AUDIT-WP-0009-T01 — derive tamper_evidence from live attestation state

The Postgres backend returned tamper_evidence=True as a constant while
docs/integrity.md permits the claim only when a live external chain-head
attestation exists. The one attestation on record is 2026-08-16 and no job
renews it, so audit-core was telling every sender it had a property whose
precondition was unverified — the §9.6 defect it twice corrected in
gate-house's doctrine, turned inward.

evaluate_tamper_evidence() derives the flag from the chain report and the
mounted attestation, distinguishing seven states. Absence, staleness,
mismatch, an undated or unreadable attestation, a chain break, and an
unwalkable chain all degrade the claim rather than leave it standing.
Unreadable is treated as absent on purpose: a malformed file must not hold
up a claim a missing file would drop.

The freshness window is 168h against an intended daily cadence — seven
cadences, so a handful of missed runs degrade the claim rather than a single
one flapping it. Window and cadence are one contract in docs/integrity.md.

Production /readyz will now report tamper_evidence: false until
AUDIT-WP-0009-T02 schedules attestation. The claim was already false; it now
says so.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0185wifnLzCxjEY2MT1XbK7L

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 713962@bnt-lap001
Assistant-Session: 2718d99d-d3ff-478f-83a2-3a30f01a02fc

SCOPE.md

@@ -97,10 +97,12 @@ effect of its own.
 **Known conformance gaps** (assessed 2026-08-29,
 `history/2026-08-29-v0.7-alignment-and-scope-assessment.md`):
 
-- `tamper_evidence=True` is returned unconditionally by the Postgres backend
-  while `docs/integrity.md` permits it only when a live external attestation
-  exists. The one attestation on record is 2026-08-16 and no job renews it.
-  This is Audit Core overclaiming its own bound — the §9.6 defect turned inward.
+- ~~`tamper_evidence=True` returned unconditionally by the Postgres backend~~
+  **Closed 2026-09-06 (`AUDIT-WP-0009-T01`).** The flag is now derived per
+  read from live chain and attestation state against a declared 168h
+  freshness window. Until `AUDIT-WP-0009-T02` schedules attestation, the
+  honest answer in production is `false` — the overclaim is gone, the
+  precondition is not yet met.
 - No cadence, heartbeat, reconciliation, or load-bearing classification exists.
   The §9.6 obligations Audit Core argued for are not yet supportable by Audit
   Core.

audit_core/integrity.py

@@ -14,6 +14,10 @@ from pathlib import Path
 from typing import Any, Iterable, Mapping
 
 SCHEMA = "audit-core.chain-head.v1"
+# How old a chain-head attestation may be before the tamper-evidence claim
+# degrades. Declared in ``docs/integrity.md`` alongside the attestation
+# cadence; the two are one contract and must move together.
+DEFAULT_ATTESTATION_MAX_AGE_HOURS = 168.0
 GENESIS = "0" * 64
 # Documented advisory-lock key so concurrent accepts cannot fork the head.
 CHAIN_LOCK_KEY = 0xA0D17007
@@ -122,6 +126,106 @@ def _apply_attestation(
     )
 
