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Security SCITT COSE statement identification reference transparency This document describes proposed requirements and test cases for identifying SCITT Signed Statements. It distinguishes reference matching from statement retrieval, signature verification, issuer authorization, and evidence of registration. It examines how alternative identification schemes treat changes to statement encodings and registration context. The requirements are input to technical discussion and do not represent working group consensus. This document does not define a common reference format, allocate a COSE header parameter, or establish a relationship vocabulary. Its examples illustrate selected properties rather than demonstrate interoperability between deployed transparency services.

Introduction A Transparency Service in the Supply Chain Integrity, Transparency, and Trust (SCITT) architecture registers Signed Statements and returns Receipts. Applications increasingly need to refer to a specific Signed Statement: an audit statement that refers to the statement it audited, a correction that refers to the statement it corrects, an evidence bundle that lists the statements it was built from. For such a reference to be usable, two implementations given the reference and a candidate statement must reach the same conclusion about whether they match. This is harder than hashing some bytes. The same logical Signed Statement can be re-encoded, can gain or lose an unprotected header, and can be registered in more than one Transparency Service, each producing its own entry handle and Receipt. A reference that is meant to survive those changes must say exactly which differences preserve identity and which do not, and must be interpretable without first understanding the application payload of the statement that carries it. The need is not specific to any one product. On 22 September 2026 one of the SCITT working group chairs asked on the scitt@ietf.org list, "On statement IDs please bring all requirements you may have for statement identification", and raised as an open question "whether or not the statement ID should be cryptographically derived from its log insertion or not" . The chair added that it has "pros and cons" and "will need real engagement to get right". This document is input to that discussion. This document states proposed requirements (Section 4), compares candidate identification objects (Section 5), and works through a small set of fully explained test cases (Section 6). It takes a position, independence of the reference-matching result from any particular registration, and argues it with use and trade-offs rather than asserting it as settled. It does not propose a wire format, a COSE header parameter, or a relationship vocabulary for standardization.

Conventions, Terminology and Scope The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. This document uses the SCITT terms Signed Statement, Transparency Service and Receipt as defined in . It adds the following working terms and keeps them apart deliberately:

Statement reference:

Data that designates a target Signed Statement together with the rule by which a candidate is judged to be that target.

Identification scheme:

The rule a reference is read under: which bytes are covered, which differences preserve identity, and, for a digest-based scheme, the hash algorithm and the exact input bytes.

Reference matching:

The operation of deciding, given a reference and a candidate statement, whether the candidate is the reference's target under the named scheme.

Entry handle (Locator):

A Transparency-Service-local identifier of a registration, such as an entry identifier returned by an API . It locates an entry in one service; it is not a portable statement identity.

Registration evidence:

A Receipt that a statement was registered. It is distinct from a reference match.

Matching a reference is not the same as retrieving a statement, verifying its signature, authorizing its Issuer, or proving it was registered. Keeping these apart is the central discipline of this document.

Relationship to existing SCITT work This document is a requirements and test-case text, not a new mechanism. The following work is adjacent; the References identify the revisions used. The architecture defines the Signed Statement envelope: Section 6.1 requires CBOR tag 18, and Section 6.3 requires the unprotected header to be an empty map before a statement is included in a Statement Sequence. It defines the statement, not a portable identifier for one statement or the bytes a statement digest covers. The Receipts document defines Verifiable Data Structures and their proofs; its Section 4.4.1 gives the registration requirements for a VDS, and its Section 5.2.1 is the RFC9162_SHA256 Receipt of Inclusion. Identity through a Receipt is scoped to a log and a VDS construction, and Section 4.4.1 leaves each VDS to define its own encoding and proofs. The payload-binding draft gives, in its Section 8, a general typed-digest-reference information model (type, purpose, digest_alg, digest) in which a carried digest and a recomputed digest are comparable only under one established referenced-artifact digest context, and in Sections 5 and 7 a derived identifier and a statement-to-Receipt binding. This document asks the narrower question of the concrete context a SCITT Signed Statement reference needs. The protected-object-binding draft defines Statement References, Receipt References and Relationship Edges with an initial relationship vocabulary (for example describes, measures, authorizes, supersedes and revokes); it couples identification with that vocabulary, whereas this document keeps matching separate from the relationship it carries. In SCRAPI Section 2.4, a GET on /entries/ followed by an EntryID answers with the Receipt (200), with 204 while registration is still running, or with 404; that EntryID is an API locator for one service, not a portable, registration-independent statement identity. The SCITT charter (charter-ietf-scitt-01, last updated 2026-03-18, retrieved 2026-10-10) lists among its non-goals to "define data formats for payload content" and asks the group to "reuse existing work from IETF WGs such as COSE and RATS, as appropriate". Where a statement-identification requirement or a small profile belongs is itself an open question (Section 10).

