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Security Secure Patterns for Internet CrEdentials cose cwt selective disclosure This document describes a data minimization technique for use with CBOR Web Token (CWT). The approach is based on Selective Disclosure JSON Web Token (SD-JWT), with changes to align with CBOR Object Signing and Encryption (COSE). About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Secure Patterns for Internet CrEdentials Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction This document updates the CBOR Web Token (CWT) specification , enabling the holder of a CWT to disclose or redact special claims marked disclosable by the issuer of a CWT. The approach is modeled after SD-JWT , with changes to align with conventions from CBOR Object Signing and Encryption (COSE) . This specification enables Holders of CWT based credentials to prove the integrity and authenticity of selected attributes asserted by an Issuer about a Subject to a Verifier. Although techniques such as one time use and batch issuance can improve the confidentiality and security characteristics of CWT based credential protocols, CWTs remain traceable. Selective Disclosure CBOR Web Tokens (SD-CWTs) are CWTs and can be deployed in protocols that are already using CWTs, even if they contain no optional to disclose claims. Credential types are distinguished by their attributes, for example a license to operate a vehicle and a license to import a product will contain different attributes. The specification of credential types is out of scope for this document, and the examples used in this document are informative. SD-CWT operates on CWT Claims Sets as described in . CWT Claims Sets contain Claim Keys and Claim Values. SD-CWT enables Issuers to mark certain Claim Keys or Claim Values mandatory or optional for a holder of a CWT to disclose. A holder cannot send redacted claim keys to a verifier who does not understand selective disclosure at all. However, Claim Keys and Claim Values which are not understood remain ignored as described in .

High level flow Figure 1: High level SD-CWT Issuance and Presentation Flow | Request Nonce | | Receive SD-CWT +-------------------------------->| | | | | |<--------------------------------+ | | Receive Nonce | | +---+ | | | | Redact Claims | | |<--+ | | | | | +---+ | | | | Demonstrate | | |<--+ Posession | | | | | | Present SD-CWT | | +-------------------------------->| | | | ]]> This diagram captures the essential details necessary to issue and present an SD-CWT. The parameters necessary to support these processes can be obtained using transports or protocols which are out of scope for this specification. However the following guidance is generally recommended, regardless of protocol or transport.

1. The issuer SHOULD confirm the holder controls all confirmation material before issuing credentials using the cnf claim.
2. To protect against replay attacks, the verifier SHOULD provide a nonce, and reject requests that do not include an acceptable an nonce (cnonce). This guidance can be ignored in cases where replay attacks are mitigated at another layer.

Terminology 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 terms from CWT , and COSE . The terms Claim Name, Claim Key and Claim Value are defined in . This document defines the following new terms:

Selective Disclosure CBOR Web Token (SD-CWT):

A CWT with claims enabling selective disclosure with key binding.

Selective Disclosure Key Binding Token (SD-CWT-KBT):

A CWT used to demonstrate possession of a confirmation method, associated to an SD-CWT.

Issuer:

An entity that produces a Selective Disclosure CBOR Web Token.

Holder:

An entity that presents a Selective Disclosure CBOR Web Token which includes a Selective Disclosure Key Binding Token.

Verifier:

An entity that validates a Partial or Full Disclosure by a holder.

Partial Disclosure:

When a subset of the original claims protected by the Issuer, are disclosed by the Holder.

Full Disclosure:

When the full set of claims protected by the Issuer, is disclosed by the Holder. An SD-CWT with no blinded claims (all claims are marked mandatory to disclose by the Issuer) is considered a Full Disclosure.

Salted Disclosed Claim:

A salted claim disclosed in the unprotected header of an SD-CWT.

Digested Salted Disclosed Claim / Blinded Claim Hash:

A hash digest of a Salted Disclosed Claim.

Blinded Claim:

Any Redacted Claim Key or Redacted Claim Element which has been replaced in the CWT payload by a Blinded Claim Hash.

Redacted Claim Key:

The hash of a claim redacted from a map data structure.

Redacted Claim Element:

The hash of an element redacted from an array data structure.

Presented Disclosed Claims Set:

The CBOR map containing zero or more Redacted Claim Keys or Redacted Claim Elements.

Validated Disclosed Claims Set:

The CBOR map containing all mandatory to disclose claims signed by the issuer, all selectively disclosed claims presented by the holder, and omitting all undisclosed instances of Redacted Claim Keys and Redacted Claim Element claims that are present in the original SD-CWT.

Overview of Selective Disclosure CWT

A CWT without Selective Disclosure Below is the payload of a standard CWT without selective disclosure. It consists of standard CWT claims, the holder confirmation key, and five specific custom claims. The payload is shown below in CBOR Extended Diagnostic Notation (EDN) . Note that some of the CWT claim map keys shown in the examples have been invented for this example and do not have registered integer keys. The custom claims deal with attributes of an inspection of the subject: the pass/fail result, the inspection location, the license number of the inspector, and a list of date when the subject was inspected.

Holder gets an SD-CWT from the Issuer Alice would like to selectively disclose some of these (custom) claims to different verifiers. Note that some of the claims may not be selectively disclosable. In our next example the pass/fail status of the inspection, the most recent inspection date, and the country of the inspection will be fixed claims (always present in the SD-CWT). After the Holder requests an SD-CWT from the issuer, the issuer generates an SD-CWT as follows:

Issued SD-CWT with all disclosures >, / CWT unprotected / { / sd_claims / 17 : [ / these are all the disclosures / <<[ /salt/ h'bae611067bb823486797da1ebbb52f83', /claim/ 501, / inspector_license_number / /value/ "ABCD-123456" ]>>, <<[ /salt/ h'8de86a012b3043ae6e4457b9e1aaab80', /value/ 1549560720 / inspected 7-Feb-2019 / ]>>, <<[ /salt/ h'7af7084b50badeb57d49ea34627c7a52', /value/ 1612560720 / inspected 4-Feb-2021 / ]>>, <<[ /salt/ h'ec615c3035d5a4ff2f5ae29ded683c8e', /claim/ "region", / region=California / /value/ "ca" ]>>, <<[ /salt/ h'37c23d4ec4db0806601e6b6dc6670df9', /claim/ "postal_code", /value/ "94188" ]>>, ] } / CWT payload / << { / iss / 1 : "https://issuer.example", / sub / 2 : "https://device.example", / exp / 4 : 1725330600, /2024-09-03T02:30:00+00:00Z/ / nbf / 5 : 1725243900, /2024-09-02T02:25:00+00:00Z/ / iat / 6 : 1725244200, /2024-09-02T02:30:00+00:00Z/ / cnf / 8 : { / cose key / 1 : { / kty / 1: 2, / EC2 / / crv / -1: 1, / P-256 / / x / -2: h'8554eb275dcd6fbd1c7ac641aa2c90d9 2022fd0d3024b5af18c7cc61ad527a2d', / y / -3: h'4dc7ae2c677e96d0cc82597655ce92d5 503f54293d87875d1e79ce4770194343' } }, /most_recent_inspection_passed/ 500: true, / redacted_claim_keys / 59(0) : [ / redacted inspector_license_number / h'0ad0f76dcb7fd812ca64c3ada3f543be 96d0e351e1e576fbab5cb659b49e599e' ], /inspection_dates/ 502 : [ / redacted inspection date 7-Feb-2019 / 60(h'f468b68dd7432030d7f33a8783acb3a4 d6afc215bbc184dce8831c64b539f335'), / redacted inspection date 4-Feb-2021 / 60(h'0839dcbdd9ed55c8aac3e573ccf59c81 0fa8d9f6ec551cb9737621faf3584e1f'), 1674004740, / 2023-01-17T17:19:00 / ], / inspection_location / 503 : { "country" : "us", / United States / / redacted_claim_keys / 59(0) : [ / redacted region / h'dff0cb7a6ff555544554c527d5e32eed ee6a9d80a1dae2e0ac611796674ce281' / redacted postal_code / h'5f5e6bfc2de598c80df58ff84090125a f8bb1ada795a37e5cfcf8f480e445755' ] } } >>, / CWT signature / h'd6b1f5143a3f6b2e54a1ea29ed98e9ed 7abb2927b34ba100fe91ed493d66062b d71716eed6b209b9d6c6861a6886ad47 5716220bcb3a0276a205002c3f6d3e86 a52d3e990f869bd38c5a476f73ee3319 64dee2cb51321b7bf2e64e62811a9b22' ]) ]]>

