SPEC-ATTESTATIONS.md

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spec: SPEC-ATTESTATIONS version: 1 status: draft audience: implementers and integrators producing or verifying native mkit attestations (in-toto v1 + DSSE)

SPEC-ATTESTATIONS — mkit v1 attestation format

Status: Draft. This document defines a native attestation primitive for mkit: the on-disk layout, the wire envelope, the signing contract, the CLI, and the transport additions. It supersedes the historical docs/NOTARY.md design note.

Scope: everything about how mkit produces, stores, transports, and verifies an attestation about a commit. Not in scope: what an attestation means — that's the business of the predicate-type URI, which is chosen by whoever makes the claim (mkit itself, Makechain, GitHub, a user's build server, etc.).

This spec is normative for the wire bytes and on-disk layout that the mkit-attest crate produces and the mkit attest / mkit verify-attest CLI consume. Items marked (planned) are spec'd here for forward compatibility but are not implemented in the current mkit-attest crate or CLI; consumers MUST NOT rely on them.

1. Design constraints

Content-addressed, same shape as everything else in mkit. An attestation has a stable hash; the hash is the on-disk name; the transport carries bytes by hash.
Standards-compliant envelope. The signed bytes MUST be consumable by any off-the-shelf tool that understands the format — today that means Sigstore cosign, GitHub gh attestation verify, the SLSA verifier, and any in-toto v1 consumer. No mkit-specific framing around the signed bytes.
Multi-signature native. One attestation envelope MAY carry signatures from multiple independent signers (repo-owner, reviewer, CI build bot, settlement service). Each signature verifies independently against its own trust root.
Predicate-agnostic. mkit never parses the predicate body. It only validates that the envelope is well-formed and at least one signature verifies.
Minimal external dep surface. JCS, DSSE PAE, BLAKE3 hashing, and the on-disk store are hand-rolled in mkit-attest. serde_json is used only to (a) parse-and-validate a caller-supplied predicate body as a JSON object before pass-through, and (b) extract the subject[].digest.blake3 field on the verify path. The canonical encoder never goes through serde_json::to_string — see §4.3. sha2 is an unconditional dependency (not feature-gated): native commit attestation subjects carry a sha256 digest alongside blake3 (§4.2) regardless of which signing-algorithm features are compiled in, so external consumers (cosign, gh attestation verify, the SLSA verifier) that only understand the in-toto/SLSA DigestSet sha256 key can still read mkit's subjects.

2. Format stack

DSSE envelope                 ← signed container (RFC-quality)
  payload = in-toto v1 Statement (JCS-canonical JSON)
                              ← says "I assert <predicate> about <subject>"
  payloadType = "application/vnd.in-toto+json"
  signatures[] = { keyid, sig }
                              ← one or more; each verified independently

DSSE is the Dead Simple Signing Envelope protocol v1. mkit implements it as written, no extensions.
in-toto Statement is in-toto v1.
JCS is RFC 8785 JSON Canonicalization Scheme.
Base64 is standard base64 (RFC 4648 §4), padded.
Hash is BLAKE3, consistent with SPEC-OBJECTS.md.
Signature algorithms — Ed25519 (COSE −19), ECDSA-P-256 (COSE −7, ES256), and ECDSA-secp256k1 (COSE −47, ES256K). All three are wired through the Signer trait (§6) and registered as trust-root variants on the verify side. Ed25519 is the built-in repo-key default; the others are reached via per-algorithm key paths or the keystore signer.

2.1 Pre-Authentication Encoding (PAE)

DSSE signs a canonical encoding of payloadType + payload, not the payload alone. Per the DSSE spec:

PAE(type, payload) =
    "DSSEv1"
    || SP || ASCII(len(type))   || SP || type
    || SP || ASCII(len(payload)) || SP || payload

Where len(x) is the decimal-ASCII length of the UTF-8 bytes of x and SP is a single ASCII space (0x20). mkit signs PAE(type, payload) directly — no further hashing wrapper. (Ed25519 internally pre-hashes with SHA-512 inside ed25519-dalek; ECDSA backends pre-hash with SHA-256. The signer trait always hands the signer the raw PAE; the signer's algorithm is responsible for any required digest.)

