[SCITT] Re: draft-noa-scitt-ai-agent-receipt-00 - ARP reconciliation run against the EMILIA and Noa corpora

[Joel Hillier <[e-mail]>]

Nenad,

Item 4 is a defect in ARP and it is closed.

-03 Section 4.9 defined the inclusion proof as carrying the leaf and said nothing about where a verifier gets the leaf it checks against. Your description transfers without adaptation: present register A's inclusion proof alongside register B's Partial Attestation, and a path-only verifier concludes B was committed. The path folds, the root matches, the signature verifies. Nothing in the walk depends on the object, so no amount of path checking detects it.

-04 adds Section 4.9.1. A verifier MUST compute the leaf from the object whose inclusion is being proved and MUST use that computed value, and no value taken from the proof, as the input to the sibling walk. Where the proof carries a leaf, the comparison happens before the walk and a mismatch refuses the proof whether or not the walk would reach the root.

One thing I got wrong first time that is worth carrying to your side. I wrote the requirement as a comparison against the leaf the proof carries. ARP permits per-register inclusion proofs to be encoded as COSE Receipts (RFC 9942), and an RFC 9942 inclusion proof carries no leaf at all -- so a requirement written only as a comparison is vacuous under exactly the encoding a SCITT-aware verifier is most likely to reach for. The requirement has to be stated over the object: compute the leaf, use the computed value as the walk input, and treat any carried leaf as a field to be refused on mismatch rather than as the thing being checked. Your reject_v2_lifted_anchor_valid_path discriminates correctly under that framing. A second vector in a receipt-style encoding, where there is no leaf field to lift and the only way to fail is to walk from a leaf the verifier did not compute, would additionally discriminate an engine that reads the requirement as comparison-only and passes yours by implementing the comparison and nothing else.

Yes to the slot, and thank you for offering the patch rather than the description. If it applies clean at your 3b59f39 I will run it against the ARP tree and report both legs.

One interaction you should know about before designing that second vector, because it is a consequence of the rule rather than of your finding. Anton Sokolov posted a measurement to this list: an ECDSA signature (r, s) has a verifying twin (r, n - s) that anyone can compute in transit without holding the key. I ran it against ARP's own constructions -- ES256 over P-256, 200 random keys -- and 200 of 200 produced a verifying twin with different bytes. Where a Merkle leaf is the canonical hash of a SIGNED envelope, as ARP's was, a substituted signature changes the leaf. Under the old text that was a silent difference. Under Section 4.9.1 it becomes a refused valid proof: the verifier recomputes the leaf from the envelope it holds, gets a value that does not match the committed one, and refuses. The rule is right, so the leaf moved: ARP now takes it over the signing input, the COSE Sig_structure, which excludes the signature by construction and still covers the protected header. If your leaf construction hashes an envelope too, the same applies, and it is cheap to check.

Item 3 is adopted as a standing rule. ARP's conformance tree already carried rule 9 -- a negative control declares the discriminator it is designed to trip and is credited only when that discriminator is what fired -- which is your observation stated as a refusal. Yours is the constructive form and the tree did not have it: index by the predicate violated, not by the stage that caught the violation, because gate placement is implementation topology and the predicate is the contract. It is rule 11, attributed to you.

It has already paid for itself twice. Applying it to NV-ARP-EO-04 in our own class showed that the control was not mis-specified -- the injected DEFECT was. It skipped the rate-limit charge on both refused arms, which is a budget bug and not an oracle, so the two arms stayed indistinguishable and the channel the row is named for was never opened. There was nothing for any discriminator to fire on. Rebuilt so the resource lookup runs before the charge, the two arms now diverge under equal bursts -- present 7x404 then 4x429, absent 11x404 -- and the row fires its own discriminator for the first time. The discriminator also had to be widened from the 404/429 transition index to the 429 mask, because the transition index is a strict subset of the predicate the specification states and classed the strongest form of the oracle, one arm limited and the other never, as a refusal for the wrong reason. Controls exercised went from six of eight to seven of seven.

Your reply to Henri today lands on the same problem from the other side, and I am taking it as confirmation rather than as a coincidence. "A verifier that proved nothing must never read as green" is, word for word, the invariant I had to add to that class this week. The aggregate could reach PASS the moment the last control fired, while the record's own does_not_establish still named open gaps -- a headline outrunning the record it summarises. It now computes over declared method limits and standing evidence gaps as well as over rows, so it cannot. And your "the authority must be impossible to bypass accidentally, or every copy is a future divergence" is the same law as Henri's instruction to sweep every identity rule in a tree rather than correct the one that was reported. Both say the defect is the second copy, not the first one you found.