Some of the claims are redacted in the payload. The corresponding disclosure is communicated in the unprotected header in the sd_claims key. For example, the inspector_license_number claim is a Salted Disclosed Claim, consisting of a per-disclosure random salt, the claim name, and claim value.

CBOR extended diagnostic notation representation of inspector_license_number disclosure >, ]]>

This is represented in CBOR pretty printed formal as follows (end of line comments and spaces inserted for clarity):

CBOR encoding of inspector_license_number disclosure

The SHA-256 hash (the hash algorithm identified in the sd_alg protected header field) of that bytes string is the Digested Salted Disclosed Claim (in hex). The digest value is included in the payload in a redacted_claim_keys field for a Redacted Claim Key (in this example), or in a named array for a Redacted Claim Element (ex: for a redacted claim element of inspection_dates).

SHA-256 hash of inspector_license_number disclosure

Finally, since this redacted claim is a map key and value, the Digested Salted Disclosed Claim is placed in a redacted_claim_keys array in the SD-CWT payload at the same level of hierarchy as the original claim. Redacted claims which are array elements are handled slightly differently, as described in .

redacted inspector_license_number claim in the issued CWT payload

Holder prepares an SD-CWT for a Verifier When the Holder wants to send an SD-CWT and disclose none, some, or all of the redacted values, it makes a list of the values to disclose and puts them in sd_claims in the unprotected header. For example, Alice decides to disclose to a verifier the inspector_license_number claim (ABCD-123456), the region claim (California), and the earliest date element in the inspection_dates array (7-Feb-2019). >, <<[ /salt/ h'8de86a012b3043ae6e4457b9e1aaab80', /value/ 1549560720 / inspected 7-Feb-2019 / ]>>, <<[ /salt/ h'ec615c3035d5a4ff2f5ae29ded683c8e', /claim/ "region", / region=California / /value/ "ca" ]>>, ] ]]> The Holder MAY fetch a nonce from the Verifier to prevent replay, or obtain a nonce acceptable to the verifier through a process similar to the method described in . Finally, the Holder generates a Selective Disclosure Key Binding Token (SD-KBT) that ties together the SD-CWT generated by the Issuer (with the disclosures the Holder chose for the Verifier in its unprotected header), the Verifier target audience and optional nonces, and proof of possession of the Holder's private key. The issued SD-CWT is placed in the kcwt (Confirmation Key CWT) protected header field (defined in ). >, / end of KBT protected header / / KBT unprotected / {}, / KBT payload / << { / cnonce / 39 : h'8c0f5f523b95bea44a9a48c649240803', / aud / 3 : "https://verifier.example/app", / iat / 6 : 1725244237, / 2024-09-02T02:30:37+00:00Z / } >>, / end of KBT payload / / KBT signature / h'feb37bac837b25d3510962ac706297cd 2ef1ace1b47a382e911124c9a4c32287 280f48931c2988d71b297d57dd9d392f 5792d8f87a95166c5ea5110a81c1bfc7' ]) / end of kbt / ]]> Together the digests in protected parts of the issued SD-CWT, and the disclosures hashed in unprotected header of the issuer_sd_cwt are used by the Verifier to confirm the disclosed claims. Since the unprotected header of the included SD-CWT is covered by the signature in the SW-KBT, the Verifier has assurance the Holder included the sent list of disclosures.

Update to the CBOR Web Token Specification The CBOR Web Token Specification (Section 1.1 of ), uses strings, negative integers, and unsigned integers as map keys. This specification relaxes that requirement, by also allowing CBOR tagged integers and text strings as map keys. CBOR maps used in a CWT cannot have duplicate keys. (An integer or string map key is distinct key from a tagged map key which wraps the corresponding integer or string value).

• When sorted, map keys in CBOR are arranged in bytewise lexicographic order of the key's deterministic encodings (see Section 4.2.1 of ). So an integer key of 3 is represented in hex as 03, an integer key of -2 is represented in hex as 21, and a tag of 60 wrapping a 3 is represented in hex as D8 3C 03

Note that holders presenting to a verifier that does not support this specification would need to present a CWT without tagged map keys. Tagged keys are not registered in the CBOR Web Token Claims IANA registry. Instead the tag provides additional information about the tagged claim key and the corresponding (untagged) value. Multiple levels of tags in a key are not permitted. Variability in serialization requirements impacts privacy. See the security considerations section for more details on the privacy impact of serialization and profiling.

SD-CWT definition SD-CWT is modeled after SD-JWT, with adjustments to align with conventions in CBOR and COSE. An SD-CWT MUST include the protected header parameter typ with either a text value "application/sd-cwt" or an uint value of "C-F-TBD" in the SD-CWT. An SD-CWT is a CWT that can contain blinded claims (each expressed as a Blinded Claim Hash) in the CWT payload, at the root level or in any arrays or maps inside that payload. It is not required to contain any blinded claims. Optionally the salted claim values (and often claim names) for the corresponding Blinded Claim Hash are actually disclosed in the sd_claims field in the unprotected header of the CWT (the disclosures). If there are no disclosures (and when no Blinded Claims Hash is present in the payload) the sd_claims field in the unprotected header is an empty array. Any party with a Salted Disclosed Claim can generate its hash, find that hash in the CWT payload, and unblind the content. However a Verifier with the hash cannot reconstruct the corresponding blinded claim without disclosure of the Salted Disclosed Claim.