Reference encoding (mkit-attest::envelope::pae_of):

PAE("hello", "body")  ==  b"DSSEv1 5 hello 4 body"
PAE("t",     "")      ==  b"DSSEv1 1 t 0 "

3. On-disk layout

.mkit/
  attestations/
    <commit-hash-hex>/                      ← one directory per subject
      <attestation-id-hex>.dsse              ← one DSSE envelope per file

<commit-hash-hex> — the 64-char lowercase hex of the commit's BLAKE3 hash. Directory is created on first attestation.
<attestation-id-hex> — the 64-char lowercase hex of BLAKE3(envelope-bytes) where the bytes are the serialized DSSE envelope (§4). Makes the filename deterministic from the envelope contents, so reposting an identical attestation is idempotent.
File contents — UTF-8 JSON bytes of the DSSE envelope (§4). No trailing newline. The attestation file ID is BLAKE3(file_bytes) hex, so any trailing whitespace would change the ID and break the content-addressing invariant.
Single-envelope size cap: 1 MiB (store::MAX_ENVELOPE_SIZE). Multi-signer envelopes in practice stay in the low kilobytes; the cap exists to bound a malformed read.
Writes are atomic: a temp file in the same directory is fsynced and rename(2)d into place, and on Unix the parent directory is fsynced after the rename to make the dirent update durable.

There is no index file, no central manifest. ls .mkit/attestations/<commit-hash>/ lists every attestation attached to that commit. This is the same discoverability model as refs/.

3.1 Relationship to objects/

Attestations are NOT objects in the objects/ tree. They are a separate resource class. Rationale:

An attestation is about a commit; it doesn't participate in the DAG. Putting it under objects/ would imply Merkle-inclusion which would be wrong — two repos with the same commit hash should be able to carry different attestations.
Attestations arrive asynchronously (a settlement service attests hours after a commit is pushed). Fetching an old commit should not require fetching every attestation ever added.

4. Envelope format

4.1 DSSE envelope JSON

JCS-canonical key order is strictly ascending UCS-2 codepoint, so the wire bytes have keys in this exact order:

{
  "payload":     "<base64(statement_json)>",
  "payloadType": "application/vnd.in-toto+json",
  "signatures":  [
    { "keyid": "<signer-identifier>", "sig": "<base64(sig_bytes)>" }
  ]
}

"payload" < "payloadType" < "signatures", and inside each signature object "keyid" < "sig". JCS sort is mandatory; the encoder builds the object members in this order and the strict decoder rejects any envelope whose bytes don't match this exact shape.
payload is base64-encoded UTF-8 of the in-toto Statement JSON (§4.2).
signatures is a non-empty array. Order is append-only by convention (mkit emits primary signer first, then each --additional-signer in the order they appeared on argv); the envelope encoder preserves the caller-supplied vector order.
keyid is an arbitrary string. Conventions in §6.3.

4.2 in-toto v1 Statement body

JCS-canonical key order. Note predicate precedes predicateType (by codepoint sort), _type is first (_ = 0x5F sorts before p = 0x70), and subject is last (s = 0x73 > p):

{
  "_type": "https://in-toto.io/Statement/v1",
  "predicate": { ... arbitrary JSON, predicate-type's own schema ... },
  "predicateType": "<uri-identifying-the-predicate>",
  "subject": [
    {
      "digest": { "blake3": "<commit-hash-hex>", "sha256": "<commit-hash-hex>" },
      "name": "<optional-human-readable-string>"
    }
  ]
}

Inside each subject entry, "digest" precedes the optional "name"; within "digest", "blake3" precedes "sha256" (JCS codepoint sort). The encoder emits name only when set.
A native commit attestation's subject[0].digest is mandatory on both keys: it carries a blake3 digest (mkit's own content-addressing hash) and a sha256 digest of the identical underlying bytes — never a second, independent artifact reference, just an additional name for the same one. sha256 exists because it's the digest algorithm the in-toto/SLSA DigestSet convention — and every widely-deployed consumer (cosign, gh attestation verify, the SLSA verifier) — actually looks for; without it those tools cannot read a mkit attestation's subjects at all. A producer that does not hold the subject bytes MUST NOT fabricate a sha256 digest. Storage-receipt advance subjects under SPEC-SERVER §15 carry blake3 only by that predicate's rule in both opaque and indexed modes. An indexed server may hold the commit bytes; that does not add sha256 to a storage-receipt advance subject. The server attests the recorded ref value, not an object attestation. Storage-receipt lease subjects, whose scope bytes are known, carry both digests.
For native commit attestations, subject[0].digest.blake3 is the commit hash. Additional subjects MAY appear (for example, a file within the commit's tree); implementations MUST NOT require them.
predicateType is the URI that tells a consumer how to interpret predicate. §6.4 lists ecosystem conventions.
predicate is opaque to mkit. The caller hands the encoder already-canonical predicate bytes (a JSON object, starting { ending }); they are pass-through verbatim into the enclosing Statement's bytes. The encoder rejects predicates that don't parse as a JSON object.
The mkit attest CLI defaults to a {} predicate body when --predicate-file is omitted.