Item 2 is open and I would rather decide it with your input than without it. ARP has the transition case in two places and joins them nowhere. Section 4.22 governs primitive rotation and Section 4.21 governs retroactive re-evaluation, and neither says what a re-digest of historical bytes under the successor suite must do. Your discriminating property is the sharp end of it: an engine that re-hashes history under the new suite agrees with the forged digest and accepts, and the agreement is what makes it look correct.

Section 4.22 is more interesting than it was a day ago, because the malleability measurement above falsified its stated premise in a second way. It argued the chain survives a rotation because the entry's bytes "are fixed at the moment the entry is appended and are never rewritten" -- fixed for the operator, and not unique for a reader. That section is rewritten regardless, and your transition question rides on the same text, so I would rather write both at once than write one and revisit.

The two shapes are a Divergence Axis, which qualifies a verdict, or a statement that suite transition belongs to the verifier contract, which refuses outright. My reading is that ARP wants the second, because a Divergence Axis qualifies a disagreement between registers about a subject, and this is not a disagreement about the subject -- it is a verifier being asked to accept a recomputation that the recorded position does not authorise. source-version-skew is corpus-state skew and rightly not a disagreement, as you say; claim-predicate-unsupported is a register's incapacity. Neither is this. But you have shipped the case and I have not, so if the axis is what your verdict set forced on you I would want to know that before I write the refusal.

Item 5 stated an open item of ours better than our own record did, and it is closed. The way it failed first is worth passing on. The first attempt fixed the expected bytes in a vector file, which is what our own record said would close it, and closed nothing: the builder had emitted those bytes from the encoder under test, so regenerating against a deliberately broken encoder produced a byte-identical file. It also contained no byte string of 24 octets or more -- and every digest ARP commits to is a 32-octet byte string -- and no map below the top level. Three separate encoder defects passed it with PASS. The rebuilt class computes every expected byte string with an independent implementation and the builder's own argument-width and ordering logic, then asserts the subject against them; all three defects now fail. Your formulation was right and my first implementation of it was not.

One measurement from it you may want: cbor2 6.1.4 in canonical mode emits the RFC 8949 Section 4.2.3 length-first map ordering, not the Section 4.2.1 bytewise ordering. For {10:1, 100:2, -1:3, "z":4, "aa":5}, whose keys encode to three different lengths, 4.2.1 gives a50a011864022003617a0462616105 and cbor2 canonical gives a50a012003186402617a0462616105. Both are well-formed and one is conforming.

Item 1 noted, and the exoneration-versus-implication asymmetry is the argument I would make for keeping the split. Opposite liability semantics under one code is a code that cannot be relied on by either party.

One last thing, since it touches leaf construction and you are working from CT. RFC 6962 is obsoleted by RFC 9162. The Merkle Tree Hash recursion is unchanged between them -- same empty case, same leaf case, same split at the largest power of two below n, same 0x00/0x01 domain separation -- with the one difference that RFC 9162 carries the hash algorithm as a log parameter where RFC 6962 fixes SHA-256. ARP -04 cites both, 9162 as the current specification and 6962 because it is what deployed logs implement. Three things measured while writing that:

- ARP's construction, which carries an odd last node up unchanged, and the CT recursive split produce identical roots for every leaf count from 1 to 4200 and identical sibling arrays for all 131,328 (leaf count, index) pairs to 512 leaves. They diverge at the empty tree alone, deliberately.

- The third convention in common use -- duplicating the last node at an odd level -- agrees with both at every leaf count that is a power of two, because no level of such a tree is ever odd. It matches at exactly 11 of the first 1024 counts and differs at the other 1013. A conformance vector taken at four or eight leaves does not detect an implementation that ported it.

- An RFC 9162 inclusion proof carries the index and the tree size, as ARP's does. The leaf is the only field that differs.

The script is conformance/runners/merkle_equiv.py, no arguments, no dependencies, about two minutes. It exists because an equivalence asserted in normative text and never executed is a gate reporting success over inputs it never examined, which is the class rules 9 and 11 are both about. That is now rule 12.

Finally, an ask, and you are the right person to refuse it if it does not suit.

Every negative control in ARP's existence-oracle class is fault injection into a read endpoint written by me, from my reading of my own text. That is the one configuration that structurally cannot surface a specification defect, and the run record says so rather than rounding it up. It is now the only thing holding that class below a clean pass. Your item 4 is the first defect in ARP found by somebody running code against it who did not write it -- which is precisely the evidence the class cannot produce. If you or anyone on your side would write an ARP Section 6.4 read endpoint from the text alone and let the class refuse it, that would close the last open gap in the record. If that is too much, the patch alone is a real contribution and I will carry it.