Types of blinded claims Salted Disclosed Claims for named claims are structured as a 128-bit salt, the name of the redacted element, and the disclosed value. For Salted Disclosed Claims of items in an array, the name is omitted. When a blinded claim is a key in a map, its blinded claim hash is added to a redacted_claim_keys array claim in the CWT payload that is at the same level of hierarchy as the key being blinded. The redacted_claim_keys key is the integer 0 (which is reserved for top-level CWT claim keys), wrapped with a CBOR tag (requested tag number 59). When blinding an individual item in an array, the value of the item is replaced with the digested salted hash as a CBOR binary string, wrapped with a CBOR tag (requested tag number 60). Blinded claims can be nested. For example, both individual keys in the inspection_location claim, and the entire inspection_location element can be separately blinded. An example nested claim is shown in . Finally, an issuer may create "decoy digests" which look like a blinded claim hash but have only a salt. Decoy digests are discussed in .

SD-CWT Issuance How the Holder communicates to the Issuer to request a CWT or an SD-CWT is out-of-scope of this specification. Likewise, how the Holder determines which claims to blind or to always disclose is a policy matter which is not discussed in this specification. This specification defines the format of an SD-CWT communicated between an Issuer and a Holder in this section, and describes the format of a Key Binding Token containing that SD-CWT communicated between a Holder and a Verifier in . The protected header MUST contain the sd_alg field identifying the algorithm (from the COSE Algorithms registry) used to hash the Salted Disclosed Claims. The unprotected header MUST contain an sd_claims section with a Salted Disclosed Claim for every blinded claim hash present anywhere in the payload, and any decoys (see ). If there are no disclosures, the sd_claims header is an empty array. The payload MUST also include a key confirmation element (cnf) for the Holder's public key. In an SD-CWT, either the subject sub/ 2 claim is present, or the redacted form of the subject is present. The issuer iss / 1 standard claim SHOULD be present, unless the protected header contains a certificate or certificate-like entity which fully identifies the issuer. All other standard CWT claims (aud/ 3, exp / 4, nbf / 5, iat / 6, and cti / 7) are OPTIONAL. The cnonce / 39 claim is OPTIONAL. The cnf claim, the cnonce claim, and the standard claims other than the subject MUST NOT be redacted. Any other claims are OPTIONAL and MAY be redacted.

Issuer generation The Issuer follows all the requirements of generating a valid CWT as updated by . The Issuer MUST implement COSE_Sign1 using an appropriate asymmetric signature algorithm / curve combination (for example ES256/P-256 or EdDSA/Ed25519) The Issuer MUST generate a unique cryptographically random salt with at least 128-bits of entropy for each Salted Disclosed Claim. If the client communicates a client-generated nonce (cnonce) when requesting the SD-CWT, the Issuer SHOULD include it in the payload.

Holder validation Upon receiving an SD-CWT from the Issuer with the Holder as the subject, the Holder verifies the following:

• the issuer (iss) and subject (sub) are correct;
• if an audience (aud) is present, it is acceptable;
• the CWT is valid according to the nbf and exp claims, if present;
• a public key under the control of the Holder is present in the cnf claim;
• the hash algorithm in the sd_alg protected header is supported by the Holder;
• if a cnonce is present, it was provided by the Holder to this Issuer and is still "fresh";
• there are no unblinded claims about the subject which violate its privacy policies;
• every blinded claim hash (some of which may be nested as in ) has a corresponding Salted Disclosed Claim, and vice versa;
• all the Salted Disclosed Claims are correct in their unblinded context in the payload.

The following informative CDDL is provided to describe the syntax for an SD-CWT issuance. A complete CDDL schema is in . "application/sd+cwt" / TBD1, &(alg: 1) ^ => int, &(sd_alg: 18) ^= int, ; -16 for sha-256 * key => any } sd-unprotected = { ? &(sd_claims: 17) ^ => salted-array, * key => any } sd-payload = { ; standard claims &(iss: 1) ^ => tstr, ; "https://issuer.example" ? &(sub: 2) ^ => tstr, ; "https://device.example" ? &(aud: 3) ^ => tstr, ; "https://verifier.example/app" ? &(exp: 4) ^ => int, ; 1883000000 ? &(nbf: 5) ^ => int, ; 1683000000 ? &(iat: 6) ^ => int, ; 1683000000 ? &(cti: 7) ^ => bstr, &(cnf: 8) ^ => { * key => any }, ; key confirmation ? &(cnonce: 39) ^ => bstr, ; ? &(redacted_keys: #6.59(0)) ^ => [ * bstr ], * key => any } ]]>

SD-CWT Presentation When a Holder presents an SD-CWT to a Verifier, it can disclose none, some, or all of its blinded claims. If the Holder wishes to disclose any claims, it includes that subset of its Salted Disclosed Claims in the sd_claims claim of the unprotected header. An SD-CWT presentation to a Verifier has the same syntax as an SD-CWT issued to a Holder, except the Holder choses the subset of disclosures included in the sd_claims claim. Since the unprotected header is not included in the signature, it will contain all the Salted Disclosed Claims when sent from the Issuer to the Holder. When sent from the Holder to the Verifier, the unprotected header will contain none, some, or all of these Claims. Finally, the SD-CWT used for presentation to a Verifier is included in a key binding token, as discsused in the next section.

Creating a Key Binding Token Regardless if it discloses any claims, the Holder sends the Verifier a unique Holder key binding (SD-KBT) for every presentation of an SD-CWT to a different Verifier. An SD-KBT is itself a type of CWT, signed using the private key corresponding to the key in the cnf claim in the presented SD-CWT. The SD-KBT contains the SD-CWT, including the Holder's choice of presented disclosures, in the kcwt protected header field in the SD-KBT. The Holder is conceptually both the subject and the issuer of the Key Binding Token. Therefore the sub and iss of an SD-KBT are implied from the cnf claim in the included SD-CWT, and are ignored for validation purposes if they are present. (A profile may define additional semantics.) The aud claim MUST be included and relevant to the Verifier. The SD-KBT payload MUST contain the issued_at (iat) claims. The protected header of the SD-KBT MUST include the typ header parameter with the value application/sd-kbt. The SD-KBT provides the following assurances to the Verifier:

• the Holder of the SD-CWT controls the confirmation method chosen by the Issuer;
• the Holder's disclosures have not been tampered with since confirmation occurred;
• the Holder intended to address the SD-CWT to the Verifier specified in the audience (aud) claim;
• the Holder's disclosure is linked to the creation time (iat) of the key binding.

The SD-KBT prevents an attacker from copying and pasting disclosures, or from adding or removing disclosures without detection. Confirmation is established according to RFC 8747, using the cnf claim in the payload of the SD-CWT. The Holder signs the SD-KBT using the key specified in the cnf claim in the SD-CWT. This proves possession of the Holder's private key. "application/kb+cwt", &(alg: 1) ^ => int, &(kcwt: 13) ^ => sd-cwt-issued, * key => any } kbt-unprotected = { * key => any } kbt-payload = { &(aud: 3) ^ => tstr, ; "https://verifier.example" ? &(exp: 4) ^ => int, ; 1883000000 ? &(nbf: 5) ^ => int, ; 1683000000 &(iat: 6) ^ => int, ; 1683000000 ? &(cnonce: 39) ^ => bstr, * key => any } ]]> The SD-KBT payload MAY include a cnonce claim. If included, the cnonce is a bstr and MUST be treated as opaque to the Holder. All other claims are OPTIONAL in an SD-KBT.

