Witness Cosignature Verifier

Verifies a C2SP tlog-checkpoint + witness-cosignature note (SPEC.md §20.2) offline: confirms the note's origin and root match a §20/§20.1 batch anchor's declared anchored_hash, then checks each pinned witness's cosignature against a k-of-n threshold. Supports both Ed25519 cosignature/v1 and ML-DSA-44 (the suite §20.2 names alongside it under the §PQC-1 reserved-extension discipline). Consumes a caller-supplied checkpoint note (e.g. from a c2sp.org/tlog-proof bundle), verify-side only, zero log operation, zero network fetch. Golden fixtures dogfood the tool's own output, including a tampered negative fixture that must fail. States only that ≥k pinned witnesses signed this root; does not establish log honesty, key ownership, or leaf inclusion (see art-280/art-279 and SPEC.md §20.1 for those checks).

Tool URL: https://ainumbers.co/chaingraph/art-424-witness-cosignature-verifier.html