4.3 Canonicalization

Both the outer DSSE envelope and the inner Statement are serialized via RFC 8785 JCS. This makes <attestation-id> (hash of envelope bytes) a stable function of the logical content. Implementation rules:

UTF-8 throughout.
Object members appear in strictly ascending byte-wise order on the key. All keys mkit emits are ASCII, so byte order coincides with UCS-2 codepoint order.
Numbers are restricted to non-negative integers (u64). JCS's float-serialization rules are deliberately not implemented — mkit never emits a non-integer number in either the envelope or the Statement frame. Predicate bodies pass through verbatim and inherit whatever number serialization the caller produced.
Strings per JCS §3.2.3 (short-form escapes for \", \\, \b, \f, \n, \r, \t; lowercase \uXXXX for the remaining control chars < 0x20; everything else, including all non-ASCII Unicode, emitted verbatim as UTF-8; / is NOT escaped).
No trailing whitespace; no trailing newline. The bytes written to disk are the bytes hashed for the attestation ID (§3).

The JCS writer lives in mkit-attest::jcs. It is restricted to the JSON subset used by DSSE + in-toto (string, u64, bool, null, object, array). serde_json::to_string is never called on the emit path — it does NOT honor JCS sort and number-format rules. serde_json IS used for parsing on the verify side (extract subject[].digest.blake3) and for validating caller-supplied predicate bodies.

The strict envelope decoder in mkit-attest::envelope::decode accepts only the exact byte shape encode emits — no whitespace tolerance, no key-order flexibility. Third-party DSSE envelopes written by other tools must be re-canonicalized before mkit's decoder will accept them. (This is intentional: an mkit producer always writes through encode, so the strict decoder gives the strongest possible "what you wrote is what came back" guarantee.)

5. Attestation lifecycle

5.1 Create

User or service constructs a predicate (arbitrary JSON object).
mkit builds the in-toto Statement (§4.2) with the commit hash as subject and the caller-supplied predicateType + predicate.
Statement is JCS-canonicalized; bytes become the envelope payload.
Signer.sign(PAE(payloadType, payload)) is called once per configured signer (primary --signer first, then each --additional-signer in order); signatures are collected into the envelope's signatures[] array in that same order.
Envelope assembled (§4.1), JCS-canonicalized, BLAKE3-hashed for the attestation id.
Envelope written to .mkit/attestations/<commit>/<att-id>.dsse.

Multi-signature envelopes are produced in a single mkit attest invocation by repeating --additional-signer (§8). Any signer failure aborts the attest with no partial envelope on disk.

5.2 Adding signatures after the fact (planned)

The current CLI does not support attaching a new signature to an already-on-disk envelope. The wire format does not preclude this — a future tool could decode, append to signatures[], re-encode, and write under the new <att-id> derived from the new bytes — but no mkit attest --add-signature <att-id> flag exists today. Producers that need multi-party attestations must collect all signatures at create time via --additional-signer.

If implemented, the new envelope MUST be written under the att-id derived from its new bytes; the previous-att-id file is NOT overwritten or deleted, so the two envelopes co-exist on disk.

5.3 Verify

For each envelope attached to the commit:

Decode envelope bytes via the strict decoder; reject if it doesn't match the canonical shape, if payloadType is not application/vnd.in-toto+json, or if signatures[] is empty.
For each signature, look up the keyid in the trust-root Registry (§6.3). On hit, run the algorithm-specific verifier over PAE(payloadType, payload) + signature bytes:
- TrustRoot::Ed25519PubKey([u8; 32]) — the SPEC-SIGNING §1 acceptance predicate (rejects malleability: non-canonical R, out-of-range S, non-canonical or low-order A/R).
- TrustRoot::P256PubKeySec1(Vec<u8>) — ECDSA-P-256/SHA-256 over a SEC1-encoded pubkey (33-byte compressed or 65-byte uncompressed).
- TrustRoot::Secp256k1PubKeySec1(Vec<u8>) — ECDSA-secp256k1/ SHA-256 over a SEC1-encoded pubkey.
- TrustRoot::SigstoreCa — placeholder; any signature dispatched to it surfaces Reason::UnsupportedTrustRoot. Real Sigstore verification (Fulcio cert chain + Rekor inclusion) is not implemented.
Each signature returns a typed Reason: Ok, UnknownKeyid, UnknownKeyidPrefix (keyid prefix not recognized by Algorithm::from_keyid), SignatureMismatch, UnsupportedTrustRoot, or AlgorithmNotEnabled (the algorithm's crypto backend was compiled out of this build).
The envelope verifies if at least one signature returns Reason::Ok. The caller is also responsible for binding the envelope to the commit being asked about — mkit-attest exposes verify::extract_primary_commit_hash to read the first subject's blake3 digest out of the Statement payload for that purpose. A signature-valid envelope whose subject digest does not match the commit actually being queried MUST NOT be treated as an attestation for that commit — the attestation directory name (.mkit/attestations/<commit-hex>/) is not itself proof of what a given envelope attests to, since it is ordinary repo-tree state any process with write access to the repo can populate.