SD-KBT and SD-CWT Verifier Validation The exact order of the following steps MAY be changed, as long as all checks are performed before deciding if an SD-CWT is valid. First the Verifier must open the protected headers of the SD-KBT and find the issuer SD-CWT present in the kcwt field. Next the Verifier must validate the SD-CWT as described in . The Verifier extract the confirmation key from the cnf claim in the SD-CWT payload. Using the confirmation key, the Verifier validates the SD-KBT as described in . Finally, the Verifier MUST extract and decode the disclosed claims from the sd_claims in the unprotected header of the SD-CWT. The decoded sd_claims are converted to an intermediate data structure called a Digest To Disclosed Claim Map which is used to transform the Presented Disclosed Claimset, into a Validated Disclosed Claimset. The Verifier MUST compute the hash of each Salted Disclosed Claim (salted), in order to match each disclosed value to each entry of the Presented Disclosed Claimset. One possible concrete representation of the intermediate data structure for the Digest To Disclosed Claim Map could be: salted } ]]> The Verifier constructs an empty cbor map called the Validated Disclosed Claimset, and initializes it with all mandatory to disclose claims from the verified Presented Disclosed Claimset. Next the Verifier performs a breadth first or depth first traversal of the Presented Disclosed Claimset, Validated Disclosed Claimset, using the Digest To Disclosed Claim Map to insert claims into the Validated Disclosed Claimset when they appear in the Presented Disclosed Claimset. By performing these steps, the recipient can cryptographically verify the integrity of the protected claims and verify they have not been tampered with. If there remain unused Digest To Disclosed Claim Map at the end of this procedure the SD-CWT MUST be considered invalid, as if the signature had failed to verify. Otherwise the SD-CWT is considered valid, and the Validated Disclosed Claimset is now a CWT Claimset with no claims marked for redaction. Further validation logic can be applied to the Validated Disclosed Claimset, as it might normally be applied to a validated CWT claimset.

Decoy Digests TODO

Encrypted Disclosures Some uses of SD-CWT involve verifiers which have internal structure. In these cases, encrypted disclosures allow more fine-grained disclosure inside a single presentation.

• In the Remote Attestation Procedures (RATS) architecture , an SD-CWT is a RATS conceptual message that represents evidence. Different evidence claims could be processed by different attesters within the same Verifier. For example, one SD-KBT could include an SD-CWT with one set of claims about the workload, and one set of claims about the platform. It would be desirable to have each RATS appraiser see a different subset of disclosures in the SD-CWT / SD-KBT.

• In the Messaging Layer Security (MLS) protocol , an SD-CWT credential could present one subset of its disclosures to the MLS Distribution Service, and a different subset of those disclosures to the other members of the MLS group.

Taking the first example disclosure from above: > ]]> The corresponding bstr is encrypted with an IANA registered Authenticated Encryption with Additional Data (AEAD) algorithm such as AEAD_AES_128_GCM, using the salt as the nonce. The salt, the algorithm used (alg), and the resulting ciphertext and mac are put in an array. The bstr encoding of the array is placed in the sd_encrypted_claims unprotected header field array of the SD-CWT. The entire SD-CWT is included in the protected header of the SD-KBT, which integrity protects both encrypted and regular disclosures alike. Neither encrypted nor regular disclosures can appear in the unprotected header of a SD-KBT. >, ... ] ]]>

• In the example above the key in hex is '1eb2d67081ee9950fb3a14c6e8896203'.

The blinded claim hash is still over the unencrypted disclosure. The receiver of an encrypted disclosure locates the appropriate key by looking up the unique salt. If the verifier is able to decrypt and verify an encrypted disclosure, the decrypted disclosure is then processed as if it where in the sd_claims unprotected header in the SD-CWT. Details of key management are left to the specific protocols which make use of encrypted disclosures. The CDDL for encrypted disclosures is described below.

• TODO: consider moving the AEAD algorithm for all encrypted disclosures into a new protected header field.

• Note: because the algorithm is in a registry which contains only AEAD algorithms, an attacker cannot replace the algorithm or the message, without a decryption verification failure.

Credential Types This specification defines the CWT claim vct (for verifiable credential type). The vct value MUST be a case-sensitive StringOrURI (see ) value serving as an identifier for the type of the SD-CWT claimset. The vct value MUST be a Collision-Resistant Name as defined in Section 2 of . This claim is defined for COSE based verifiable credentials, similar to the JOSE based verifiable credentials claim (vct) described in Section 3.2.2.1.1 of . Profiles built on this specification are also encouraged to use more specific media types, as described in .

Examples

Minimal spanning example The following example contains claims needed to demonstrate redaction of key-value pairs and array elements.

An EDN Example >, / CWT unprotected / { / sd_claims / 17 : [ / these are the disclosures / <<[ /salt/ h'bae611067bb823486797da1ebbb52f83', /claim/ 501, / inspector_license_number / /value/ "ABCD-123456" ]>>, <<[ /salt/ h'8de86a012b3043ae6e4457b9e1aaab80', /value/ 1549560720 / inspected 7-Feb-2019 / ]>>, <<[ /salt/ h'ec615c3035d5a4ff2f5ae29ded683c8e', /claim/ "region", / region=California / /value/ "ca" ]>>, ] } / CWT payload / << { / iss / 1 : "https://issuer.example", / sub / 2 : "https://device.example", / exp / 4 : 1725330600, /2024-09-03T02:30:00+00:00Z/ / nbf / 5 : 1725243900, /2024-09-02T02:25:00+00:00Z/ / iat / 6 : 1725244200, /2024-09-02T02:30:00+00:00Z/ / cnf / 8 : { / cose key / 1 : { / kty / 1: 2, / EC2 / / crv / -1: 1, / P-256 / / x / -2: h'8554eb275dcd6fbd1c7ac641aa2c90d9 2022fd0d3024b5af18c7cc61ad527a2d', / y / -3: h'4dc7ae2c677e96d0cc82597655ce92d5 503f54293d87875d1e79ce4770194343' } }, /most_recent_inspection_passed/ 500: true, / redacted_claim_keys / 59(0) : [ / redacted inspector_license_number / h'0ad0f76dcb7fd812ca64c3ada3f543be 96d0e351e1e576fbab5cb659b49e599e' ], /inspection_dates/ 502 : [ / redacted inspection date 7-Feb-2019 / 60(h'f468b68dd7432030d7f33a8783acb3a4 d6afc215bbc184dce8831c64b539f335'), / redacted inspection date 4-Feb-2021 / 60(h'0839dcbdd9ed55c8aac3e573ccf59c81 0fa8d9f6ec551cb9737621faf3584e1f'), 1674004740, / 2023-01-17T17:19:00 / ], / inspection_location / 503 : { "country" : "us", / United States / / redacted_claim_keys / 59(0) : [ / redacted region / h'dff0cb7a6ff555544554c527d5e32eed ee6a9d80a1dae2e0ac611796674ce281' / redacted postal_code / h'5f5e6bfc2de598c80df58ff84090125a f8bb1ada795a37e5cfcf8f480e445755' ] } } >>, / CWT signature / h'd6b1f5143a3f6b2e54a1ea29ed98e9ed 7abb2927b34ba100fe91ed493d66062b d71716eed6b209b9d6c6861a6886ad47 5716220bcb3a0276a205002c3f6d3e86 a52d3e990f869bd38c5a476f73ee3319 64dee2cb51321b7bf2e64e62811a9b22' ]) / end of issuer SD-CWT / } >>, / end of KBT protected header / / KBT unprotected / {}, / KBT payload / << { / cnonce / 39 : h'8c0f5f523b95bea44a9a48c649240803', / aud / 3 : "https://verifier.example/app", / iat / 6 : 1725244237, / 2024-09-02T02:30:37+00:00Z / } >>, / end of KBT payload / / KBT signature / h'feb37bac837b25d3510962ac706297cd 2ef1ace1b47a382e911124c9a4c32287 280f48931c2988d71b297d57dd9d392f 5792d8f87a95166c5ea5110a81c1bfc7' ]) / end of kbt / ]]>

