fix: remove ambiguous encodings from hash inputs

[jmecom]

problem

Buzz has several instances where the encoding of hash inputs is non-injective. This means that two semantically distinct inputs can produce the same hash. The core issue is lack of proper separation. For example, consider a protocol which hashes a first name and last name: hash("Jane"||"Doe") is equivalent to hash("Jan"||"eDoe"). Although the names are distinct, the input to the hash function is identical.

The correct approach is to, for example, use TLV-encoding or another unambiguous encoding. Each input is transformed:

First name: "Jane" -> {tag: "first", len: 4, value: "Jane"} -> "first::4::Jane"
Last name: "Doe" -> {tag: "last", len: 3, value: "Doe"} -> "last::3::Doe"

First name: "Jan" -> {tag: "first", len: 3, value: "Jan"} -> "first::3::Jan"
Last name: "eDoe" -> {tag: "last", len: 4, value: "eDoe"} -> "last::4::eDoe"

`hash("first::4::Jane"||"last::3::Doe") != hash("first::3::Jan"||"last::4::eDoe")`

fix

Replace ambiguous hash inputs in four places, with one fix per commit:

• Audit rows use version 2 TLV encoding. Legacy verification checks cryptographic field widths and restricts object IDs to the UUID/hash forms used by existing producers.
• Git signatures use version 2 with an attestation presence byte and field lengths. Legacy verification requires a commit or tag header, preventing two accepted interpretations of the same signed bytes.
• OAuth cache identities use a versioned JSON tuple, preserving field and scope boundaries. Old caches are not imported because their configuration binding is ambiguous.
• Deletion manifests reject embedded newlines at the digest helper, including the legacy v4 chunk-validation path. Valid v4/v5 digests stay unchanged.

Apply audit migration 0045 and upgrade audit/Git verifiers before deploying new writers. Existing audit hashes are retained; mixed-version chains verify. OAuth users must sign in once after upgrading. Legacy audit and Git checks cannot establish the original interpretation of arbitrary inputs hashed before the upgrade; the compatibility assumptions are documented.

Checked with the audit, Git signing, agent, and database suites (903 passed), including real Git commit/tag signing, OAuth integration tests, collision regressions, and PostgreSQL test discovery. Workspace formatting and Clippy pass.

Full repository checks are still running. Live PostgreSQL/Redis tests are blocked by Docker Desktop's organization sign-in requirement.

[github-actions]

🔐 Codex Security Review

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