 
+@dataclass(frozen=True)
+class TamperEvidenceState:
+    """Whether the ``tamper_evidence`` claim is currently earned.
+
+    ``docs/integrity.md`` permits the claim only while both preconditions are
+    live. Absence or staleness degrades it — it does not leave it standing.
+    """
+
+    claimed: bool
+    reason: str
+    observed_at: str | None = None
+    age_seconds: float | None = None
+    max_age_seconds: float | None = None
+
+    def as_dict(self) -> dict[str, Any]:
+        return {
+            "tamper_evidence": self.claimed,
+            "reason": self.reason,
+            "attestation_observed_at": self.observed_at,
+            "attestation_age_seconds": self.age_seconds,
+            "attestation_max_age_seconds": self.max_age_seconds,
+        }
+
+
+def _parse_iso(value: str | None) -> datetime | None:
+    if not value:
+        return None
+    try:
+        parsed = datetime.fromisoformat(str(value))
+    except ValueError:
+        return None
+    if parsed.tzinfo is None:
+        parsed = parsed.replace(tzinfo=timezone.utc)
+    return parsed
+
+
+def evaluate_tamper_evidence(
+    report: ChainReport | None,
+    attestation: Mapping[str, Any] | None,
+    *,
+    max_age_hours: float = DEFAULT_ATTESTATION_MAX_AGE_HOURS,
+    now: datetime | None = None,
+) -> TamperEvidenceState:
+    """Derive the tamper-evidence claim from live state.
+
+    ``report`` must be the chain report produced *against* ``attestation``, so
+    that ``attestation_match`` reflects the cited head. ``report is None``
+    means the chain could not be walked at all, which is not a licence to keep
+    claiming.
+    """
+    max_age_seconds = float(max_age_hours) * 3600.0
+    if report is None:
+        return TamperEvidenceState(
+            False, "chain_unreadable", max_age_seconds=max_age_seconds
+        )
+    if not report.intact:
+        return TamperEvidenceState(
+            False, "chain_break", max_age_seconds=max_age_seconds
+        )
+    if attestation is None:
+        return TamperEvidenceState(
+            False, "no_attestation", max_age_seconds=max_age_seconds
+        )
+    observed_at = str(attestation.get("observed_at") or "") or None
+    if report.attestation_match is not True:
+        return TamperEvidenceState(
+            False,
+            "attestation_mismatch",
+            observed_at=observed_at,
+            max_age_seconds=max_age_seconds,
+        )
+    stamped = _parse_iso(observed_at)
+    if stamped is None:
+        return TamperEvidenceState(
+            False,
+            "attestation_undated",
+            observed_at=observed_at,
+            max_age_seconds=max_age_seconds,
+        )
+    reference = now or datetime.now(timezone.utc)
+    if reference.tzinfo is None:
+        reference = reference.replace(tzinfo=timezone.utc)
+    age_seconds = (reference - stamped).total_seconds()
+    if age_seconds > max_age_seconds:
+        return TamperEvidenceState(
+            False,
+            "attestation_stale",
+            observed_at=observed_at,
+            age_seconds=age_seconds,
+            max_age_seconds=max_age_seconds,
+        )
+    return TamperEvidenceState(
+        True,
+        "attested",
+        observed_at=observed_at,
+        age_seconds=age_seconds,
+        max_age_seconds=max_age_seconds,
+    )
+
+
 def attestation_from_report(report: ChainReport, *, observed_at: str | None = None) -> dict[str, Any]:
     return {
         "schema": SCHEMA,

audit_core/postgres_backend.py

@@ -20,6 +20,7 @@ from __future__ import annotations
 
 import json
 import os
+import time
 from datetime import datetime, timezone
 from typing import Any
 
