]> Council of AI (CSOAI Ltd)

3rd Floor, 86-90 Paul Street London EC2A 4NE United Kingdom nicholas@csoai.org https://councilof.ai/

Security SCITT SCITT COSE CBOR A signed statement conveyed as a COSE_Sign1 object may be serialized into many distinct byte sequences that all decode to the same data item. Where a protocol identifies such a statement by a digest computed over its wire octets (referred to here as a data-hash), the identifier is sensitive to that framing while the signature over the statement is not. This document reports a measurement of the size of that class. Taking one 165-octet COSE_Sign1 object and re-emitting it under every combination of six CBOR encoding freedoms yields 64 distinct octet sequences. All 64 carry an identical Sig_structure and therefore an identical, valid signature. All 64 produce distinct data-hash values, with no collisions. A stock CBOR decoder rejected none of them, and 31 were silently repaired into the original form by the act of being read. This document specifies nothing and proposes no wording. It reports a measurement, publishes the reproduction recipe, and identifies the prior work that already addresses the problem it measures.

Introduction Transparency protocols in the SCITT architecture frequently need a stable identifier for a signed statement. A natural choice is a digest over the octets of the statement as it appeared on the wire, since that is what a client fetches and can byte-compare. This document refers to such a digest as a data-hash. The CCF profile for COSE receipts commits each ledger leaf to a data-hash of the registered Signed Statement (its Section 2). The SCITT Reference APIs leave the form of an entry identifier to the Transparency Service, which may derive it in the same way. The measurement reported here is offered as input to that work rather than as a comment on either document. CBOR permits a single data item to be encoded in more than one way. A COSE_Sign1 object may be emitted with or without its tag, with definite or indefinite length arrays and maps, and with byte strings emitted whole or in chunks. Each such choice changes the octets without changing the decoded value. The signature does not observe those choices. Per , Section 4.4, the Sig_structure covers the protected header and the payload; it does not cover the framing of the enclosing container. The consequence is an asymmetry: two parties may agree that a signature is valid and still disagree on the data-hash of the statement they have both just verified. Two instances of this asymmetry were raised independently on the SCITT mailing list within two days of each other in September 2026: one concerning the CBOR tag, one concerning the outer array. That coincidence prompted the question this document answers: not "which framings exist", but "how large is the class they belong to".

Conventions and Definitions This document is Informational and contains no normative requirements.

data-hash:

A digest computed over the octet sequence of a signed statement as transmitted, used as an identifier for that statement.

framing:

Those aspects of a CBOR encoding that may vary without changing the decoded data item.

Method A single COSE_Sign1 object, referred to here as A, was taken as the starting point. A is 165 octets and has the SHA-256 digest 8595e4a4c8b93e7b1b7b798dc302a2b7d2890021f7eff372d79b32f78867e4ac. A was decoded once, and the resulting data item re-emitted under every combination of six independent encoding freedoms: presence or absence of CBOR tag 18; definite or indefinite length outer array; definite or indefinite length unprotected header map; protected header byte string emitted whole or chunked; payload byte string emitted whole or chunked; signature byte string emitted whole or chunked. Six binary freedoms give 64 combinations. No element value was altered at any point; only its encoding. A chunked byte string is emitted as an indefinite-length string of consecutive chunks of half its length, rounded down (two or three chunks), as the published script does. Each resulting octet sequence was then decoded with a stock CBOR decoder, and both its data-hash and its Sig_structure were computed. The measurement was first made with cbor2 6.1.3 on CPython 3.13, macOS arm64, and repeated for this revision with the published script, unchanged, using cbor2 5.6.5 on CPython 3.11, Linux x86_64. Both runs produced the figures reported in -00; the row counting re-serialization to any form, and the size range, come from the second run only. The measurement, its inputs and the script are published ().