Conformance requirement: every caller-facing verification entry point MUST perform the binding check in the previous paragraph itself — it MUST NOT be an opt-in step callers can forget. mkit verify-attest (the CLI) already does this correctly: it calls extract_primary_commit_hash and rejects any envelope whose subject hash does not equal the commit hex the directory is keyed under (mkit-cli's verify_attest command), before considering the envelope's signature at all. The WASM binding (mkit_wasm::attest::attest_verify) does not — it takes no commit-hash parameter and returns whether any signature verifies, with no subject-binding check, despite being a public, documented API surface. That is a real, narrow gap in one entry point, not a property of mkit-attest's verification model in general. Closing it requires adding a required commit-hash parameter to attest_verify and performing the same check internally; this document stays Draft until that lands.

The mkit verify-attest CLI enumerates every envelope under .mkit/attestations/<commit>/, prints a per-signature verdict, and exits 0 iff every envelope has at least one verifying signature whose subject binds to that commit.

There is no revocation list. Revocation is: stop trusting the keyid. Transparency-log-backed signatures would solve this differently by binding signatures to timestamps; that's future work gated on a real Sigstore backend.

6. Signers, trust roots, and predicate types

6.1 `Signer` trait

The Signer trait (mkit_attest::Signer) has three methods:

algorithm() -> Algorithm — the COSE-tagged signing algorithm this signer produces (Ed25519 / Secp256k1 / P256). Callers use this to pick the right verifier path on the receive side without reparsing the keyid.
keyid() -> Result<String, Error> — identifies which trust root verifies this signer's output. May fail before the first sign call for signers that learn their keyid from the signer process (for example, ExternalSigner returns KeyIdNotKnownUntilFirstSign).
sign(pae: &[u8]) -> Result<Vec<u8>, Error> — signs the DSSE PAE. Implementation may prompt, network-call, subprocess, etc. Returns raw signature bytes (the envelope encoder base64s on the way out).

Verification is symmetric but NOT attached to the same trait, because the verifier often has no knowledge of how the signature was produced (for example, for a future Sigstore-keyless signature the verifier is Fulcio's cert chain, which no signer owns). Verifiers dispatch on keyid (§6.3) and the resolved TrustRoot variant.

6.2 Built-in signers

mkit ships three production signer kinds, plus a scaffold for Sigstore. Each is opt-in; the CLI picks the signer via --signer flag or the attest.signer config key.

Signer name	What it signs with	Where the trust root lives
`repo-key` (default)	Ed25519/secp256k1/P-256 over a raw 32-byte secret on disk under `.mkit/keys/`	The corresponding public key, registered by `keyid` in the trust-roots file.
`keystore`	A user-scoped `mkit-keystore` key reference (the keystore performs the signing)	The keystore backend's canonical key id.
`external`	Subprocess wire conversation per SPEC-EXTERNAL-SIGNER	Whatever the external signer's trust model is.
`sigstore` (scaffold)	—	— — `Error::SigstoreNotImplemented` is returned. Not selectable from the CLI factory.

repo-key covers all three algorithms:

Ed25519 reuses the existing commit-signing keypair at .mkit/keys/default.key (path is configurable via signing_key).
secp256k1 and P-256 each read a raw 32-byte secret from attest.secp256k1_key_path / attest.p256_key_path (defaults .mkit/keys/secp256k1.key, .mkit/keys/p256.key). Neither path is auto-generated — mkit keygen --algorithm <alg> must run first.

external is the extension point an HSM, Secure Enclave, browser- crypto wallet, Makechain, etc. wraps. The full wire contract — invocation, request/response shapes, capability discovery, PIN prompts, error semantics, size caps, timeout, determinism, versioning — is specified normatively in SPEC-EXTERNAL-SIGNER.md, with the underlying length-prefixed buffa framing defined in SPEC-RPC.md. Protocol version is v1; that document is the source of truth for any new signer implementation.

In summary, the wire is length-prefixed SignerFrame protobuf messages (NOT one-line JSON) carrying:

parent → child:   Hello { protocol = v1, want_capabilities = true, ... }
child  → parent:  HelloResponse { protocol = v1, capabilities, ... }
parent → child:   SignRequest { algorithm, key_form, key_ref, payload }
child  → parent:  SignResponse { signature, public_key, key_id }
                    OR Error { code, message }

The binary path comes from user-scoped attest.external_signer_path and MUST be absolute (relative paths are rejected at construction to close a $PATH resolution race). The child receives the PAE inside SignRequest.payload on stdin — never on argv.