@@ -35,10 +36,14 @@ from audit_core.interface import (
 from audit_core.credentials import CredentialDirectory
 from audit_core.integrity import (
     CHAIN_LOCK_KEY,
+    DEFAULT_ATTESTATION_MAX_AGE_HOURS,
     GENESIS,
     ChainRow,
+    TamperEvidenceState,
     attestation_from_report,
     chain_link,
+    evaluate_tamper_evidence,
+    load_attestation,
     verify_rows,
 )
 from audit_core.redaction import Finding
@@ -177,6 +182,9 @@ class PostgresAuditBackend:
         statement_timeout_ms: int = 30_000,
         migrate: bool = True,
         credential_dir: str | None = None,
+        attestation_path: str | None = None,
+        attestation_max_age_hours: float = DEFAULT_ATTESTATION_MAX_AGE_HOURS,
+        attestation_cache_seconds: float = 60.0,
     ) -> None:
         # A mounted credential directory takes precedence: it is the only
         # source that can change while the process runs, which is what dynamic
@@ -202,6 +210,19 @@ class PostgresAuditBackend:
         self.recoverable_days = recoverable_days
         self.recoverable_source = recoverable_source
         self.recoverable_basis = recoverable_basis
+        # The chain-head attestation lives outside this database by design
+        # (docs/integrity.md): a copy restored with the table proves nothing.
+        # Its path is mounted, not configured here, so a rotated attestation
+        # takes effect without a restart.
+        self.attestation_path = (
+            attestation_path
+            if attestation_path is not None
+            else os.environ.get("AUDIT_CORE_ATTESTATION_PATH") or None
+        )
+        self.attestation_max_age_hours = float(attestation_max_age_hours)
+        self._attestation_cache_seconds = float(attestation_cache_seconds)
+        self._tamper_state: TamperEvidenceState | None = None
+        self._tamper_state_at = 0.0
         base_kwargs = {
             "autocommit": True,
             # A stalled write must surface as unavailable rather than hold a
@@ -288,10 +309,12 @@ class PostgresAuditBackend:
         ``immutable`` is True because migration 0002 installs a trigger that
         rejects UPDATE and DELETE, so no consumer credential can alter a stored
         record. It is not a claim against the database owner or a superuser,
-        who can drop the trigger. ``tamper_evidence`` is True because a
-        hash chain plus verify detects a rewritten payload, and a chain-head
-        attestation outside this database detects a suffix rewrite that
-        stays inside Postgres. It is not WORM or ``data.archive``.
+        who can drop the trigger. ``tamper_evidence`` is *derived*, not
+        declared: a hash chain plus verify detects a rewritten payload only
+        while a live chain-head attestation outside this database also
+        detects a suffix rewrite that stays inside Postgres. Absent, stale,
+        or mismatched attestation degrades the claim to False — see
+        :meth:`tamper_evidence_state`. It is never WORM or ``data.archive``.
 
         ``custody_class`` is ``operational``, not ``archive``. This store is
         durable append-only Postgres recovered through the platform
@@ -303,7 +326,7 @@ class PostgresAuditBackend:
             custody_class="operational",
             retention_days=self.retention_days,
             immutable=True,
-            tamper_evidence=True,
+            tamper_evidence=self.tamper_evidence_state().claimed,
             durable=True,
             recoverable_days=self.recoverable_days,
             recoverable_source=self.recoverable_source,
@@ -492,6 +515,44 @@ class PostgresAuditBackend:
     def attest_chain(self) -> dict:
         return attestation_from_report(self.verify_chain())
 
+    def load_current_attestation(self) -> dict | None:
+        """The mounted chain-head attestation, or None if absent/unreadable.
+
+        Unreadable is treated as absent on purpose: a malformed file must not
+        be able to keep a claim standing that a missing file would drop.
+        """
+        if not self.attestation_path:
+            return None
+        try:
+            return load_attestation(self.attestation_path)
+        except (OSError, ValueError):
+            return None
+
+    def tamper_evidence_state(self) -> TamperEvidenceState:
+        """Evaluate the tamper-evidence preconditions against live state.
+
+        Cached briefly because ``/readyz`` reads it on every probe and the
+        evaluation walks the chain. The cache only ever delays a *change* of
+        state; it cannot manufacture one.
+        """
+        now = time.monotonic()
+        cached = self._tamper_state
+        if cached is not None and now - self._tamper_state_at < self._attestation_cache_seconds:
+            return cached
+        attestation = self.load_current_attestation()
+        try:
+            report = self.verify_chain(attestation)
+        except Exception:  # backend unavailable, and so is the claim
+            report = None
+        state = evaluate_tamper_evidence(
+            report,
+            attestation,
+            max_age_hours=self.attestation_max_age_hours,
+        )
+        self._tamper_state = state
+        self._tamper_state_at = now
+        return state
+
     def health(self) -> None:
         self._query("SELECT 1", ())
 