Use Cases The following cases need different identification properties; they motivate the requirements without selecting a scheme. Audit reference. An auditor registers a statement that refers to the statement it audited. A reader must match the reference to the audited statement even if the audited statement has since been re-registered elsewhere. Correction reference. An Issuer registers a later statement that corrects an earlier one, keeping the same subject. The subject groups the two; the reference must still select the specific earlier statement, not merely the subject. Offline evidence bundle. A bundle lists the statements it was built from and is checked by a relying party that holds the bytes but queries no service. Matching must not require a service round-trip. Cross-service presence. The same statement is registered in two Transparency Services. A reference that is meant to be portable must designate the same statement across both, distinguishing identity-bearing changes from encoding changes the scheme excludes.

Proposed Requirements These requirements are proposed input, not working group consensus. They are taken from an existing requirements note (see Section 11) and are stated here with their differing roles made explicit: A1 and A2 are properties of a reference, A3 is a design position, A4 is its acceptance test, and A5 is a conditional dependency of schemes that rely on a Receipt, not a fifth independent rule on every reference. The BCP 14 keywords in A1 to A5 address a future identification scheme or profile; they are not obligations this document places on an existing protocol. Where a requirement restates an existing obligation it says so and cites it: the CBOR tag 18 of Section 6.1, the empty unprotected header of Section 6.3, and the VDS registration requirements of Section 4.4.1 are existing obligations this document relies on, not new ones.

A1. A reference has an unambiguous target and matching rule Given a reference and a candidate Signed Statement, a verifier can decide whether they match under a named identification scheme. The scheme states which differences preserve identity, including differences in signatures, headers and encodings. For a digest-based scheme it also defines the hash algorithm, the input bytes, and their encoding. These definitions MAY be supplied by a profile rather than repeated in each reference. The Issuer and Subject pair groups related statements but does not select an individual statement from that group ( Section 6 uses iss and sub to identify the Artifact; the architecture defines no statement identifier). Acceptance: two implementations using the same scheme agree on matching and non-matching candidate vectors; missing inputs or unsupported rules cannot produce a successful match; a ToBeSigned scheme must specify external_aad and the source of any detached payload.

A2. A reference can be read without interpreting the carrier's payload A verifier can locate and interpret a reference without interpreting the application payload of the Signed Statement that carries it. Recomputing a target digest MAY still require the target's payload bytes; that is a property of the target, not of reading the reference from the carrier. This is a proposed scope choice: without it, extracting a reference could require payload-specific parsing or decryption of the carrier.

A3. Matching a statement reference is independent of registration Proposed requirement: matching a reference to a candidate does not depend on registration with a particular Transparency Service or on a particular insertion event. Service-local entry handles and Receipts may coexist with the reference and may differ. This is one proposed answer to that open question. It is a design position, argued from the offline-bundle and cross-service use cases, not a claim that a log-derived identifier cannot work. A log-dependent identifier does not by itself prevent offline verification when the needed bytes, keys and trust anchors are already held; the property asked for here is independence from the registration context, not offline capability in general (Section 5.2).

A4. Acceptance test for A3 Under the same identification scheme, registration in either of two Transparency Services preserves the identity A1 defines. The test must distinguish identity-bearing changes from changes the scheme explicitly excludes. No two-service registration is measured in this document (Section 6 and Section 11); the illustrative cases instead apply local transformations that a registration may cause (an added unprotected parameter; a changed outer encoding) and show which leave the chosen identity unchanged. The tag-removal case (C9) is a negative format control, not evidence of successful registration in two services.