Bounded execution. mkit bounds the whole signer conversation with a single wall-clock deadline (spawn → request-write → response-read → stderr-drain → child-exit) and concurrently drains the child's stderr so a stderr-heavy signer cannot deadlock against a blocked stdout read. On timeout the child is killed and reaped. The deadline defaults to 120s (generous for hardware touch/PIN/biometric) and is configurable via the user-scoped attest.external_signer_timeout_secs. See SPEC-EXTERNAL-SIGNER §2.1. WebAuthn assertions are additionally validated against a configurable ceremony policy (RP-ID, origin, UP/UV, crossOrigin, counter) — see SPEC-EXTERNAL-SIGNER §6.1.

Signer argv. The subprocess is spawned with an optional argv vector in addition to the protobuf request on stdin. By default the vector is empty. There are three ways to populate it, each overriding the previous:

User-scoped attest.external_signer_args — a pipe-separated list (sign|--tag|prod). Pipe instead of comma because the multi-sig spec already uses , as its key=value separator.
--external-signer-arg <V> on mkit attest, repeatable. When any instance appears, the collected values REPLACE (not append to) the config value — per-invocation override for "sign with tag X just this once."
args=<a>|<b>|<c> on --additional-signer for the multi-sig envelope — see §6.2.1 below.

Every token is passed verbatim to Command::args — no shell interpolation, no splitting on whitespace.

6.2.1 Multi-sig spec: `--additional-signer` clause

mkit attest --additional-signer takes a comma-separated key=value spec. The recognized keys are:

Key	Meaning
`algorithm`	`ed25519` / `secp256k1` / `p256`. Required.
`signer`	`repo-key` / `external`. Required.
`path`	Overrides key path (repo-key) or binary path (external).
`args`	Pipe-separated argv for external signers only (optional).

Keystore-backed signing is available as the primary attest.signer. --additional-signer is currently limited to repo-key and external until the multi-sig grammar grows a key-ref field for keystore selection.

Example:

mkit attest --additional-signer \
  "algorithm=p256,signer=external,path=/usr/local/bin/mkit-sign-se,args=sign|--tag|demo"

| is used inside args= because , is the outer separator. When args= is present it overrides attest.external_signer_args for this signer only; absent means "fall through to config." An explicit args= with no value is a valid "zero argv" override.

6.3 `keyid` conventions

Free-form UTF-8. Strongly recommended canonical shapes:

Ed25519 repo-key — blake3:<hex> where <hex> is the 64-char lowercase hex of BLAKE3(pubkey_bytes) (32 bytes → 64 hex chars). The full keyid string is 71 bytes long (blake3: + 64 hex). The legacy blake3: prefix is preserved for backward compatibility with attestations produced before the multi-algorithm split; the verifier maps it to Ed25519 via Algorithm::from_keyid.
secp256k1/P-256 repo-key — secp256k1:<hex> / p256:<hex> of the SEC1-encoded compressed pubkey (33 bytes → 66 hex chars).
Keystore-backed keys — whatever the keystore backend returns from KeySigner::keyid(). Conventionally prefixed by its algorithm.
External — whatever the external signer returns in SignResponse.key_id; by convention scheme-prefixed (x509:<spki-hash>, makechain:0x…, etc.).
Sigstore keyless (planned) — sigstore:<cert-san> (for example, sigstore:https://github.com/user/repo/.github/workflows/release.yml@refs/tags/v1).

The keyid is purely a dispatch key into the verifier registry. It is NOT authoritative — the signature itself is the only thing that proves identity. Algorithm::from_keyid recognizes the prefixes ed25519:, secp256k1:, p256:, and the legacy blake3:; anything else surfaces Reason::UnknownKeyidPrefix on the verify side.

6.4 Predicate types — conventions, not requirements

mkit knows about zero predicate types and validates none of them. Ecosystem conventions to encourage (non-normative, for users to opt into):

https://slsa.dev/provenance/v1 — SLSA build provenance.
https://in-toto.io/attestation/vuln/v0.1 — vuln-scan results.
https://github.com/officialunofficial/mkit/spec/predicate/review/v1 — code review sign-off (planned; not yet defined).
https://github.com/officialunofficial/mkit/spec/predicate/storage-receipt/v1 — server-signed evidence of a committed live advance or lease change (SPEC-SERVER §15).
Third-party predicates (for example, a Makechain settlement predicate at a yet-to-be-provisioned URI such as tag:github.com,2024:officialunofficial/mkit/settlement/v1) are entirely opaque to mkit.

6.5 Trust-roots file

The verify path resolves a keyid to a TrustRoot variant via a user-scoped trust-roots file. By default this lives at $XDG_CONFIG_HOME/mkit/trust-roots.toml (typically ~/.config/mkit/trust-roots.toml). mkit verify-attest --trust-roots <path> overrides the location.