docs/audit-backend-contract.md

@@ -120,7 +120,7 @@ Compatibility rules (not yet implemented):
 | `custody_class` | `development`, `operational`, `archive`, or `hot_search` |
 | `retention_days` | Maximum age before eligible deletion; `None` is a lifecycle statement (no expiry), not a recovery guarantee |
 | `immutable` | Whether stored records are protected from in-place alteration |
-| `tamper_evidence` | Whether manifests, hash chains, or signatures exist |
+| `tamper_evidence` | Whether manifests, hash chains, or signatures exist **and their preconditions are currently live** — backends derive this, they do not declare it |
 | `durable` | Whether survival is expected across process restarts and host reboots |
 | `recoverable_days` | Cited platform backup window; `None` if not declared |
 | `recoverable_source` | Where the recoverable window is cited from |
@@ -165,7 +165,10 @@ integrity proofs, or survival of `/tmp` across reboots.
 - `custody_class`: `operational`
 - `retention_days`: unset in production (the service does not expire rows)
 - `immutable`: true (trigger `events_append_only`; not a claim against the database owner)
-- `tamper_evidence`: true (hash chain + verify + external head attestation; not WORM)
+- `tamper_evidence`: **derived per read**, not constant — true only while the
+  chain verifies and a fresh, matching external head attestation is mounted
+  (`docs/integrity.md`, freshness window 168h; not WORM). Absent, stale,
+  unreadable, or mismatched attestation reports false.
 - `durable`: true
 - `recoverable_days`: 30, cited from the platform `data.backup` provision
 - `recoverable_source`: `resource-control/data/capability/platform-audit-storage.json#provisions[capability=data.backup]`

docs/integrity.md

@@ -39,10 +39,39 @@ owner can. Tamper evidence against that class of attacker requires a
 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
+3. that attestation is **fresh** — its `observed_at` is within the
+   freshness window below
 
 It still does not mean WORM, object lock, or ITC-CAP `data.archive`.
 It does not raise provision maturity to D5.
 
+## Attestation cadence and freshness window
+
+An attestation dated once is not a live precondition. The claim is
+derived from the mounted attestation on every read, not declared:
+
+| Setting | Value |
+| --- | --- |
+| Intended attestation cadence | daily |
+| Freshness window | **168 hours (7 days)** |
+| Mount path | `AUDIT_CORE_ATTESTATION_PATH` |
+
+The window is seven times the cadence so that a handful of missed runs
+degrade the claim rather than a single one flapping it. Widening the
+window without shortening the cadence weakens the claim and is a
+change to this contract, not a tuning knob.
+
+`tamper_evidence` degrades to `False`, with the reason recorded, when
+the attestation is absent, unreadable, undated, older than the window,
+or cites a head the live chain does not carry — and when the chain
+itself is broken or cannot be walked. Unreadable is treated as absent
+deliberately: a malformed file must not hold up a claim that a missing
+file would drop.
+
+Until `AUDIT-WP-0009-T02` schedules the attestation job, no attestation
+is mounted in production and `/readyz` reports `tamper_evidence: false`.
+That is the honest reading of the current state, not a regression.
+
 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