Results Quantity Value Encoding freedoms varied 6 Distinct octet sequences produced 64 Distinct data-hash values 64 Data-hash collisions 0 Rejected by the stock decoder 0 Silently re-serialized to A on read (excluding A) 31 Silently re-serialized on read, to any form 62 Distinct Sig_structures across all 64 1 The Sig_structure rows and the re-serialization rows carry the substance of the result. There is exactly one Sig_structure across all 64 encodings: 109 octets, with the SHA-256 digest 60b4c76b84c456ed0604305075f51f6d050476177ef17be70a13a7b9abfb370f. A single signature therefore verifies over every one of the 64 encodings, while each of those encodings has a different data-hash. A decode-then-re-encode cycle leaves only two of the 64 unchanged: A itself, and the untagged, fully definite encoding referred to here as C (164 octets). The 31 other tagged encodings come back as exactly A's octets; the 31 other untagged encodings come back as exactly C's octets. An implementation that stores a re-serialization of what it received, rather than the received octets themselves, will therefore compute a data-hash it cannot later reproduce from what it published, and no error is raised at any point in that sequence. Revision -00 reported the first of these two groups (31) and not the second. With the chunk rule above, the 64 encodings range from 164 to 179 octets. The seven variants named in the published artifact range from 164 to 170 octets; revision -00 gave that narrower range for all 64, which was an error. This measurement does not vary integer-width encoding or map key ordering. The true class is therefore larger than 64.

Discussion The observation this document supports is narrow: a rule that enumerates disallowed framings cannot close a combinatorial class, because the class is larger than the set of framings anyone has happened to notice. Two axes were found by accident in one week; six freedoms give 64 encodings; and the six tested are not all of them. A requirement to use the core deterministic encoding of , Section 4.2.1, would exclude indefinite-length items, and with them five of the six freedoms measured here. It would not settle the sixth: , Section 2, defines both tagged and untagged COSE_Sign1 messages, so two encodings of this object, A and C, would remain, with two data-hash values and one signature. This document does not propose that data-hash be made framing-invariant. The wire octets are what a client fetches and byte-compares, so hashing them is the appropriate choice. The open question is only which octets are meant. A consequence worth stating explicitly, because it is an implementation requirement rather than a wording question: a service that publishes a data-hash cannot reproduce the identifier it published unless it retains the octets it received rather than a re-serialization of them. That 62 of 64 encodings are rewritten by a decode-and-re-encode cycle indicates that such a storage path will encounter this in practice rather than in theory.

Relationship to Existing Work This document discovers nothing. It measures the size of a class whose members were identified by others, and the approaches that address it were published before this measurement was made. registers an as-transmitted algorithm (its Section 4.4), under which no canonicalization is applied and the pre-image is the exact octet sequence, and records two earlier canonicalization algorithms as withdrawn. That approach closes the whole class measured here, and predates this measurement. The outer-array framing axis was raised on the SCITT mailing list on 2026-09-04 by Konrad Gruszka, together with an independent recomputation performed on a different platform with a different reader and no COSE library. The tag axis was raised on the list on 2026-09-03 by the author of this document. The tag axis surfaced only because Emek Can Dogru recomputed a previously published vector rather than reading it. This document is offered because the measurement is reproducible and the magnitude may be useful to those weighing the alternatives, not because the alternatives are in doubt.

Security Considerations The behaviour reported here is a naming and reproducibility hazard rather than a break of any cryptographic primitive. No signature is forged, no digest is collided, and the signature scheme behaves exactly as specifies. The hazard is that a verifier and an issuer may both be correct and still disagree. Where a data-hash is used as a lookup key, an index, a deduplication key, or as the subject of a further attestation, an adversary able to influence the framing of a statement in transit, without invalidating its signature, can cause that statement to be filed under an identifier the issuer does not expect, or to appear absent from a service that in fact holds it. Retaining received octets verbatim, as describes, removes this class of hazard. Enumerating individual disallowed framings does not, since the enumeration cannot be shown to be complete.

IANA Considerations This document has no IANA actions.

