Makes the paper's pin-store safety proposition true in the deployed code:
- the full signed STH is retained at pin and at each advance (not just
size+root), so on equivocation both conflicting heads are transferable,
signature-verifiable evidence;
- any equivocation (same-size root mismatch, or a consistency anchor
disagreeing with the pin) persists poisoned=true with both signed
heads and refuses ALL further operations for that log, restart-proof
and independent of the update flag;
- rollback unchanged. 102 tests (was 100): poison persistence, evidence
transferability, full-head retention.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
A transparency log without split-view defense is just a signature with
extra steps: the provider could serve one tree to the agent and another
to the world, or roll the log back, and standalone receipt verification
would never notice. The primitives (RFC 9162 consistency proofs) were
already implemented and correct; this closes the loop on the AGENT side.
- src/pacta/sthstore.py: a local STH pin store. Unknown log -> pin
(trust-on-first-use, recorded as such). Same tree size -> the root
must match the pin byte-for-byte; a mismatch is named EQUIVOCATION
and is a hard rejection. Larger tree -> a consistency proof FROM THE
PINNED SIZE is required and verified before the pin advances
(receipts already embed a from-previous anchor; the anchor's root is
itself checked against the pin so a lying anchor cannot bridge a
split view). Smaller tree -> LOG ROLLBACK, hard rejection.
- Freshness policy: --max-sth-age-seconds rejects stale (or
future-dated) tree heads - an old-but-valid STH can hide later
entries.
- Wired into receipt-verify, claims, and agent (--sth-store,
--consistency-proof, --max-sth-age-seconds); evidence records the
pin action; any accountability failure fails the receipt closed.
- Provider: log-consistency --from-size N (serve proofs for pinning
agents whose pin is older than the receipt's embedded anchor) and
log-audit (monitor self-check: recompute the tree, verify the stored
STH and per-entry leaf hashes).
Live drill in this commit's validation: pin-on-first-use -> matched ->
grown-with-proof advance -> a real forged same-size split view REJECTED
with the equivocation diagnostic -> freshness rejection -> clean
self-audit. tests/test_sthstore.py covers pin/match/equivocation,
growth-without-proof, lying consistency anchors, rollback, freshness.
45/45 tests green.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
