size = 2, len(raw)
+        while index + 1 < size:
+            if raw[index] != 0xFF:
+                return head  # not a marker boundary: the walk is lost, keep everything
+            marker = raw[index + 1]
+            if marker in (0xDA, 0xD9):  # SOS / EOI: the coded scan follows
+                end = raw.rfind(b"\xff\xd9")
+                return raw[:index] + (raw[end + 2 :] if end >= index else b"") + appended
+            if 0xD0 <= marker <= 0xD7 or marker == 0x01:
+                index += 2
+                continue
+            if index + 4 > size:
+                break
+            length = int.from_bytes(raw[index + 2 : index + 4], "big")
+            if length < 2 or index + 2 + length > size:
+                break
+            index += 2 + length
+        return head  # ran out of buffer before the scan: nothing was skipped anyway
+    if raw[:8] == b"\x89PNG\r\n\x1a\n":
+        out = bytearray()
+        position, size, saw_idat = 8, len(raw), False
+        while position + 8 <= size:
+            (length,) = struct.unpack(">I", raw[position : position + 4])
+            chunk_type = raw[position + 4 : position + 8]
+            start = position + 8
+            if chunk_type == b"IDAT":
+                saw_idat = True
+            else:
+                out += chunk_type + raw[start : start + min(length, size - start)]
+            position = start + length + 4
+            if chunk_type == b"IEND":
+                out += raw[position:]
+                break
+        return bytes(out) + appended if saw_idat else head
+    return head
+
+
 def _first_token_match(head: bytes, table: tuple[tuple[bytes, str], ...]) -> str | None:
     """First platform in ``table`` whose token appears in ``head``, else None.
 
@@ -915,12 +982,16 @@ def _identify_from_evidence(
     # score, the latter can be a by-product of our own SDXL removal pass, so
     # neither is a trustworthy "the generator stamped its identity" claim.
     ai_vendor_claims: dict[str, str] = {}
-    camera_label = _device_platform(head)
-    signer_label = _signer_platform(head)
+    # The vendor registries match short raw substrings, so they read the container's
+    # metadata rather than its pixels -- see `_metadata_region`. Every other check
+    # below keeps the full buffer: their markers are long and distinctive.
+    region = _metadata_region(head)
+    camera_label = _device_platform(region)
+    signer_label = _signer_platform(region)
 
     # ── C2PA Content Credentials ────────────────────────────────────
     has_c2pa = bool(info) or c2pa_marker_in(head)
-    issuers = [info["issuer"]] if info.get("issuer") else _issuers_in(head)
+    issuers = [info["issuer"]] if info.get("issuer") else _issuers_in(region)
     # Full AI generation (trainedAlgorithmicMedia) vs an AI-enhanced real photo
     # (compositeWithTrainedAlgorithmicMedia). The structured kind is parsed once in
     # _internal.c2pa._populate_registry_fields (covers PNG + any container the c2pa-python
@@ -947,7 +1018,7 @@ def _identify_from_evidence(
     generator = (
         info.get("claim_generator")
         or cbor_text_after(head, b"claim_generator")
-        or (", ".join(tools) if (tools := _ai_tools_in(head)) else None)
+        or (", ".join(tools) if (tools := _ai_tools_in(region)) else None)
     )
     # Platform: a distinctive device/camera token in the manifest wins (it is the
     # signer/producer), then an editing-app/AI-device signer (Samsung Galaxy,
@@ -998,7 +1069,7 @@ def _identify_from_evidence(
     # reusing the derived `has_c2pa` / `c2pa_source_kind` above, which are broader:
     # the file path's answer must not move.
     trained_source = b"trainedAlgorithmicMedia" in head or b"TrainedAlgorithmicMedia" in head
-    if not synthid and trained_source and c2pa_marker_in(head) and (vendors := synthid_vendors_in(head)):
+    if not synthid and trained_source and c2pa_marker_in(head) and (vendors := synthid_vendors_in(region)):
         synthid = synthid_verdict(", ".join(vendors))
     if synthid:
         watermarks.append(f"SynthID watermark, inferred from C2PA metadata ({synthid})")
@@ -1011,7 +1082,7 @@ def _identify_from_evidence(
     # ── C2PA soft-binding: a named forensic/third-party watermark vendor ─
     # (Adobe TrustMark, Digimarc, Imatag, ...). Present in the manifest even when
     # the watermark itself can't be decoded; names whose watermark stamped the pixels.
-    soft_binding = meta.get("soft_binding") or (", ".join(v) if (v := soft_binding_vendors_in(head)) else None)
+    soft_binding = meta.get("soft_binding") or (", ".join(v) if (v := soft_binding_vendors_in(region)) else None)
     if soft_binding:
         signals.append(Signal("soft_binding", f"C2PA soft binding: {soft_binding}", "high"))
         watermarks.append(f"Forensic watermark soft binding ({soft_binding})")