# Security: audit chain, identity, policy

The HMAC-chained audit log, Ed25519 install identity, and approval / policy enforcement. Other subsystems anchor receipts to the chain, so its write path is load-bearing.

## Key files

| File | Purpose |
|---|---|
| `audit.py` | Immutable HMAC-chained audit log; daily JSONL rotation; chain crosses file boundaries |
| `audit_chain.py` | `AuditChainStore` facade plus the `EVENT_*` type constants |
| `audit_receipt.py` | Offline-verifiable receipt projection (COSE / in-toto) over a chain range |
| `agent_card_keystore.py` | Ed25519 install-identity keystore |
| `intent_capsule.py` | Signed task-goal capsules with drift escalation |
| `sigstore_attestation.py` | Rekor attestation with a local Ed25519 fallback; verifies local bundles |
| `result_receipt_bundle.py` | Result receipt bundle with offline DSSE verification |
| `capability_delta.py` | Detects capability-widening changes across workflows and permission configs |
| `surface_grant_delta.py` | Lineage gate grant analysis across diffs |
| `key_derivation.py` | HKDF-SHA256 per-store key derivation with scheme versioning |

## Invariants

- The HMAC key lives OUTSIDE the audit log directory at mode `0600`; anything more readable is a hard error at load time (`audit.py`).
- Stores never share a key: each derives its own from the master by HKDF-SHA256
  under a domain tag that is also prefixed into the chain hash preimage, so one
  store's record cannot be replayed against another (`key_derivation.py` v2).
- Event-type constants are append-only: add `EVENT_*` names, never edit or reuse existing ones (`audit_chain.py`).
- Chain helpers take the chain as a parameter (no singletons) and log through
  `log_with_prev_digest`, so `prev_chain_digest` is in the payload before the HMAC.
- The audit chain is opt-in at runtime (`BERNSTEIN_AUDIT=1`, read in `../orchestration/orchestrator.py`); features degrade without it.
- Untrusted paths are opened, never compared: a comparison validates one lookup while the open performs another (`sigstore_attestation.py`).
- Same rule for tenant writes: the whole subtree is created and opened through
  `TenantPaths.anchor` (`../persistence/anchored_write.py`), rotation included. It
  needs `dir_fd` + `O_NOFOLLOW`; lacking either, the refusal narrows to the final
  component or vanishes, never weakens (`ANCHORED_{WRITE,ROTATE}_SUPPORTED`).

## Testing

Single files only: `uv run pytest tests/unit/test_audit.py -x -q`.

<!-- Reviewed 2026-08-27 against this subtree; the notes above still hold. -->