A5. Receipt-verification dependency Where identification depends on a leaf hash or a Receipt, the selected Verifiable Data Structure specification MUST define the entry-to-leaf transformation, serialization and tree hashing sufficiently for independent implementations to reproduce the result. This is a dependency of such schemes and of interoperable Receipt verification, not a fifth independent requirement on every reference. Section 4.4.1 states that "Each VDS specification applying for inclusion in this registry MUST define how to encode the VDS identifier and its Proof Types in CBOR" and that "Each specification MUST define how to produce and consume the supported Proof Types"; its Section 5.2.1 is the Receipt of Inclusion of the specific RFC9162_SHA256 construction, which begins "In a signed proof, the payload is the Merkle Tree root that corresponds to the log at size tree-size". Different VDS constructions differ in their leaf and node hashing (for example the 0x00/0x01 prefixes of Section 2.1.1); that difference does not make either construction ambiguous. This document does not test leaf hashing or Receipts.

Design Alternatives and Trade-offs The central design decision is not the hash algorithm but the identification object: which differences count as identity-changing. The following is the authors' analysis of candidate objects; it is not a selection. Identification object Possible benefit Price to be settled Whole transmitted object bytes Simple exact byte identity Transport or envelope changes produce different values even when other checks still pass An explicitly defined canonical object encoding Equal treatment of selected encoding variants Extra rules, cost, and the risk of diverging normalization The COSE signing input ToBeSigned Binds to the actually signed content and its signature context Does not itself identify the signature value or the unprotected envelope; external inputs must be available and fixed The payload only Fits when the content, not the signed assertion, is meant Issuer, header and signature differences are not necessarily distinguished A VDS leaf or service entry Close tie to a concrete registration Depends on the VDS construction or the service context For COSE, Section 4.3 (externally supplied data) and Section 4.4 (the Sig_structure) are the relevant definitions, and Section 9 constrains the CBOR encoding. A successful signature verification does not by itself fix which universal statement identity an application should use.

The ToBeSigned object, as used in the examples The examples in Section 6 use SHA-256 over ToBeSigned, the Sig_structure ["Signature1", body_protected, external_aad, payload] of Sections 4.4 and 9, with the payload embedded. The scheme names external_aad as an explicit input: it is the empty byte string unless a specific case states otherwise, and a case that signs over a non-empty external_aad lists those exact bytes (Appendix A) and applies them to both matching and signature verification. The body_protected value is the original protected-header byte string, not a decoded and re-encoded map. This object names a class of signed inputs: it excludes the signature value, the unprotected header and the outer wrapper, so changes confined to those parts preserve it, and it does not normalize the bytes inside the protected header or payload. It is used here as one illustrative rule. This document does not propose it as the statement identity defined by the architecture; defines no such identity, and a later profile is free to choose a different object.

Offline is not the same as registration-independent (A3) A scheme can be checked offline when the needed bytes, keys and trust assumptions are already held; a service-dependent identifier is not online merely because it is service-dependent. A3 therefore argues for independence from the registration context, not for a blanket impossibility of offline checking of other models.