tests/test_integrity.py

@@ -137,3 +137,182 @@ def test_http_integrity_requires_read(tmp_path):
     assert body["intact"] is True
     assert body["events"] == 1
     assert "record" not in body
+
+
+# --- AUDIT-WP-0009-T01: tamper_evidence is derived, not declared -----------
+
+
+def _report(**kw):
+    from audit_core.integrity import ChainReport
+
+    fields = dict(
+        intact=True,
+        events=2,
+        head="a" * 64,
+        head_event_id="e2",
+        head_accepted_at="2026-09-01T00:00:00+00:00",
+        first_break=None,
+        attestation_match=True,
+    )
+    fields.update(kw)
+    return ChainReport(**fields)
+
+
+def _at(hours_ago: float):
+    from datetime import datetime, timedelta, timezone
+
+    stamp = datetime(2026, 9, 6, tzinfo=timezone.utc) - timedelta(hours=hours_ago)
+    return stamp.isoformat()
+
+
+def _now():
+    from datetime import datetime, timezone
+
+    return datetime(2026, 9, 6, tzinfo=timezone.utc)
+
+
+def test_fresh_matching_attestation_earns_the_claim():
+    from audit_core.integrity import evaluate_tamper_evidence
+
+    state = evaluate_tamper_evidence(
+        _report(), {"chain_hash": "a" * 64, "observed_at": _at(6)}, now=_now()
+    )
+    assert state.claimed is True
+    assert state.reason == "attested"
+
+
+def test_absent_attestation_degrades_the_claim():
+    from audit_core.integrity import evaluate_tamper_evidence
+
+    state = evaluate_tamper_evidence(_report(attestation_match=None), None, now=_now())
+    assert state.claimed is False
+    assert state.reason == "no_attestation"
+
+
+def test_stale_attestation_degrades_the_claim():
+    from audit_core.integrity import evaluate_tamper_evidence
+
+    # The one attestation on record when this task was written was 21 days old.
+    state = evaluate_tamper_evidence(
+        _report(), {"chain_hash": "a" * 64, "observed_at": _at(21 * 24)}, now=_now()
+    )
+    assert state.claimed is False
+    assert state.reason == "attestation_stale"
+    assert state.age_seconds > state.max_age_seconds
+
+
+def test_attestation_just_inside_the_window_still_counts():
+    from audit_core.integrity import evaluate_tamper_evidence
+
+    state = evaluate_tamper_evidence(
+        _report(), {"chain_hash": "a" * 64, "observed_at": _at(167)}, now=_now()
+    )
+    assert state.claimed is True
+
+
+def test_mismatched_attestation_degrades_the_claim():
+    from audit_core.integrity import evaluate_tamper_evidence
+
+    state = evaluate_tamper_evidence(
+        _report(attestation_match=False),
+        {"chain_hash": "b" * 64, "observed_at": _at(1)},
+        now=_now(),
+    )
+    assert state.claimed is False
+    assert state.reason == "attestation_mismatch"
+
+
+def test_undated_attestation_degrades_the_claim():
+    from audit_core.integrity import evaluate_tamper_evidence
+
+    state = evaluate_tamper_evidence(_report(), {"chain_hash": "a" * 64}, now=_now())
+    assert state.claimed is False
+    assert state.reason == "attestation_undated"
+
+
+def test_broken_chain_degrades_the_claim_even_with_fresh_attestation():
+    from audit_core.integrity import evaluate_tamper_evidence
+
+    state = evaluate_tamper_evidence(
+        _report(intact=False, first_break="e2"),
+        {"chain_hash": "a" * 64, "observed_at": _at(1)},
+        now=_now(),
+    )
+    assert state.claimed is False
+    assert state.reason == "chain_break"
+
+
+def test_unreadable_chain_degrades_the_claim():
+    from audit_core.integrity import evaluate_tamper_evidence
+
+    state = evaluate_tamper_evidence(
+        None, {"chain_hash": "a" * 64, "observed_at": _at(1)}, now=_now()
+    )
+    assert state.claimed is False
+    assert state.reason == "chain_unreadable"
+
+
+def _bare_postgres_backend(tmp_path, attestation_path=None):
+    """A PostgresAuditBackend with no pool — retention_policy needs no I/O."""
+    import pytest
+
+    pytest.importorskip("psycopg", reason="needs psycopg to import the backend")
+    from audit_core.integrity import DEFAULT_ATTESTATION_MAX_AGE_HOURS
+    from audit_core.postgres_backend import PostgresAuditBackend
+
+    backend = object.__new__(PostgresAuditBackend)
+    backend.retention_days = None
+    backend.recoverable_days = 30
+    backend.recoverable_source = "test"
+    backend.recoverable_basis = "measured"
+    backend.attestation_path = str(attestation_path) if attestation_path else None
+    backend.attestation_max_age_hours = DEFAULT_ATTESTATION_MAX_AGE_HOURS
+    backend._attestation_cache_seconds = 0.0
+    backend._tamper_state = None
+    backend._tamper_state_at = 0.0
+    return backend
+
+
+def test_postgres_policy_drops_the_claim_without_an_attestation(tmp_path):
+    backend = _bare_postgres_backend(tmp_path)
+    backend.verify_chain = lambda attestation=None: _report(attestation_match=None)
+    policy = backend.retention_policy
+    assert policy.tamper_evidence is False
+    assert policy.immutable is True
+    assert backend.tamper_evidence_state().reason == "no_attestation"
+
+
+def test_postgres_policy_earns_the_claim_from_a_fresh_attestation(tmp_path):
+    from audit_core.integrity import utc_now
+
+    path = tmp_path / "chain-head.json"
+    path.write_text(json.dumps({"chain_hash": "a" * 64, "observed_at": utc_now()}))
+    backend = _bare_postgres_backend(tmp_path, attestation_path=path)
+    backend.verify_chain = lambda attestation=None: _report(
+        attestation_match=attestation is not None
+    )
+    assert backend.retention_policy.tamper_evidence is True
+
+
+def test_postgres_policy_treats_an_unreadable_attestation_as_absent(tmp_path):
+    path = tmp_path / "chain-head.json"
+    path.write_text("{not json")
+    backend = _bare_postgres_backend(tmp_path, attestation_path=path)
+    backend.verify_chain = lambda attestation=None: _report(attestation_match=None)
+    assert backend.retention_policy.tamper_evidence is False
+    assert backend.tamper_evidence_state().reason == "no_attestation"
+
+
+def test_postgres_policy_drops_the_claim_when_the_chain_cannot_be_walked(tmp_path):
+    from audit_core.integrity import utc_now
+
+    path = tmp_path / "chain-head.json"
+    path.write_text(json.dumps({"chain_hash": "a" * 64, "observed_at": utc_now()}))
+    backend = _bare_postgres_backend(tmp_path, attestation_path=path)
+
+    def _unavailable(attestation=None):
+        raise RuntimeError("backend unavailable")
+
+    backend.verify_chain = _unavailable
+    assert backend.retention_policy.tamper_evidence is False
+    assert backend.tamper_evidence_state().reason == "chain_unreadable"