Joel


________________________________________
From: Nenad Vasic <nenadvasic=[e-mail]>
Sent: Monday, 17 August 2026 15:11:04
To: [e-mail]
Subject: [SCITT] Re: draft-noa-scitt-ai-agent-receipt-00 - ARP reconciliation run against the EMILIA and Noa corpora

Joel, -03 answers most of what this thread left open, so this is implementation
experience against the new registry rather than a late answer to the July
questions. We maintain an adjacent layer in the same seat (post-execution action
evidence with principal binding, post-quantum signatures) and have spent recent
weeks in the same exercise against a third-party conformance corpus, so each
point below is grounded in shipped behaviour rather than opinion.

1. The agent-axis split in Section 3 is the one our verdict set forced on us,
and we would support it surviving registry review unchanged. In shipped code, a
credential reference that resolves to nothing binds the signer alone and
*exonerates* the named principal — no one authorized anything — while a
resolved-but-insufficient authorization implicates the delegation chain that
granted what it shouldn't. Opposite liability semantics cannot share a code,
and agent-credential-absent / agent-action-scope-divergence keeps them apart.
The server-recorded marking on agent-action-scope-divergence also matches our
experience: scope-versus-conduct is decidable only where post-execution
evidence lives, never at the register that attested the grant.

2. One case the registry may still want — or may want to declare out of scope:
a digest-suite transition of the evidence itself. source-version-skew is
corpus-state skew and rightly "not a disagreement"; claim-predicate-unsupported
covers what a register cannot evaluate. Neither carries the transition case:
records under a predecessor suite remain valid at their recorded positions,
while a retroactive re-digest of the same bytes under the successor must
refuse. The discriminating property, from the suite-transition pair we
published inline on the web-bot-auth list on 2026-08-09 (p20/n29): a naive
engine that re-hashes history under the new suite *agrees with the forged
digest* and accepts. If Divergence Axes qualify verdicts over long-lived
evidence, that wants either an axis or an explicit statement that suite
transition belongs to the verifier contract.

3. Canonicalization: the -03 registry carries no parse or canonicality axes. If
the malformed classes from your run graduate into it, our experience supports
indexing by the predicate violated (parse-validity, I-JSON profile predicate,
representation canonicality) rather than by the stage that caught the
violation — your own coverage note had seven of nine malformed/* files refused
at the shape gate rather than the purpose-built check. Where a violation is
caught is implementation topology, not contract; one vector per predicate keeps
engines comparable even when their gate placement differs.

4. The vector this thread asked for in July is built.
anchor-leaf-not-bound-to-payload is real, and it is the more dangerous side of
its pair — a broken proof fails loudly everywhere, while a valid proof for the
wrong leaf passes every path check and fails only if the verifier must
recompute the leaf from the payload bytes it actually holds. The vector lifts
accept_with_merkle_anchor_v2's own anchor verbatim — same leaf_hash, same
merkle_root — onto a receipt with a different payload: the Merkle path folds
and the signature verifies while the leaf is bound to another receipt's bytes.
A path-only verifier accepts it; a conformant one rejects on the
leaf == SHA-256(0x00||canon(payload)) self-check alone. Your existing
reject-unbound vector cannot isolate this — its leaf also fails the path walk,
so both checks fire and a path-only engine still refuses. Confirmed with your
own packages/verify at your pin 125e4f43 and re-verified today at 3b59f39
(applies clean; JavaScript and Python legs agree, zero disagreements; the Go
leg is your CI's):

add('reject_v2_lifted_anchor_valid_path', '…the leaf must equal SHA-256(0x00||canon(payload)) of THIS receipt…', false, 'anchor_leaf_unbound', KEY.pub,
receipt({ receipt_id: 'tr_v2_lifted_anchor', issuer: 'ep:demo' }, { anchor: { alg: 'EP-MERKLE-v2', leaf_hash: v2Leaf /* accept_with_merkle_anchor_v2's own leaf */, merkle_proof: [{ hash: v2Sibling, position: 'right' }], merkle_root: v2Root } }));

A git-apply-able patch (generator + regenerated receipts.v1.json, existing
vectors byte-identical) is ready if you want the slot. Registry-wise this is
also a candidate axis the current set lacks; the verifier ordering it
enforces — leaf recomputation before path walk — is how our light-client
verification ships, and it is the ordering that makes reliance on Section
4.18-style ledger heads safe. (The mih-sokolov payload-binding draft's
leaf-construction rule in its 6.1 is the same discipline statement-side; the
two compose.)

5. For Iman's INDETERMINATE positive control, one addition when the preimage
rule is published: spec-supplied bytes-plus-expected-digest vectors are what
make the upgrade durable — deployment-authored vectors measure
self-consistency, not conformance. Our discipline for the same problem: the
preimage is the signed wire bytes themselves, never a re-serialization, frozen
by known-answer tests.

Implementation for the claims above (delegation-chain verdicts, leaf-first
light-client verification, transition contract), offline-verifiable:
https://github.com/navigatorbuilds/elara-mesh

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