Conformance and Illustrative Test Cases Each check is reported on separate axes: ref_usable indicates whether the reference is well-formed and compatible with the named scheme; match is MATCH or NO_MATCH when the comparison can be completed, and INDETERMINATE when the reference is unusable or a required input is absent; sig_valid indicates whether the candidate's signature verifies under the stated key; and is_scitt indicates whether the candidate is a tagged COSE_Sign1 ( Section 6.1 requires CBOR tag 18). A match establishes none of signature validity, issuer trust, payload truth, or registration. A candidate is read as a COSE_Sign1 ( Section 4.2), bare or with tag 18: an array of exactly four elements: the protected header, a byte string that is empty or encodes one map; the unprotected header, a map; the payload, a byte string or nil; and the signature, a byte string. For these illustrative axes, this read check covers only the outer tag, the array shape, the stated element types and the basic CBOR validity rules below. It does not validate header labels or header parameter semantics. The is_scitt axis reports this structural check with tag 18, not complete conformance to the SCITT envelope profile. The candidate and its protected header are each one CBOR data item that is well-formed and valid under Section 5.3.1: every text string is valid UTF-8, and no map holds two keys that are equal under the key equivalence of Section 5.6.1, so that the integer 1 and true are two keys and 0.0 and -0.0 are one. Whether the content of a tag inside the COSE_Sign1 is valid for that tag ( Section 5.3.2) is not checked; the data items inside a tag are checked like any other data item. The deterministic encoding is not required. A candidate that breaks any of this, for example with a text string that is not valid UTF-8 in either header, is not a statement: its is_scitt is False and its sig_valid is None, not evaluated; its match is NO_MATCH under a usable reference and INDETERMINATE under an unusable one. The sig_valid axis is computed with one rule for every case. n*P is the point P multiplied by the integer n. A is the 32-byte public key of the stated key, derived from its seed in Appendix A.2 as in Section 5.1.5; R is the first and S the second 32 bytes of the 64-byte signature, S read as a little-endian integer; B' is the base point and L the order of B' ( Section 5.1); and ToBeSigned is the object of Section 5.1 of this document with the case's external_aad and payload. sig_valid is True if and only if all four rules hold: A and R are canonical encodings: decoding each as in Section 5.1.3 succeeds. A and R have order L: L*A and L*R are the neutral element, and A and R are not the neutral element. S is below L. With k = SHA-512(R || A || ToBeSigned) read as a little-endian integer and reduced modulo L, the cofactorless equation S*B' = R + k*A holds. R enters the hash exactly as received, A as its 32-byte encoding. After the candidate passes the read checks above, a signature that is not 64 bytes gives sig_valid False. Otherwise, if the payload is absent and not supplied, there is no ToBeSigned and sig_valid is None, not evaluated (C8). Otherwise, evaluate the four rules above. Because A and R have order L, the cofactorless equation of rule 4 and the cofactored equation 8*S*B' = 8*R + 8*k*A of Section 5.1.7 accept exactly the same signatures. This document defines individual verification only. A randomized batch result MUST NOT substitute for checking all four rules for each signature. Case C11 has an R of mixed order, for which the two equations differ; rule 2 refuses it before either is checked. The rule fixes the sig_valid axis of these cases only; the match axis does not depend on it. The cases use three synthetic signed statements about one subject and the reference between them, under the ToBeSigned scheme of Section 5.1. Each case is checked against the reference digest given in Appendix A.1. The full bytes, keys and per-case results are in Appendix A; the keys are pure test keys. The fixtures use the fully specified Ed25519 algorithm (COSE algorithm -19); the source package uses EdDSA (-8). Identification is independent of the signature algorithm. ID Req ref_usable match is_scitt sig_valid What it shows C1 A1 True MATCH True True positive control C2 A1 True NO_MATCH True True subject grouping is not single identity C3 A1 True NO_MATCH True True changed target payload C4 A1 True NO_MATCH True True changed protected header C5 A4 True MATCH True True added unprotected parameter (envelope) C6 A1 True MATCH True False signature replaced, same signed input C7 A1 True NO_MATCH True True non-empty external_aad C8 A1 True INDETERMINATE True None detached payload not supplied C9 A1 True MATCH False True tag 18 removed C10 A1 False INDETERMINATE True True scheme/algorithm mismatch C11 A1 True MATCH True False R of mixed order The following cases illustrate the scope and limits of the chosen identity rule. Case C5 adds one unprotected parameter after signing: the ToBeSigned digest and the signature are unchanged while the whole-object digest changes, so a whole-bytes identifier and a ToBeSigned identifier disagree exactly where a registration may rewrite the envelope. C9 reports MATCH under the illustrative ToBeSigned rule and is_scitt = False because tag 18 is absent; it is a negative format control, not evidence of successful registration in two services. Case C8 withholds a detached payload: a missing input yields INDETERMINATE, never a match. Case C6 replaces the signature with one from another key over the same signing input: the ToBeSigned reference still matches, but the signature does not verify under the stated key, which is why the two axes are reported separately. An empty check set is not a PASS: a case for which no axis could be evaluated is reported as INDETERMINATE, not as a match.

Security Considerations This section considers the attacker and the risks this work raises, in the spirit of . A successful reference match is not a trust decision. It does not establish that the candidate's signature is valid, that its Issuer is authorized, that its payload is true, or that it was registered. Relying parties MUST keep these results separate and MUST NOT treat a match as any of them. Collapsing them into a single boolean is the main foreseeable misuse. The identification object determines the attack surface. A whole-bytes identifier changes under any envelope rewrite, so a party expecting stability may fail to match a legitimately re-registered statement. A ToBeSigned identifier is stable across such rewrites but, by construction, does not distinguish two objects that share a signing input and differ only in the signature value or unprotected header; a party that needs to pin the exact signature instance needs a whole-object identifier instead. A scheme MUST state external_aad and the source of any detached payload; a missing input MUST NOT silently become an empty input or a successful match, and a mismatch between a scheme's declared algorithm and a reference's algorithm MUST NOT fall back silently. Tag discipline matters: a generic COSE_Sign1 without CBOR tag 18 is not a SCITT Signed Statement ( Section 6.1), and a test that drops the tag MUST NOT be presented as a valid SCITT case. A stable reference shows only that a designated statement exists and matches; it does not show that no newer relevant statement has appeared, so a reference match MUST NOT be read as current validity or as completeness of a history. A verifier that recomputes a target digest reads externally supplied bytes; large, deeply nested, or cyclically referenced inputs can exhaust resources. An implementation should bound the input size and nesting it will process rather than hashing unbounded input, and should treat a set of references as a list to check, not a graph to traverse. Digest-based identification depends on the collision and second-preimage resistance of the selected hash algorithm.