An in-repo trust-roots file is REJECTED unless --trust-roots is explicitly passed. A hostile clone could otherwise ship .mkit/attest-trust-roots.toml listing attacker keys and the verify loop would print "ok" against attacker-controlled signatures.

Schema (a strict subset of TOML — the parser is hand-rolled and recognizes only this exact grammar; unknown keys inside a block are tolerated and ignored):

# Repeat one block per trusted key.
[[trust_root]]
keyid      = "ed25519:9f2c7e1a4b6d0358f7a12e9c3d5b8064af731c0e29d4b8f651a3c7e0d9b2f4a6"   # exact match against the keyid carried in the DSSE signature
kind       = "ed25519"             # see table below
pubkey_hex = "3a7c9e1f4b2d6058a9c3e7f1b4d6082a5c9e3f7b1d4602a8c5e9f3b7d1042a6c"         # raw public-key bytes, lowercase hex

`kind`	Maps to `TrustRoot` variant	`pubkey_hex` expects
`ed25519`	`Ed25519PubKey([u8; 32])`	exactly 32 bytes (64 hex chars)
`p256-sec1` or `p256`	`P256PubKeySec1(Vec<u8>)`	SEC1: 33 bytes compressed (66 hex) or 65 bytes uncompressed (130 hex)
`secp256k1` or `secp256k1-sec1`	`Secp256k1PubKeySec1(Vec<u8>)`	SEC1: 33 bytes compressed (66 hex) or 65 bytes uncompressed (130 hex)
anything else	silently skipped	—

Missing file ⇒ empty registry ⇒ every signature surfaces UnknownKeyid ⇒ no envelope verifies. The CLI prints a note: trust-roots file not found at ... hint in that case.

7. Integration with existing mkit primitives

7.1 Commits

No change to the commit object format. An attestation is a separate resource; a commit with zero attestations and a commit with five attestations are indistinguishable at the commit layer.

7.2 Signing code (`SPEC-SIGNING.md`)

The Ed25519 DSSE signer for repo-key reuses ed25519-dalek and the same .mkit/keys/default.key as commit signing. It signs the DSSE PAE per §2.1 — note this is a DIFFERENT signed-bytes domain from commit signing:

COMMIT_DOMAIN       = "mkit.commit\x00"      (SPEC-SIGNING §2)
REMIX_DOMAIN        = "mkit.remix\x00"       (SPEC-SIGNING §2)
DSSE signing        = "DSSEv1 ..." (PAE, §2.1)

The PAE prefix "DSSEv1 " cannot collide with either domain prefix (no \x00 byte, different leading bytes). Cross-domain signature confusion (SPEC-SIGNING §2's R-17) is therefore extended to cover attestations with no additional work.

7.3 Transport (planned)

Push/pull of attestations across the wire is not yet implemented in any of the mkit-transport-{file,http,s3,ssh,memory} backends, and the SPEC-RPC opcode table does not yet carry attestation verbs. mkit push / mkit pull / mkit fetch move only the object DAG today; attestations remain repo-local until a future transport revision adds them.

When that revision lands it MUST:

Allow uploading an envelope as (commit-hash, envelope-bytes), with the server computing <att-id> = BLAKE3(envelope-bytes) and writing .mkit/attestations/<commit>/<att-id>.dsse.
Allow downloading by <att-id> (returning envelope bytes).
Allow listing every <att-id> attached to a given <commit>, returning the same sorted-by-att-id order store::list produces.
Degrade cleanly: a pre-attestation-aware server returns whatever the SPEC-RPC unsupported-verb status is, and the client emits a warning: line to stderr and continues without attestations.

Opcode numbers and exact frame shapes will be specified in SPEC-RPC once the wire revision is cut.

8. CLI surface

mkit attest [--commit <hash>]
            [--algorithm ed25519|secp256k1|p256]
            [--signer repo-key|external|keystore]
            [--predicate-type <URI>]
            [--predicate-file <path>]
            [--external-signer-arg <V>]...
            [--additional-signer "<spec>"]...
    Produce a signed DSSE attestation for a commit. Prints the
    att-id (64 hex chars) on success.

mkit verify-attest [--commit <hash>]
                   [--trust-roots <path>]
                   [--algorithm <filter>]
    Verify every attestation attached to a commit. Exits 0 iff every
    listed attestation has at least one verifying signature. The
    --algorithm filter narrows the per-signature display to one of
    ed25519 / secp256k1 / p256; the verdict is unchanged.