Nested example Instead of the structure from the previous example, imagine the payload contains an inspection history log with the following structure. It could be blinded at multiple levels of the claim set hierarchy. For example, looking at the nested disclosures below, the first disclosure unblinds the entire January 2023 inspection record. However, when the record is disclosed, the inspector license number and inspection location are redacted inside the record. The next disclosure unblinds the inspector_license_number, and the next disclosure unblinds the inspection location record, but the region and postcode claims inside the location record are also individually blinded. The fourth disclosure unblinds the inspection region. The fifth disclosure unblinds the earliest inspection record, and the last disclosure unblinds the inspector_license_number for that record. Verifiers start unblinding claims for which they have blinded claim hashes. They continue descending until there are no blinded claim hashes at any level of the hierarchy for which they have a corresponding disclosure. >, <<[ /salt/ h'bae611067bb823486797da1ebbb52f83', /claim/ 501, / inspector_license_number / /value/ "ABCD-123456" ]>>, <<[ /salt/ h'7d2505257e7850b70295a87b3c8748e5', /claim/ 503, / San Francisco location / /value/ { 1: "us", simple(59): [ h'de03a7a0b4359511a7dc0edd8f4ebc00 b5783d8a0d36e715679e23c703011d16', h'5a98ac2381cb59dee7a43daa073eab48 9773e2830a0b9c4e1efd55737dbb1c06' ] } ]>>, <<[ /salt/ h'52da9de5dc61b33775f9348b991d3d78', /claim/ 2, / region=California / /value/ "ca" ]>>, <<[ /salt/ h'b52272341715f2a0b476e33e55ce7501', /value/ { 500: True, 502: 1549560720, simple(59): [ h'cd88763edb2485b8109613546051f606 e6b822456da1bf09f604b886e1def45a', h'0a45eb75de44741bea78dc48b1898d40 09601dbf567279f3042a24cee9fdcab5' ] } / inspection 7-Feb-2019 / ]>>, <<[ /salt/ h'591eb2081b05be2dcbb6f8459cc0fe51', /claim/ 501, / inspector_license_number / /value/ "DCBA-101777" ]>> ] ]]> After applying the disclosures of the nested structure above, the disclosed claim set visible to the verifier would look like the following:

Implementation Status Note to RFC Editor: Please remove this section as well as references to before AUTH48. This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in . The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist. According to , "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit".

Transmute Prototype Organization: Transmute Industries Inc Name: github.com/transmute-industries/sd-cwt Description: An open source implementation of this draft. Maturity: Prototype Coverage: The current version ('main') implements functionality similar to that described in this document, and will be revised, with breaking changes to support the generation of example data to support this specification. License: Apache-2.0 Implementation Experience: No interop testing has been done yet. The code works as proof of concept, but is not yet production ready. Contact: Orie Steele (orie@transmute.industries)

Security Considerations Security considerations from COSE {(RFC9052)} and CWT apply to this specification.

Transparency Verification of an SD-CWT requires that the verifier have access to a verification key (public key) that is associated with the issuer. Compromise of the issuer's signing key enables an attacker to forge credentials for any subject associated with the issuer. Certificate transparency as described in , or key transparency as described in can enable the observation of incorrectly issued certificates or fraudulent bindings between verification keys and issuer identifiers. Issuers choose which claims to include in an SD-CWT, and whether they are mandatory to disclose, including self asserted claims such as "iss". All mandatory to disclose data elements are visible to the verifier as part of verification, some of these elements reveal information about the issuer, such as key or certificate thumbprints, supported digital signature algorithms, or operational windows which can be inferred from analysis of timestamps.

Traceability Presentations of the same SD-CWT to multiple verifiers can be correlated by matching on the signature component of the COSE_Sign1. Signature based linkability can be mitigated by leveraging batch issuance of single use tokens, for a credential management complexity cost. Any claim value with sufficiently low "anonymity set" can be used track the subject. For example, a high precision issuance time might match the issuance of only a few credentials for a given issuer, and as such any presentation of a credential issued at that time can be determined to be associated with the set of credentials issued at that time, for those subjects.

Credential Types The mandatory and optional to disclose data elements in an SD-CWT are credential type specific. Several distinct credential types might be applicable to a given use case. Issuers MUST perform a privacy and confidentiality assessment regarding each credential type they intend to issue prior to issuance.

Determinism & Augmentation It is possible to encode additional information through the choices made during the serialization stage of producing an SD-CWT, for example, by adjusting the order of CBOR map keys, or by choosing different numeric encodings for certain data elements. provides guidance for constructing application profiles that constrain serialization optionality beyond CBOR Common Deterministic Encoding rulesets (CDE). The construction of such profiles has a significant impact on the privacy properties of a credential type.

Threat Model Each use case will have a unique threat model which MUST be considered before the applicability of SD-CWT based credential types can be determined. This section provides a non exahustive list of topics to be consider when developing a threat model for applying SD-CWT to a given use case. Has there been a t-closeness, k-anonymity and l-diverity assessment (see ) assuming compromise of the one or more issuers, verifiers or holders, for all relevant credential types? How many issuers exist for the credential type? Is the size of the set of issuers growing or shrinking over time? For a given credential type, will subjects be able to hold instances of the same credential type from multiple issuers, or just a single issuer? Does the credential type require or offer the ability to disclose a globally unique identifier? Does the credential type require high precision time or other claims that have sufficient entropy such that they can serve as a unique fingerprint for a specific subject. Does the credential type contain Personally Identifiable Information (PII), or other sensitive information which might have value in a market. How many verifiers exist for the credential type? Is the size of the set of verifiers growing or shrinking over time? Are the verifiers a superset, subset, or disjoint set of the issuers or subjects? Are there any legally required reporting or disclosure requirements associated with the verifiers? Is there reason to believe that a verifier's historic data will be aggregated and analyzed? Assuming multiple verifiers are simultaneously compromised, what knowledge regarding subjects can be inferred from analyzing the resulting dataset? How many subjects exist for the credential type? Is the size of the set of subjects growing or shrinking over time? Does the credential type require specific hardware, or algorithms that limit the set of possible subjects to owners of a specific device or subscribers to a given service.