Privacy Considerations A stable, portable reference is a correlator. The same identifier appearing in two contexts links them. Schemes and deployments SHOULD consider whether a reference needs to be stable across services at all, and whether salted or context-scoped references are more appropriate where linkage is a concern. The subject identifier in a statement groups statements about one Artifact and is itself a correlator independent of the reference scheme. A digest is not by itself a confidentiality mechanism. When the covered content is drawn from a small or well-known set of possibilities, a party that holds the reference can confirm a guess by hashing each candidate and comparing, even if the target statement is never disclosed. A scheme that intends a reference to hide its target needs enough entropy in the covered content (for example a salt) that this guessing attack is infeasible.

IANA Considerations This document has no IANA actions. It allocates no COSE header parameter and defines no registry. The COSE header label -70001 used by the examples in Appendix A is in the Private Use range of the COSE Header Parameters registry (integer values less than -65536); it is a local convention of the example package, not a requested or assigned value.

Open Questions Concrete feedback is sought on the following. The identification object. Should a SCITT statement reference bind to ToBeSigned, to a canonical object encoding, to the whole object bytes, or to a VDS leaf; and should the choice be fixed by the architecture or left to profiles? Registration derivation. That open question stands: should a statement identifier be cryptographically derived from its log insertion, and what are the trade-offs against a registration-independent identifier (A3)? Venue. Whether statement-identification requirements, or a small profile, belong in the SCITT architecture, in a Receipts or payload-binding document, or in a separate document; the charter's payload-format non-goal (Section 2.1) bears on this. Relationship vocabulary. This document deliberately excludes one; whether the identification work needs any relationship terms at all is open.

Implementation Status This section is given per and is to be removed by the RFC Editor before publication. An open-source project, proofbundle, carries the requirements note this document is drawn from and a small synthetic example package, at commit eb1b7eeb28a4f77a4fe9fb1eadc21be0fb03746d (files docs/scitt/statement_identification/requirements.md and the package under docs/scitt/statement_identification/package/). That package signs three synthetic statements and checks the ToBeSigned references between them offline with a CBOR library and a general cryptography library. The illustrative examples here sign with the fully-specified Ed25519 algorithm -19 of ; the package at that commit uses EdDSA -8, which lists as deprecated, and the ToBeSigned identification does not depend on the signature algorithm. The three example signatures were decoded and verified independently in this work by decoding the COSE_Sign1 with a CBOR library and verifying the Ed25519 signature over the Sig_structure with a general cryptography library. pycose 1.1.0, the COSE library present in the run environment, cannot decode algorithm -19; no other independent COSE library was available, so whether a second COSE library decodes and verifies -19 was not tested. The examples were generated and checked with cbor2 5.4.6 and cryptography 41.0.7. The illustrative cases of Section 6 and Appendix A were generated and checked in this work; the fixtures are synthetic and the keys are test keys. One own measurement motivates the choice of hash input under A1 and is reported here as a measured fact, not an endorsement. Against a locally run transparency ledger (microsoft/scitt-ccf-ledger at commit 00101f76, CCF 7.0.17), of 62 registered rows, 39 had a SHA-256 of the submitted bytes that differed from the data-hash in the returned Receipt, because the service sets the unprotected header to an empty map and rewrites the outer encoding before inclusion. This measurement shows why a reference scheme must specify whether it covers submitted bytes or the representation included in the Statement Sequence; it does not by itself show that the identifier depends on the service or insertion event. The corpus matrix is in the proofbundle repository at tools/scitt_ccf_external/differential_corpus_round3/README.md (freeze commit c5ff0a72e28aa087eecd3e2b426577be3cf2a80f) and the replay at tools/scitt_ccf_external/replay_earlier_build/results_table.md (commit 3ca368164b68a09a863f7ec12b2f9fa1b21123ec). Only the repository evidence is cited here. This information is given per and does not constitute endorsement.