Defaults:

--commit — HEAD.
--algorithm — attest.default_algorithm from config, else ed25519.
--signer — attest.signer from config, else repo-key.
--predicate-type — https://github.com/officialunofficial/mkit/spec/predicate/empty/v1 (placeholder; real callers pass their own URI).
--predicate-file — omitted ⇒ predicate body is {}.
--trust-roots — $XDG_CONFIG_HOME/mkit/trust-roots.toml.

The following subcommands appear in earlier drafts but are not implemented:

mkit attest verify — folded into the top-level mkit verify-attest.
mkit attest ls / show / rm — list/inspect/delete by hand against .mkit/attestations/<commit>/<att-id>.dsse.
mkit attest --add-signature <att-id> — see §5.2; multi-signer envelopes must currently be produced in a single invocation.
--require-predicate / --require-signer filters on mkit verify-attest — the verify command currently exits 0 if every envelope verifies under at least one trust-rooted signature, without further policy filtering.

9. Config

Keys in the merged config (.mkit/config repo-local + user-scoped $XDG_CONFIG_HOME/mkit/config). Security-sensitive keys are user-scoped only: setting them in repo-local config is rejected with a stderr warning. See THREAT-MODEL.md for the motivation.

[attest]
default_algorithm    = "ed25519" | "secp256k1" | "p256"   (user-scoped)
signer               = "repo-key" | "external" | "keystore"  (user-scoped)
external_signer_path = /abs/path/to/binary               (user-scoped; required when signer = external)
external_signer_args = a|b|c                             (user-scoped; pipe-separated argv, optional)
external_signer_timeout_secs = 120                       (user-scoped; whole-conversation deadline, default 120)
secp256k1_key_path   = .mkit/keys/secp256k1.key          (user-scoped; default shown)
p256_key_path        = .mkit/keys/p256.key               (user-scoped; default shown)

[key]                                                    # see SPEC-KEYSTORE
ed25519_ref     = "<backend>:<label>"                    (user-scoped)
secp256k1_ref   = "<backend>:<label>"                    (user-scoped)
p256_ref        = "<backend>:<label>"                    (user-scoped)

signer = "keystore" signs with the keystore reference selected for the configured algorithm (key.<alg>_ref, falling back to key.default_ref). Repo-local config MUST NOT select keystore or populate any key.*_ref — a hostile clone would otherwise pin the victim's signing identity to an attacker-chosen key alias.

attest.auto_sign_commit (planned). Earlier drafts proposed an "auto-attest every commit with a minimal commit-signed predicate" toggle. This is not implemented; mkit commit does not invoke the attest pipeline today. Producers that want commit-level attestations must run mkit attest explicitly after the commit lands.

10. Security properties (what this does and doesn't protect)

Protects against:

Silent substitution of attestation content. JCS + BLAKE3 content- addressing means any byte change produces a new att-id.
Cross-domain signature reuse. DSSE PAE prefix is distinct from commit-signing PAE (§7.2).
Single-signer compromise. Multi-sig lets you require N-of-M policy at verify time (callers must implement the N-of-M check themselves on top of the per-signature Reason results — the envelope verifies as long as any signature returns Ok).
Predicate-type sprawl. mkit doesn't parse predicates, so a future unsafe predicate-type can't subvert mkit's invariants.
In-repo trust-roots planting. mkit verify-attest refuses to read a trust-roots file under the repo's .mkit/ directory unless the user passes --trust-roots explicitly.
Ed25519 malleability. Verify uses verify_strict (rejects non-canonical R/A and s ≥ ℓ), so honest verifiers reach the same verdict on the same signature bytes.

Does NOT protect against:

A malicious predicate body. Interpreting the predicate is the consumer's job; mkit just moves the bytes.
Revocation timing. Without a transparency log the "when was this signature made" is the signer's claim. Real Sigstore-style auditable timestamping is future work.
A malicious signer impl. An external signer can return whatever it wants. The keyid/trust root has to be checked by whoever consumes the envelope.

11. Non-goals

Revocation lists. Out of scope; relies on transparency log or signer-level key rotation.
Signed predicates as typed structs. mkit never parses predicate beyond "is this a JSON object?" on the produce side.
Automatic SLSA provenance generation. Separate feature; if shipped, it'd be a predicate-type-specific emitter that drives mkit attest at build time.
C2PA/JUMBF support. Different ecosystem, different shape.
Binary-only DSSE variant. Defeats interop.
Float number support in JCS. mkit never emits one.

12. Stability

This document stays draft while the §5.3 WASM-verification gap remains open. Once that closes, any breaking change to the on-disk layout or wire envelope requires a version bump and a SPEC-ATTESTATIONS-v2.md (same pattern as SPEC-OBJECTS). The DSSE + in-toto bodies are governed by their upstream specs; mkit tracks those as they evolve.

