# Integrity contract (AUDIT-WP-0007)

Operational custody already stores `payload_hash` and rejects UPDATE/DELETE
for the runtime role. That is not tamper evidence: a database owner can
drop the trigger and rewrite rows. This contract says what the hash chain
proves, and what it does not.

## Chain

Each accepted event stores:

- `payload_hash` — SHA-256 of the canonical event record (already stored)
- `chain_prev` — the previous event's `chain_hash`, or genesis
- `chain_hash` — `SHA-256(chain_prev | payload_hash | event_id)`

Genesis is 64 ASCII zeros (`audit_core.integrity.GENESIS`). The delimiter
is `|`. The chain is **one stream per schema**, not per tenant. A
per-tenant chain would hide a cross-tenant rewrite.

The first accept of an empty store uses genesis as `chain_prev`. A
duplicate accept must not mint a second link. A conflict must not
advance the head. Concurrent first-accepts of different events take an
advisory lock (`CHAIN_LOCK_KEY`) so they cannot fork.

Verify walks `ORDER BY accepted_at, event_id`, recomputes every link,
and reports the first event id whose stored hashes do not match. A
break is a **custody defect**, not a sender error. Retrying the event
will not repair it.

## Proof bound

A chain *inside* the same database detects a rewritten `payload_hash`
**if the attacker does not also recompute the suffix**. A database
owner can. Tamper evidence against that class of attacker requires a
**chain-head attestation** stored outside `platform-pg`.

`tamper_evidence=True` is allowed only when:

1. `verify` exists and fails on a rewritten row
2. an external head attestation exists and verify-against-attestation
   reports a missing cited head as a break

It still does not mean WORM, object lock, or ITC-CAP `data.archive`.
It does not raise provision maturity to D5.

Do not write the attestation into the Barman prefix
(`platform-pg/` on `resource:platform:audit-storage`). That copy is
restored with the table. A second copy may follow the logical-offsite
path already used by RESOURCE-WP-0002-T06 (`rapp-postgres` /
Nextcloud + age); cite it, do not invent a new bucket.

## Tests this contract names

| Test | What it asserts |
| --- | --- |
| `test_first_accept_sets_genesis` | First `chain_prev` is genesis |
| `test_second_accept_links` | Second `chain_prev` is the first `chain_hash` |
| `test_duplicate_does_not_fork` | Replay does not add a link |
| `test_verify_clean_on_fresh_store` | Empty and two-event stores verify |
| `test_rewritten_payload_fails_verify` | Superuser rewrite of `payload_hash` is a break |
| `test_attestation_mismatch` | Cited head absent from the live chain is a break |

## Operator surface

```bash
python -m audit_core verify-chain
python -m audit_core verify-chain --against docs/evidence/chain-head-<ts>.json
python -m audit_core attest-chain --output docs/evidence/chain-head-<ts>.json
```

`GET /v1/integrity` (`may_read`) returns
`{intact, events, head, head_event_id, first_break, attestation_match}`
and never event payloads.