Random Numbers Each salt used to protect disclosed claims MUST be generated independently from the salts of other claims. The salts MUST be generated from a source of entropy that is acceptable to the issuer. Poor choice of salts can lead to brute force attacks that can reveal redacted claims.

Binding the KBT and the CWT The issuer claim in the SD-CWT is self-asserted by the issuer. Because confirmation is mandatory, the subject claim of an SD-CWT, when present, is always related directly to the confirmation claim. There might be many subject claims and many confirmation keys that identify the same entity or that are controlled by the same entity, while the identifiers and keys are distinct values. Reusing an identifier or key enables traceability, but MUST be evaluated in terms of the confidential and privacy constraints associated with the credential type. Conceptually, the Holder is both the issuer and the subject of the SD-KBT, even if the issuer or subject claims are not present. If they are present, they are self-asserted by the Holder. All three are represented by the confirmation (public) key in the SD-CWT. As with any self-assigned identifiers, Verifiers need to take care to verify that the SD-KBT issuer and subject claims match the subject in the SD-KBT, and are a valid representation of the Holder and correspond to the Holder's confirmation key. Extra care should be taken in case the SD-CWT subject claim is redacted. Likewise, Holders and Verifiers need to verify that the issuer claim of the SD-CWT corresponds to the Issuer and the key described in the protected header of the SD-CWT.

IANA Considerations

COSE Header Parameters IANA is requested to add the following entries to the CWT claims registry (https://www.iana.org/assignments/cose/cose.xhtml#header-parameters).

sd_claims The following completed registration template per RFC8152 is provided:

• Name: sd_claims
• Label: TBD1 (requested assignment 17)
• Value Type: bstr
• Value Registry: (empty)
• Description: A list of selectively disclosed claims, which were originally redacted, then later disclosed at the discretion of the sender.
• Reference: RFC XXXX

sd_alg The following completed registration template per RFC8152 is provided:

• Name: sd_alg
• Label: TBD2 (requested assignment 18)
• Value Type: int
• Value Registry: COSE Algorithms
• Description: The hash algorithm used for redacting disclosures.
• Reference: RFC XXXX

sd_encrypted_claims The following completed registration template per RFC8152 is provided:

• Name: sd_encrypted claims
• Label: TBD6 (requested assignment 19)
• Value Type: bstr
• Value Registry: (empty)
• Description: A list of encrypted selectively disclosed claims, which were originally redacted, then later disclosed at the discretion of the sender.
• Reference: RFC XXXX

CBOR Tags

To be redacted tag The array claim element, or map key and value inside the "To be redacted" tag is intended to be redacted using selective disclosure.

• Tag: TBD3 (requested assignment 58)
• Data Item: (any)
• Semantics: An array claim element, or map key and value intended to be redacted.
• Specification Document(s): RFC XXXX

Redacted claim keys tag This tag encloses the integer claim key 0 (reserved as a CWT claim key). It indicates that the claim value is an array of redacted claim keys at the same level.

• Tag: TBD4 (requested assignment 59)
• Data Item: unsigned integer 0
• Semantics: Tags the claim key 0. The value of the key is an array of selective disclosure redacted claim keys.
• Specification Document(s): RFC XXXX

Redacted claim element tag The binary string inside the tag is a selective disclosure redacted claim element of an array.

• Tag: TBD5 (requested assignment 60)
• Data Item: byte string
• Semantics: A selective disclosure redacted (array) claim element.
• Specification Document(s): RFC XXXX

CBOR Web Token (CWT) Claims IANA is requested to add the following entries to the CWT claims registry (https://www.iana.org/assignments/cwt/cwt.xhtml).

vct The following completed registration template per RFC8392 is provided:

• Claim Name: vct
• Claim Description: Verifiable credential type
• JWT Claim Name: vct
• Claim Key: TBD6 (request assignment 11)
• Claim Value Type(s): bstr
• Change Controller: IETF
• Specification Document(s): RFC XXXX

Media Types This section requests the registration of new media types in https://www.iana.org/assignments/media-types/media-types.xhtml.

application/sd+cwt IANA is requested to add the following entry to the media types registry in accordance with RFC6838, RFC4289, and RFC6657. The following completed registration template is provided:

• Type name: application
• Subtype name: sd+cwt
• Required parameters: n/a
• Optional parameters: n/a
• Encoding considerations: binary
• Security considerations: See the Security Considerations section of RFC XXXX, and
• Interoperability considerations: n/a
• Published specification: RFC XXXX
• Applications that use this media type: TBD
• Fragment identifier considerations: n/a
• Additional information:
  • Magic number(s): n/a
  • File extension(s): n/a
  • Macintosh file type code(s): n/a
• Person & email address to contact for further information: SPICE WG mailing list (spice@ietf.org) or IETF Security Area (saag@ietf.org)
• Intended usage: COMMON
• Restrictions on usage: none
• Author: See Author's Addresses section
• Change controller: IETF
• Provisional registration? No

application/kb+cwt IANA is requested to add the following entry to the media types registry in accordance with RFC6838, RFC4289, and RFC6657. The following completed registration template is provided:

• Type name: application
• Subtype name: kb+cwt
• Required parameters: n/a
• Optional parameters: n/a
• Encoding considerations: binary
• Security considerations: See the Security Considerations section of RFC XXXX, and
• Interoperability considerations: n/a
• Published specification: RFC XXXX
• Applications that use this media type: TBD
• Fragment identifier considerations: n/a
• Additional information:
  • Magic number(s): n/a
  • File extension(s): n/a
  • Macintosh file type code(s): n/a
• Person & email address to contact for further information: SPICE WG mailing list (spice@ietf.org) or IETF Security Area (saag@ietf.org)
• Intended usage: COMMON
• Restrictions on usage: none
• Author: See Author's Addresses section
• Change controller: IETF
• Provisional registration? No

Content-Formats RFC Editor: Please replace "RFCthis" with the RFC number assigned to this document. RFC Editor: IANA is requested to register the following Content-Format numbers in the "CoAP Content-Formats" registry, within the "Constrained RESTful Environments (CoRE) Parameters" registry group : New CoAP Content Formats

Content-Type	Content Coding	ID	Reference
application/sd+cwt	-	TBD1	RFCthis
application/kb+cwt	-	TBD2	RFCthis

If possible, TBD1 and TBD2 should be assigned in the 256..9999 range.