Illustrative Test Cases: Bytes and Keys All byte strings are hex. Each of the three base statements is a tagged COSE_Sign1 (first byte 0xd2, CBOR tag 18) signed with the fully-specified Ed25519 algorithm (COSE algorithm -19, ). The private-use label carries a list of references; in these examples the list holds exactly one, the array [-16, "ToBeSigned", digest], which references statement 01, and the digest is SHA-256 over the ToBeSigned of statement 01. The test keys are pure test keys given as fixed seeds and MUST NOT be used for any real statement.

Base statements and the reference All three statements share the subject pkg:generic/example-widget@1.0.0. COSE header label -70001 (Private Use) carries a list of references; in these examples the list holds exactly one, [-16, "ToBeSigned", digest], which references statement 01, and its digest is SHA-256 over the ToBeSigned of 01. Reference digest (SHA-256 over ToBeSigned of 01): c3f5f13764679b9f77918bb52782480bc222be5ae4bce37f51384a6b1faa9dbc 01 ToBeSigned SHA-256: c3f5f13764679b9f77918bb52782480bc222be5ae4bce37f51384a6b1faa9dbc Statement 01 (iss https://issuer.example; 330 bytes; SHA-256 9c7253617b9fe51ba25eb0f957dadea493ca03e98c4a81806237757f13d054c6):

Statement 02 (iss https://auditor.example, refers to 01; 365 bytes; SHA-256 c4758ea9ae4d85461a93ee1dbb302fb698c922b1fbcb4139ebc706e3aeccbebd):

Statement 03 (iss https://issuer.example, refers to 01; 450 bytes; SHA-256 f5afbb3b0c7d822b00741e5eacd9128b723922bf3cad6ed5a3ab00e884b58e99):

Test keys PURE TEST KEYS given as fixed seeds. They MUST NOT be used for any real statement. issuer: Ed25519 seed (hex, 32 bytes): 51bdccce75df50a4bcee21a2f730ccdcd4917a0a4d20dd2ab3d2e5ffad3f6fd3 kid (hex, SHA-256 of the SubjectPublicKeyInfo): 0e6e86cd20c07df4e95cb4a07a286192d29fc74567b41e726f1c7f7bd202eb5b auditor: Ed25519 seed (hex, 32 bytes): 2a8b1161e212cc24681c06e6d7486daa0910253e54903d41ae964aaee2bfc681 kid (hex, SHA-256 of the SubjectPublicKeyInfo): a2820b29631a0e52ec5dbe41f948a9e5725c6b8d8c0346bd3ef5483cab27de58

Cases Each case below gives the candidate's full bytes and its SHA-256, so a reader can confirm the exact bytes the case was run on. The expected axes are in the table of Section 6; the measured axes are given here.

C1 (requirement A1) Unchanged reference and candidate (the reference target 01). candidate SHA-256 (whole COSE_Sign1): 9c7253617b9fe51ba25eb0f957dadea493ca03e98c4a81806237757f13d054c6 (330 bytes) measured: ref_usable = True, match = MATCH, is_scitt = True, sig_valid = True

C2 (requirement A1) Same issuer and subject, different statement (03): subject grouping does not select 01. candidate SHA-256 (whole COSE_Sign1): f5afbb3b0c7d822b00741e5eacd9128b723922bf3cad6ed5a3ab00e884b58e99 (450 bytes) measured: ref_usable = True, match = NO_MATCH, is_scitt = True, sig_valid = True

C3 (requirement A1) Changed target payload (re-signed): ToBeSigned differs. candidate SHA-256 (whole COSE_Sign1): 63723353fdf05662cfcc618e8b47fd4142dce4a71e84b61a6bdbdde988e837ab (339 bytes) measured: ref_usable = True, match = NO_MATCH, is_scitt = True, sig_valid = True

C4 (requirement A1) Changed protected header (content type), re-signed: ToBeSigned differs. candidate SHA-256 (whole COSE_Sign1): 65eb1e45177add009aef819b438c910aae934c3a05b7966ef73614e11ff07bcc (330 bytes) measured: ref_usable = True, match = NO_MATCH, is_scitt = True, sig_valid = True