The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. These design goals make it different from earlier binary serializations such as ASN.1 and MessagePack. This document obsoletes RFC 7049, providing editorial improvements, new details, and errata fixes while keeping full compatibility with the interchange format of RFC 7049. It does not create a new version of the format. Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. There is a need to be able to define basic security services for this data format. This document defines the CBOR Object Signing and Encryption (COSE) protocol. This specification describes how to create and process signatures, message authentication codes, and encryption using CBOR for serialization. This specification additionally describes how to represent cryptographic keys using CBOR. This document, along with RFC 9053, obsoletes RFC 8152. Traceability in supply chains is a growing security concern. While Verifiable Data Structures (VDSs) have addressed specific issues, such as equivocation over digital certificates, they lack a universal architecture for all supply chains. This document defines such an architecture for single-issuer signed statement transparency. It ensures extensibility and interoperability between different transparency services as well as compliance with various auditing procedures and regulatory requirements. Action State Group, Inc. Tyche Institute Independently written systems that anchor records to a SCITT Transparency Service repeatedly need the same construction: a canonical form of structured content, a content-addressed identifier derived from that form, binding to a SCITT Signed Statement and Receipt, and references that cite external artifacts by digest. This document, referred to as CPB, specifies that construction as declarations rather than as a payload format. A payload profile declares its canonicalization algorithm and exclusion set and thereby obtains a reproducible derived identifier. A CPB Signed Statement carries either the complete statement content as specified by RFC 9943 or a digest of content held elsewhere using the COSE Hash Envelope of RFC 9995. CPB also defines an abstract typed digest reference information model and one optional protected-header encoding, cpb-refs; a payload profile may instead define its own reference serialization. An IANA registry assigns the canonicalization algorithm identifiers that these declarations name. CPB does not define payload content formats, establish or require a universal artifact-type registry, or require either typed-reference carrier. Fraunhofer SIT Bowball Technologies Ltd Microsoft Research This document specifies a REST API with the HTTP resources, request and response messages, and error handling needed for an interoperable implementation of a SCITT Transparency Service, as defined by the Supply Chain Integrity, Transparency, and Trust (SCITT) Architecture. Fraunhofer SIT Microsoft Research Microsoft Research Microsoft Research This document defines a new verifiable data structure (VDS) type for COSE Receipts and the associated inclusion and consistency proofs, specifically designed for append-only logs produced by the Confidential Consortium Framework (CCF) to provide stronger tamper- evidence guarantees.

Reproducing the Measurement The published artifacts contain the starting object A, the seven named variants with their sizes and digests, the finding, and the script that mints and checks them: https://councilof.ai/interop/scrapi-ccf/data-hash-framing-space.json https://councilof.ai/interop/scrapi-ccf/data-hash-vector.json https://councilof.ai/interop/scrapi-ccf/mint_framing_space.py The starting object A is field A_signed_statement_as_registered.bytes_hex of the second file. Its SHA-256 is recorded in the first. Placing the three files in one directory and running the script with any recent cbor2 re-emits A under the six freedoms listed in Section 3, hashes each result and checks the figures against the published finding.

Changes from -00 RFC Editor: please remove this section before publication. Corrected the size range: 164 to 170 octets is the range of the seven named variants; across all 64 it is 164 to 179. Reported the second re-serialization group: the 31 other untagged encodings re-serialize to C, so 62 of 64 are rewritten on read. The figure of 31 re-serialized to A is unchanged. Added the effect of RFC 8949 core deterministic encoding (two encodings remain). Stated the chunk rule, recorded a second run on a different platform and decoder version, and listed the script's URL. Updated references: draft-mih-sokolov-scitt-payload-binding-05 (now titled "Canonicalization Declaration for SCITT Signed Statements"), draft-ietf-scitt-receipts-ccf-profile-05; described the SCITT Reference APIs' entry identifier more precisely.

Acknowledgements Konrad Gruszka raised the outer-array framing axis and independently recomputed the vector on a separate platform. Emek Can Dogru recomputed a published vector rather than reading it, which is what surfaced the tag axis. Anton Sokolov's registration of an as-transmitted binding reached the conclusion this measurement supports before the measurement was made.