References Normative References JSON Web Signature (JWS) represents content secured with digital signatures or Message Authentication Codes (MACs) using JSON-based data structures. Cryptographic algorithms and identifiers for use with this specification are described in the separate JSON Web Algorithms (JWA) specification and an IANA registry defined by that specification. Related encryption capabilities are described in the separate JSON Web Encryption (JWE) specification. JSON Web Token (JWT) is a compact, URL-safe means of representing claims to be transferred between two parties. The claims in a JWT are encoded as a JSON object that is used as the payload of a JSON Web Signature (JWS) structure or as the plaintext of a JSON Web Encryption (JWE) structure, enabling the claims to be digitally signed or integrity protected with a Message Authentication Code (MAC) and/or encrypted. CBOR Web Token (CWT) is a compact means of representing claims to be transferred between two parties. The claims in a CWT are encoded in the Concise Binary Object Representation (CBOR), and CBOR Object Signing and Encryption (COSE) is used for added application-layer security protection. A claim is a piece of information asserted about a subject and is represented as a name/value pair consisting of a claim name and a claim value. CWT is derived from JSON Web Token (JWT) but uses CBOR rather than JSON. 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. 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. This document describes a simple process that allows authors of Internet-Drafts to record the status of known implementations by including an Implementation Status section. This will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. This process is not mandatory. Authors of Internet-Drafts are encouraged to consider using the process for their documents, and working groups are invited to think about applying the process to all of their protocol specifications. This document obsoletes RFC 6982, advancing it to a Best Current Practice. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. 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 a set of algorithms that can be used with the CBOR Object Signing and Encryption (COSE) protocol (RFC 9052). This document, along with RFC 9052, obsoletes RFC 8152. Universität Bremen TZI This document formalizes and consolidates the definition of the Extended Diagnostic Notation (EDN) of the Concise Binary Object Representation (CBOR), addressing implementer experience. Replacing EDN's previous informal descriptions, it updates RFC 8949, obsoleting its Section 8, and RFC 8610, obsoleting its Appendix G. It also specifies and uses registry-based extension points, using one to support text representations of epoch-based dates/times and of IP addresses and prefixes. // (This cref will be removed by the RFC editor:) The present // revision (–16) addresses the first half of the WGLC comments, // except for the issues around the specific way how to best achieve // pluggable ABNF grammars for application-extensions. It is // intended for use as a reference document for the mid-WGLC CBOR WG // interim meeting on 2025-01-08. This document specifies Ephemeral Diffie-Hellman Over COSE (EDHOC), a very compact and lightweight authenticated Diffie-Hellman key exchange with ephemeral keys. EDHOC provides mutual authentication, forward secrecy, and identity protection. EDHOC is intended for usage in constrained scenarios, and a main use case is to establish an Object Security for Constrained RESTful Environments (OSCORE) security context. By reusing CBOR Object Signing and Encryption (COSE) for cryptography, Concise Binary Object Representation (CBOR) for encoding, and Constrained Application Protocol (CoAP) for transport, the additional code size can be kept very low. This specification adds the equivalent of the JSON Object Signing and Encryption (JOSE) "typ" (type) header parameter to CBOR Object Signing and Encryption (COSE). This enables the benefits of explicit typing (as defined in RFC 8725, "JSON Web Token Best Current Practices") to be brought to COSE objects. The syntax of the COSE type header parameter value is the same as the existing COSE content type header parameter. This specification describes how to declare in a CBOR Web Token (CWT) (which is defined by RFC 8392) that the presenter of the CWT possesses a particular proof-of-possession key. Being able to prove possession of a key is also sometimes described as being the holder-of-key. This specification provides equivalent functionality to "Proof-of-Possession Key Semantics for JSON Web Tokens (JWTs)" (RFC 7800) but using Concise Binary Object Representation (CBOR) and CWTs rather than JavaScript Object Notation (JSON) and JSON Web Tokens (JWTs). This document defines algorithms for Authenticated Encryption with Associated Data (AEAD), and defines a uniform interface and a registry for such algorithms. The interface and registry can be used as an application-independent set of cryptoalgorithm suites. This approach provides advantages in efficiency and security, and promotes the reuse of crypto implementations. [STANDARDS-TRACK] IANA This specification defines a method for computing a hash value over a CBOR Object Signing and Encryption (COSE) Key. It specifies which fields within the COSE Key structure are included in the cryptographic hash computation, the process for creating a canonical representation of these fields, and how to hash the resulting byte sequence. The resulting hash value, referred to as a "thumbprint", can be used to identify or select the corresponding key. Informative References Authlete Keio University Ping Identity This specification defines a mechanism for the selective disclosure of individual elements of a JSON-encoded data structure used as the payload of a JSON Web Signature (JWS). The primary use case is the selective disclosure of JSON Web Token (JWT) claims. MATTR Authlete Inc. Ping Identity This specification describes data formats as well as validation and processing rules to express Verifiable Credentials with JSON payloads with and without selective disclosure based on the SD-JWT [I-D.ietf-oauth-selective-disclosure-jwt] format. Universität Bremen TZI CBOR (STD 94, RFC 8949) defines "Deterministically Encoded CBOR" in its Section 4.2, providing some flexibility for application specific decisions. To facilitate Deterministic Encoding to be offered as a selectable feature of generic encoders, the present document defines a CBOR Common Deterministic Encoding (CDE) Profile that can be shared by a large set of applications with potentially diverging detailed requirements. It also defines "Basic Serialization", which stops short of the potentially more onerous requirements that make CDE fully deterministic, while employing most of its reductions of the variability needing to be handled by decoders. This document describes version 2.0 of the Certificate Transparency (CT) protocol for publicly logging the existence of Transport Layer Security (TLS) server certificates as they are issued or observed, in a manner that allows anyone to audit certification authority (CA) activity and notice the issuance of suspect certificates as well as to audit the certificate logs themselves. The intent is that eventually clients would refuse to honor certificates that do not appear in a log, effectively forcing CAs to add all issued certificates to the logs. This document obsoletes RFC 6962. It also specifies a new TLS extension that is used to send various CT log artifacts. Logs are network services that implement the protocol operations for submissions and queries that are defined in this document. While there are several established protocols for end-to-end encryption, relatively little attention has been given to securely distributing the end-user public keys for such encryption. As a result, these protocols are often still vulnerable to eavesdropping by active attackers. Key Transparency is a protocol for distributing sensitive cryptographic information, such as public keys, in a way that reliably either prevents interference or detects that it occurred in a timely manner. Google LLC Guardian Project Cloudflare Inc. Most HTTP authentication schemes are probeable in the sense that it is possible for an unauthenticated client to probe whether an origin serves resources that require authentication. It is possible for an origin to hide the fact that it requires authentication by not generating Unauthorized status codes, however that only works with non-cryptographic authentication schemes: cryptographic signatures require a fresh nonce to be signed. Prior to this document, there was no existing way for the origin to share such a nonce without exposing the fact that it serves resources that require authentication. This document defines a new non-probeable cryptographic authentication scheme. In network protocol exchanges, it is often useful for one end of a communication to know whether the other end is in an intended operating state. This document provides an architectural overview of the entities involved that make such tests possible through the process of generating, conveying, and evaluating evidentiary Claims. It provides a model that is neutral toward processor architectures, the content of Claims, and protocols. Messaging applications are increasingly making use of end-to-end security mechanisms to ensure that messages are only accessible to the communicating endpoints, and not to any servers involved in delivering messages. Establishing keys to provide such protections is challenging for group chat settings, in which more than two clients need to agree on a key but may not be online at the same time. In this document, we specify a key establishment protocol that provides efficient asynchronous group key establishment with forward secrecy (FS) and post-compromise security (PCS) for groups in size ranging from two to thousands. Rohan Mahy Consulting Services The Messaging Layer Security (MLS) protocol contains Credentials used to authenticate an MLS client with a signature key pair. Selective Disclosure CBOR Web Tokens (SD-CWT) and Selective Disclosure JSON Web Tokens (SD-JWT) define token formats where the holder can selectively reveal claims about itself with strong integrity protection and cryptographic binding to the holder's key. This document defines MLS credentials for both these token types.