C5 (requirement A4) Added one unprotected parameter after signing (tag 18 kept): ToBeSigned and signature unchanged, whole-object digest changes. candidate SHA-256 (whole COSE_Sign1): fbab2b2b1c572ca374458ba2590fa5ca0da056a6ce47666f7220f1bca4250ab8 (355 bytes) measured: ref_usable = True, match = MATCH, is_scitt = True, sig_valid = True

C6 (requirement A1) Same ToBeSigned, signature replaced with one from a different key: matches the ToBeSigned reference, but does not verify under the issuer key. candidate SHA-256 (whole COSE_Sign1): f8bf927260ae6d9c6edadeb79e0295e39dc30341b21b1a61d6cf31b35d778bf8 (330 bytes) measured: ref_usable = True, match = MATCH, is_scitt = True, sig_valid = False

C7 (requirement A1) Target signed over a non-empty external_aad. The scheme applies the supplied AAD bytes (given below) to both reference matching and signature verification, so the empty-external_aad reference does not match while the signature verifies. The same candidate bytes under the fixed empty-external_aad rule are reported separately below. A missing external context is not assumed empty. candidate SHA-256 (whole COSE_Sign1): 0a5390069e431e6dd83764ee276b28f69d34986a9e64d82f05ef3322bc66d61c (330 bytes) external_aad (hex, 17 bytes): 6374783a6465706c6f796d656e742d3432 measured: ref_usable = True, match = NO_MATCH, is_scitt = True, sig_valid = True the same candidate bytes under the fixed empty external_aad rule: match = MATCH, sig_valid = False

C8 (requirement A1) Detached payload (payload field is nil) and the detached bytes are not supplied: the match is INDETERMINATE, never a success. candidate SHA-256 (whole COSE_Sign1): 7ceb07265f8b803a1ff96924da0b3a48b8355ec9daed0a9a31cd36cddaf34fdb (195 bytes) measured: ref_usable = True, match = INDETERMINATE, is_scitt = True, sig_valid = None note: detached payload not supplied; a missing input is never a match

C9 (requirement A1/RFC9943-6.1) Tag 18 removed, nothing else changed: the ToBeSigned digest still matches, but the object is not a tagged COSE_Sign1, so it is not a valid SCITT Signed Statement (RFC 9943 Section 6.1 requires tag 18). candidate SHA-256 (whole COSE_Sign1): c205d745619c3087ad1f83f6c1f7883f4fff616f8f90ecbe64c4aa08854b28e6 (329 bytes) measured: ref_usable = True, match = MATCH, is_scitt = False, sig_valid = True

C10 (requirement A1) The scheme names SHA-512 (-44), while the reference carries SHA-256 (-16), so ref_usable is False and match is INDETERMINATE; no fallback is performed. candidate SHA-256 (whole COSE_Sign1): 9c7253617b9fe51ba25eb0f957dadea493ca03e98c4a81806237757f13d054c6 (330 bytes) measured: ref_usable = False, match = INDETERMINATE, is_scitt = True, sig_valid = True

C11 (requirement A1) Statement 01 with its 64 signature bytes replaced: R is a canonical encoding of a point of mixed order, and S is below L. Rule 2 refuses R, so sig_valid is False while the ToBeSigned reference matches. With rule 2 left out, the cofactored equation holds and the cofactorless equation does not. candidate SHA-256 (whole COSE_Sign1): d1e77e708d759c9b393e93708968887df1ef0e56872166877f68d4ff9e43afe1 (330 bytes) measured: ref_usable = True, match = MATCH, is_scitt = True, sig_valid = False with rule 2 left out: the cofactorless equation gives sig_valid = False, the cofactored equation gives sig_valid = True

Acknowledgments The author used AI tools to help draft this document and to generate and cross-check its examples. The author reviewed the entire text and is responsible for it. Citing adjacent work in the References does not imply that its authors endorse this document.

Provenance The requirements in Section 4 are drawn from the requirements note at the commit cited in Section 11. The illustrative cases in Appendix A were generated for this document; Section 11 separately identifies the sources of the reported measurement.

Open Items (to be removed before publication) Reference verification. The references were checked against the published documents on 2026-10-10; re-check at submission time, including whether has been published as an RFC, and run idnits with network access. List note. If the observation note behind Open Question 2 is published on the SCITT list before submission, add its archive link to the Implementation Status section.