mkit-attest

DSSE and in-toto v1 attestations for mkit, with multi-algorithm signers (Ed25519, secp256k1, P-256) and an RFC 8785 JCS encoder.

The wire format and on-disk layout this crate produces are defined, normatively, in docs/specs/SPEC-ATTESTATIONS.md — any change here must update the spec in the same PR.

Layering

• jcs — RFC 8785 JSON Canonicalization writer for the subset of JSON DSSE and in-toto need (string, uint, bool, null, array, pre-sorted ASCII-keyed object).
• statement — in-toto v1 Statement encoder. Predicate bodies are passed through as already-canonical bytes; this crate never parses predicates.
• envelope — DSSE envelope encoder and strict decoder, PAE, and attestation_id derivation.
• signer — the common Signer trait every backend implements.
• signer_repo_key — Ed25519 over the repo key (the default signer).
• signer_external — drives an external signer subprocess over the length-prefixed buffa-generated SignerFrame wire (see contrib/signers/README.md and docs/specs/SPEC-EXTERNAL-SIGNER.md).

Attestations are stored under .mkit/attestations/<commit-hash>/<att-id>.dsse and are a first-class object type, not a side-channel — see the top-level README's "Attestations" section for the end-to-end model.