feat(common): encode the SNARK aggregate signature as bytes in the certificate message

[jpraynaud]

Content

This PR includes the encoding of the certificate message aggregate signature by type, so SNARK and IVC proofs use the compact bytes-hex encoding while concatenation proofs remain backward compatible:

• Encode the certificate message multi_signature field by aggregate signature type: JSON-hex for concatenation proofs (backward compatible with all deployed clients) and bytes-hex for SNARK and IVC proofs.
• Route the genesis signatures through the native ProtocolKey codec (String::try_from) instead of an explicit bytes-hex call.
• Keep the certificate hash and chain walking JSON-hex based, so certificate hashes stay byte-identical.

Pre-submit checklist

• Branch
  • Tests are provided (if possible)
  • Crates versions are updated (if relevant)
  • CHANGELOG file is updated (if relevant)
  • Commit sequence broadly makes sense
  • Key commits have useful messages
• PR
  • All check jobs of the CI have succeeded
  • Self-reviewed the diff
  • Useful pull request description
  • Reviewer requested

Issue(s)

Closes #3379

[github-actions]

Test Results

     5 files  ± 0     209 suites  ±0   2h 18m 1s ⏱️ - 53m 49s
 3 369 tests  - 32   3 369 ✅  - 32  0 💤 ±0  0 ❌ ±0 
11 124 runs   - 31  11 124 ✅  - 31  0 💤 ±0  0 ❌ ±0 

Results for commit a37adb3. ± Comparison against base commit bc9d8c1.

This pull request removes 35 and adds 3 tests. Note that renamed tests count towards both.

mithril-stm ‑ circuits::halo2::tests::golden::cases::negative::slow::index_out_of_bounds
mithril-stm ‑ circuits::halo2::tests::golden::cases::negative::slow::index_too_large_for_circuit_range
mithril-stm ‑ circuits::halo2::tests::golden::cases::negative::slow::indices_not_increasing
mithril-stm ‑ circuits::halo2::tests::golden::cases::negative::slow::leaf_merkle_path_mismatch
mithril-stm ‑ circuits::halo2::tests::golden::cases::negative::slow::leaf_swap_keep_merkle_path
mithril-stm ‑ circuits::halo2::tests::golden::cases::negative::slow::leaf_wrong_verification_key
mithril-stm ‑ circuits::halo2::tests::golden::cases::negative::slow::merkle_path_corrupt_sibling
mithril-stm ‑ circuits::halo2::tests::golden::cases::negative::slow::merkle_path_flip_position
mithril-stm ‑ circuits::halo2::tests::golden::cases::negative::slow::merkle_path_length_long
mithril-stm ‑ circuits::halo2::tests::golden::cases::negative::slow::merkle_path_length_short
…

mithril-aggregator ‑ database::record::certificate::tests::snark_multi_signature::snark_multi_signature_is_stored_as_bytes_hex
mithril-common ‑ messages::certificate::tests::multi_signature_encoding::concatenation_multi_signature_is_encoded_as_json_hex
mithril-common ‑ messages::certificate::tests::multi_signature_encoding::snark_multi_signature_is_encoded_as_bytes_hex

♻️ This comment has been updated with latest results.

[Copilot]

Pull request overview

This PR updates how certificate-related cryptographic payloads are encoded so that SNARK/IVC proofs use a more compact byte-string CBOR representation (and bytes-hex on the wire where applicable), while keeping concatenation proofs backward compatible via JSON-hex encoding. This aligns certificate message/record serialization with the ProtocolKey codec to ensure consistent encoding decisions by aggregate signature type.

Changes:

• Emit CBOR byte strings for proof byte vectors by using serde_bytes for SNARK/IVC proof fields and the aggregate signature CBOR envelope.
• Route certificate message and DB record encoding through String::try_from(&ProtocolKey<...>) to centralize encoding decisions in the ProtocolKeyCodec.
• Introduce a custom ProtocolKeyCodec for AggregateSignature that encodes concatenation as JSON-hex and (when enabled) SNARK/IVC as bytes-hex.

Reviewed changes

Copilot reviewed 6 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
mithril-stm/src/protocol/aggregate_signature/signature.rs	Encodes CBOR envelope proof_bytes as a CBOR byte string and updates golden CBOR fixtures accordingly.
mithril-stm/src/proof_system/halo2_snark/proof.rs	Serializes SNARK circuit_proof with serde_bytes for compact CBOR bytes.
mithril-stm/src/circuits/halo2_ivc/types.rs	Serializes IVC proof byte wrapper with serde_bytes for compact CBOR bytes.
mithril-stm/Cargo.toml	Adds serde_bytes dependency (and ignores it for cargo-machete).
mithril-common/src/messages/certificate.rs	Uses ProtocolKey codec (String::try_from) for certificate crypto fields; adds tests for multi-signature encoding by type.
mithril-common/src/crypto_helper/types/wrappers.rs	Implements custom ProtocolKeyCodec for AggregateSignature to encode by signature type (JSON-hex vs bytes-hex).
mithril-aggregator/src/database/record/certificate.rs	Stores certificate signatures/keys via ProtocolKey codec (String::try_from) and adds a test for SNARK multi-signature storage encoding.
Cargo.lock	Locks the new serde_bytes dependency.

[damrobi]

LGTM 👍