Items currently marked (planned) — §5.2 (--add-signature), §7.3 (transport verbs), §8 (mkit attest ls/show/rm, --require-predicate/--require-signer), §9 (attest.auto_sign_commit), §6.4 review predicate URI — are forward-compatible additions and will not require a version bump when implemented.

13. Invariants

§10 is the authoritative statement of what this format does and does not protect. This table maps each guarantee to its enforcement point.

Invariant	Enforced by
Any byte change to an envelope yields a new att-id	att-id = `BLAKE3(envelope-bytes)`; JCS makes the bytes a stable function of content (§3, §4.3)
Reposting an identical attestation is idempotent	filename derived from the envelope hash (§3)
An envelope round-trips only if byte-identical to what `encode` emits	strict decoder: exact key order, no whitespace tolerance (§4.3)
A DSSE signature can never be replayed as a commit/remix/tag signature	PAE prefix `"DSSEv1 "` is disjoint from every SPEC-SIGNING domain (§7.2)
`keyid` alone never authenticates	keyid is a dispatch key only; trust-root lookup + signature verification decide (§6.3, §5.3)
A hostile clone cannot plant trust roots	in-repo trust-roots file rejected unless `--trust-roots` is explicit (§6.5, §10)
Missing trust roots fail closed	empty registry ⇒ every signature `UnknownKeyid` ⇒ no envelope verifies (§6.5)
A hostile repo cannot redirect signing	`attest.*` signer/key/path keys are user-scoped only (§9)
Ed25519 verdicts are identical across honest verifiers	`verify_strict` rejects malleable encodings (§5.3, §10)
A malicious predicate body cannot subvert mkit	the predicate is never parsed beyond "is a JSON object" (§1, §10)
No partial envelope reaches disk	any signer failure aborts before write; writes are tmp + fsync + rename atomic (§5.1, §3)
Malformed reads are bounded	1 MiB envelope size cap (§3)

What the format does NOT guarantee — revocation timing, predicate semantics, honesty of an external signer — is enumerated in §10 and is not restated here.

Preview

spec: SPEC-ATTESTATIONS version: 1 status: draft audience: implementers and integrators producing or verifying native mkit attestations (in-toto v1 + DSSE)

SPEC-ATTESTATIONS — mkit v1 attestation format

1. Design constraints

2. Format stack

2.1 Pre-Authentication Encoding (PAE)

3. On-disk layout

3.1 Relationship to objects/

4. Envelope format

4.1 DSSE envelope JSON

4.2 in-toto v1 Statement body

4.3 Canonicalization

5. Attestation lifecycle

5.1 Create

5.2 Adding signatures after the fact (planned)

5.3 Verify

6. Signers, trust roots, and predicate types

6.1 Signer trait

6.2 Built-in signers

6.2.1 Multi-sig spec: --additional-signer clause

6.3 keyid conventions

6.4 Predicate types — conventions, not requirements

6.5 Trust-roots file

7. Integration with existing mkit primitives

7.1 Commits

7.2 Signing code (SPEC-SIGNING.md)

7.3 Transport (planned)

8. CLI surface

9. Config

10. Security properties (what this does and doesn't protect)

11. Non-goals

12. Stability

13. Invariants

Files

spec: SPEC-ATTESTATIONS version: 1 status: draft audience: implementers and integrators producing or verifying native mkit attestations (in-toto v1 + DSSE)

SPEC-ATTESTATIONS — mkit v1 attestation format

1. Design constraints

2. Format stack

2.1 Pre-Authentication Encoding (PAE)

3. On-disk layout

3.1 Relationship to objects/

4. Envelope format

4.1 DSSE envelope JSON

4.2 in-toto v1 Statement body

4.3 Canonicalization

5. Attestation lifecycle

5.1 Create

5.2 Adding signatures after the fact (planned)

5.3 Verify

6. Signers, trust roots, and predicate types

6.1 Signer trait

6.2 Built-in signers

6.2.1 Multi-sig spec: --additional-signer clause

6.3 keyid conventions

6.4 Predicate types — conventions, not requirements

6.5 Trust-roots file

7. Integration with existing mkit primitives

7.1 Commits

7.2 Signing code (SPEC-SIGNING.md)

7.3 Transport (planned)

8. CLI surface

9. Config

10. Security properties (what this does and doesn't protect)

11. Non-goals

12. Stability

13. Invariants

6.1 `Signer` trait

6.2.1 Multi-sig spec: `--additional-signer` clause

6.3 `keyid` conventions

7.2 Signing code (`SPEC-SIGNING.md`)

6.1 `Signer` trait

6.2.1 Multi-sig spec: `--additional-signer` clause

6.3 `keyid` conventions

7.2 Signing code (`SPEC-SIGNING.md`)