Closes the cryptographic-attestation gap in ADR-116 §2.2: every
witness event can now be signed by the Seed's Ed25519 key, with
verify available to any auditor holding the public key.
Module shape (`src/witness_signing.rs`, kept separate from
`witness::` so the hash chain stays usable without dalek linked
in — important for the wasm32 audit-verifier variant we'll ship
later):
* sign_event(event, &SigningKey) -> Signature
* verify_signature(event, &Signature, &VerifyingKey)
-> Result<(), SignatureVerifyError>
* signature_to_hex / signature_from_hex (128-char lowercase,
matches the witness hex convention)
* SignatureVerifyError::Invalid
* SignatureParseError::{Length, Hex}
Key design point: signature covers the SAME canonical bytes
witness::hash_event hashes. That means:
1. A signed event commits to the entire event content (kind,
payload, timestamp, seq, prev_hash) — no field can be
retroactively changed without invalidating both the hash AND
the signature.
2. The signature implicitly commits to the event's *chain
position* via prev_hash — splicing a signed event into a
different chain breaks verification.
Adds `ed25519-dalek = "2.1"` to cog-ha-matter (already in
workspace via ruv-neural, version kept aligned).
9 new tests:
* sign_and_verify_round_trip
* verify_rejects_signature_under_wrong_key
* verify_rejects_tampered_event (mutate payload after sign)
* verify_rejects_event_with_wrong_prev_hash (splice attack)
* signature_hex_round_trip
* signature_from_hex_rejects_wrong_length
* signature_from_hex_rejects_non_hex
* signature_is_deterministic_for_same_event_and_key
(locks Ed25519's determinism — catches future accidental
swap to a randomized scheme)
* different_events_produce_different_signatures
60/60 cog tests green (51 → 60). Key management is intentionally
out of scope here — the cog runtime reads the Seed's key from the
Cognitum control plane's secure store (separate concern).
ADR-116 P4 now ⁵⁄₆: ✅ mDNS record, ✅ chain, ✅ JSONL, ✅ file
persistence, ✅ Ed25519 signing; ⏳ responder + embedded broker.
Co-Authored-By: claude-flow <ruv@ruv.net>
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