workplans/AUDIT-WP-0009-evidence-role-conformance.md

@@ -57,7 +57,7 @@ Fixed by the statute; not deferred, not ours:
 
 ```task
 id: AUDIT-WP-0009-T01
-status: todo
+status: done
 priority: high
 state_hub_task_id: "f545b0e4-8c99-5186-affd-ce9a41209ed7"
 ```
@@ -69,6 +69,24 @@ leave it standing. Assert the degradation with a test, and state the freshness
 window in `docs/integrity.md` alongside the two existing preconditions, which
 today are documented but unenforced.
 
+Done 2026-09-06. `evaluate_tamper_evidence` in `audit_core/integrity.py` derives
+the claim from a `ChainReport` and the mounted attestation; `PostgresAuditBackend`
+reads it per `retention_policy` call through `tamper_evidence_state()`, briefly
+cached because `/readyz` probes it. Seven degradation reasons are distinguished
+and asserted — `no_attestation`, `attestation_stale`, `attestation_mismatch`,
+`attestation_undated`, `chain_break`, `chain_unreadable`, against `attested`.
+Unreadable is treated as absent deliberately: a malformed file must not hold up
+a claim a missing file would drop. The freshness window is **168h** against an
+intended daily cadence, declared in `docs/integrity.md` with the reasoning that
+the window is seven cadences so missed runs degrade rather than flap. Twelve
+tests in `tests/test_integrity.py`; `docs/audit-backend-contract.md` now states
+the field is derived rather than declared.
+
+Consequence to state plainly: production `/readyz` will report
+`tamper_evidence: false` until T02 mounts a renewed attestation. The 2026-08-16
+attestation is 21 days old against a 7-day window. That is the overclaim being
+removed, not a regression — the claim was false before and now says so.
+
 ```task
 id: AUDIT-WP-0009-T02
 status: todo