The deployed consistency verifier implemented the RFC 9162 2.1.4.2
bit-navigation loop but its final return checked only the two
reconstructed roots, omitting the terminal condition that the new-size
navigation counter reach zero. That condition couples the consumed proof
length to the claimed tree sizes; without it, a valid proof for one
transition verifies under a lied (power-of-two) old size. Flagship: a
valid 2->3 proof is accepted under the false claim 1->3 with the size-2
root.
Fix: add `and sn == 0` to the final return.
This is the corpus's Known Gap 14 (3,867 deployed-accepts-only cases in a
pinned 73,573-case family, recorded in public log entry 13). It was
found by the project's own differential harness; a post-appeal review
round added a faithful RFC oracle as a third comparison, which showed
the deployed verifier — not the mechanized model — was the one deviating
from RFC 9162, and traced it to the missing terminal check.
Scope: verify_consistency's only production caller is the consumer-side
pin store, reached only behind a verified head signature. Generation is
RFC-correct and unaffected; the live provider service does not run this
verifier; the published standalone verify.py has no consistency verifier.
An empirical search found 0 realizable pin-advance poisons against an
honestly pinned consumer, consistent with Known Gap 14's non-claim.
Verification:
- New fail-first three-way regression test
test_consistency_lied_size_three_way_agreement (deployed / recursive
ConsRec model / independent faithful RFC 9162 transliteration) over the
honest AND lied-size families; fails pre-fix, passes post-fix.
- Historical differential tests (164,479 inclusion; 164,224 consistency)
unchanged — the fix rejects nothing honest.
- Full suite: 145 passed, 0 failed.
Public log entry 13, the attested accumulator commit, and the IACR
submission PDF are all unchanged. Vulnerable state tagged
vulnerable/sn0-consistency-fd2f6ba. See
docs/security-2026-07-23-consistency-terminal-check.md.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
