feat: Implement live video section 19.4 — Verifiable Segment Info

[N1Knight]

Summary

⚠️ Depends on the §19.3 PR — please merge that one first. This PR adds only the section 19.4 work on top.

Implements C2PA Live Video §19.4 — Verifiable Segment Info (VSI), the second validation method for live video streams. Instead of embedding a full C2PA manifest in each segment, the init segment carries a c2pa.session-keys assertion with one or more session keys (COSE_Key / RFC 9052), and each media segment carries a COSE_Sign1_Tagged signature in an ISO BMFF emsg box.

This PR also wires up full VSI signing support using caller‑provided Ed25519 session keys, including incremental signing across process runs via resume_from_segment and CLI flags --session-key and --previous-segment.

Also implements §19.7.3 signerBinding verification: each session key's COSE_Sign1_Tagged signerBinding is now cryptographically verified against the manifest signer's end-entity X.509 certificate, ensuring session keys were issued by the same entity that signed the manifest.

High-Level Changes

• VSI & Session Key Validation (sdk/src/live_video/): Core logic to validate VSI fields (sequenceNumber, bmffHash, signerBinding), enforce sequence bounds, and verify session keys (including mandatory kid checks).

• Cryptographic Bindings (session_key_validation.rs): Implements signerBinding verification. Extracts the SPKI from the end-entity certificate (via x509-parser) and verifies the COSE_Sign1_Tagged signature produced by the session key.

• New Assertions (sdk/src/assertions/session_keys.rs): Adds typed structs and serde support for the c2pa.session-keys assertion.

• VSI signing from explicit session keys (sdk/src/live_video/vsi_signing.rs): Replaces the ephemeral key mode with LiveVideoVsiSigner::from_signing_key, which takes a caller‑provided Ed25519 session key and builds a spec‑compliant c2pa.session-keys assertion (including signerBinding) into the init segment.

• Incremental VSI signing (resume_from_segment): Adds LiveVideoVsiSigner::resume_from_segment, which parses a previously signed VSI emsg box, reads the sequenceNumber, and sets next_sequence_number = sequenceNumber + 1 so segments can be signed incrementally across separate process runs.

• CLI wiring for VSI (cli/src/live_video_sign.rs, cli/src/main.rs): The live-video-sign subcommand now requires --session-key when --method vsi, always re‑signs the init segment for VSI, and supports --previous-segment to resume the VSI sequence number from an earlier segment using the same session key.

Key design decisions

• COSE_Key integer map keys: The c2pa.session-keys assertion stores keys as raw c2pa_cbor::Value with integer CBOR keys (RFC 9052 §7.1). The SDK's manifest read path transcodes CBOR through JSON, converting integer keys to text strings and byte values to integer arrays. cose_key.rs handles both representations transparently.

• Session key resolution: validate_verifiable_segment_info finds the matching session key by comparing the kid in the COSE_Sign1 unprotected header against the keys registered via validate_session_keys.

• signerBinding bytes extraction: The internal CBOR→JSON→CBOR roundtrip (via ManifestAssertion::as_json_object) loses the CBOR Bytes type. extract_signer_binding_bytes handles all resulting representations (Value::Bytes, Value::Text base64, Value::Array of integers) to support both direct CBOR access and the current JSON-transcoded path.

• Certificate from manifest: The end-entity certificate for signerBinding verification is extracted from the manifest's own signature info (Manifest::signature_info().cert_chain()), requiring no additional CLI arguments.

• bmffHash verification: The bmffHash field in the VSI SegmentInfoMap is deserialized into the existing BmffHash struct via c2pa_cbor::value::from_value. Verification reuses BmffHash::verify_in_memory_hash, which internally calls bmff_to_jumbf_exclusions to resolve /emsg xpath exclusions into byte ranges, then hashes the segment data excluding those ranges. When bmffHash is Null, verification is skipped for backward compatibility.

Test

• cargo test --features="file_io" -p c2pa -- live_video

End to end test

The caller provides an Ed25519 session key (--session-key, raw 32-byte seed). The init segment carries a c2pa.session-keys assertion; each media segment receives a COSE_Sign1 in an emsg box signed with that session key.

--init and --session-key are both required for VSI.

Sign all segments at once

cargo run --bin c2patool -- live_video_demo/input live-video-sign \
  --segments_glob "seg_*.m4s" \
  --output live_video_demo/signed_vsi \
  --manifest live_video_demo/manifest.json \
  --init init.mp4 \
  --method vsi \
  --session-key live_video_demo/ed25519_session.key

Sign segments one at a time (simulating a live ingest pipeline)

First segment:

cargo run --bin c2patool -- live_video_demo/input live-video-sign \
  --segments_glob "seg_001.m4s" \
  --output live_video_demo/signed_vsi \
  --manifest live_video_demo/manifest.json \
  --init init.mp4 \
  --method vsi \
  --session-key live_video_demo/ed25519_session.key

Subsequent segments (use --previous-segment to resume the sequence number):

cargo run --bin c2patool -- live_video_demo/input live-video-sign \
  --segments_glob "seg_002.m4s" \
  --output live_video_demo/signed_vsi \
  --manifest live_video_demo/manifest.json \
  --init init.mp4 \
  --method vsi \
  --session-key live_video_demo/ed25519_session.key \
  --previous-segment live_video_demo/signed_vsi/seg_001.m4s

Verify all segments

The validator auto-detects the method from the init segment manifest
(c2pa.session-keys assertion):

cargo run --bin c2patool -- live_video_demo/signed_vsi/init.mp4 live-video \
  --segments_glob "seg_*.m4s"

Verify a single segment

cargo run --bin c2patool -- live_video_demo/signed_vsi/init.mp4 live-video \
  --segments_glob "seg_001.m4s"

Spec references

Section	Title
§18.25	c2pa.session-keys assertion
§19.4	Verifiable Segment Info
§19.7.3	Verifiable Segment Info Validation
RFC 9052 §7.1	COSE_Key map parameter labels