Complete CDDL Schema

A complete CDDL description of SD-CWT "application/sd+cwt", &(alg: 1) ^ => int, &(sd_alg: TBD2) ^= int, ; -16 for sha-256 * key => any } kbt-protected = { &(typ: 16) ^ => "application/kb+cwt", &(alg: 1) ^ => int, &(kcwt: 13) ^ => sd-cwt-issued, * key => any } sd-unprotected = { ? &(sd_claims: TBD1) ^ => salted-array, ? &(sd_encrypted_claims: TBD6) ^ => encrypted-array * key => any } kbt-unprotected = { * key => any } sd-payload = { ; standard claims &(iss: 1) ^ => tstr, ; "https://issuer.example" ? &(sub: 2) ^ => tstr, ; "https://device.example" ? &(aud: 3) ^ => tstr, ; "https://verifier.example" ? &(exp: 4) ^ => int, ; 1883000000 ? &(nbf: 5) ^ => int, ; 1683000000 ? &(iat: 6) ^ => int, ; 1683000000 ? &(cti: 7) ^ => bstr, &(cnf: 8) ^ => { * key => any }, ; key confirmation ? &(cnonce: 39) ^ => bstr, ; ? &(redacted_claim_keys: TBD3) ^ => [ * bstr ], * key => any } kbt-payload = { &(aud: 3) ^ => tstr, ; "https://verifier.example" ? &(exp: 4) ^ => int, ; 1883000000 ? &(nbf: 5) ^ => int, ; 1683000000 &(iat: 6) ^ => int, ; 1683000000 ? &(cnonce: 39) ^ => bstr, * key => any } salted-array = [ +bstr .cbor salted ] salted = salted-claim / salted-element / decoy salted-claim = [ bstr .size 16, ; 128-bit salt (int / text), ; claim name any ; claim value ] salted-element = [ bstr .size 16, ; 128-bit salt any ; claim value ] decoy = [ bstr .size 16 ; 128-bit salt ] encrypted-array = [ +bstr .cbor encrypted ] encrypted = [ bstr .size 16, ; 128-bit salt uint16, ; IANA AEAD Algorithm number bstr, ; the ciphertext output of a bstr-encoded-salted ; with a matching salt bstr ; the corresponding MAC ] ;bstr-encoded-salted = bstr .cbor salted key = int / text TBD1 = 17 TBD2 = 18 ;TBD3 = #6.58(any) ; CBOR tag wrapping to-be-redacted keys or elements TBD4 = #6.59(0) ; CBOR tag 59 wrapping reserved integer claim key 0 ;TBD5 = #6.60( bstr ) ;redacted_claim_element TBD6 = 19 ]]>

Comparison to SD-JWT SD-CWT is modeled after SD-JWT, with adjustments to align with conventions in CBOR and COSE.

Media Types The COSE equivalent of application/sd-jwt is application/sd+cwt. THe COSE equivalent of application/kb+jwt is application/kb+cwt.

Redaction Claims The COSE equivalent of _sd is a CBOR tag (requested assignment 59) of the claim key 0. The corresponding claim value is an array of the redacted claim keys. The COSE equivalent of ... is a CBOR tag (requested assignment 60) of the digested salted claim.

Issuance The issuance process for SD-CWT is similar to SD-JWT, with the exception that a confirmation claim is REQUIRED.

Presentation The presentation process for SD-CWT is similar to SD-JWT, except that a Key Binding Token is REQUIRED. The Key Binding Token then includes the issued SD-CWT, including the Holder-selected disclosures. Because the entire SD-CWT is included as a claim in the SD-KBT, the disclosures are covered by the Holder's signature in the SD-KBT, but not by the Issuer's signature in the SD-CWT.

Validation The validation process for SD-CWT is similar to SD-JWT, however, JSON Objects are replaced with CBOR Maps which can contain integer keys and CBOR Tags.

Keys used in the examples

Subject / Holder Holder COSE key pair in EDN format The fields necessary for the COSE Key Thumbprint in EDN format: The same map in CBOR pretty printing The COSE thumbprint (in hexadecimal)--SHA256 hash of the thumbprint fields: Holder key pair in JWK format Input to Holder public JWK thumbprint (ignore line breaks) SHA-256 of the Holder public JWK input string (in hex) Holder public JWK thumbprint Holder public key in PEM format Holder private key in PEM format

Issuer Issuer COSE key pair in Extended Diagnostic Notation (EDN) The fields necessary for the COSE Key Thumbprint in EDN format: The same map in CBOR pretty printing The COSE thumbprint (in hexadecimal)--SHA256 hash of the thumbprint fields: Issuer key pair in JWK format Input to Issuer JWK thumbprint (ignore line breaks) SHA-256 of the Issuer JWK input string (in hex) Issuer JWK thumbprint Issuer public key in PEM format Issuer private key in PEM format

Document History Note: RFC Editor, please remove this entire section on publication.

draft-ietf-spice-sd-cwt-03

• remove bstr encoding from sd_claims array (but not the individual disclosures)
• clarify which claims are optional/mandatory
• correct that an SD-CWT may have zero redacted claims
• improve the walkthrough of computing a disclosure
• clarify that duplicate map keys are not allowed, and how tagged keys are represented.
• added security considerations section (#42) and text about privacy and linkability risks (#43)
• register SD-CWT and SD-KBT as content formats in CoAP registry (#39)
• updated media types registrations to have more useful contacts (#44)
• build most of the values (signatures/salts/hashes/dates) in the examples automatically using a script that implements SD-CWT
• regenerate all examples with correct signatures
• add nested example
• add optional encrypted disclosures
• add description of decoy digests TODO
• provide test vectors TODO

draft-ietf-spice-sd-cwt-02

• KBT now includes the entire SD-CWT in the Confirmation Key CWT (kcwt) existing COSE protected header. Has algorithm now specified in new sd_alg COSE protected header. No more sd_hash claim. (PR #34, 32)
• Introduced tags for redacted and to-be-redacted claim keys and elements. (PR#31, 28)
• Updated example to be a generic inspection certificate. (PR#33)
• Add section saying SD-CWT updates the CWT spec (RFC8392). (PR#29)

draft-ietf-spice-sd-cwt-01

• Added Overview section
• Rewrote the main normative section
• Made redaacted_claim_keys use an unlikely to collide claim key integer
• Make cnonce optional (it now says SHOULD)
• Made most standard claims optional.
• Consistently avoid use of bare term "key" - to make crypto keys and map keys clear
• Make clear issued SD-CWT can contain zero or more redactions; presented SD-CWT can disclose zero, some, or all redacted claims.
• Clarified use of sd_hash for issuer to holder case._
• Lots of editorial cleanup
• Added Rohan as an author and Brian Campbell to Acknowledgements
• Updated implementation status section to be BCP205-compatible
• Updated draft metadata

draft-ietf-spice-sd-cwt-00

• Initial working group version based on draft-prorock-spice-cose-sd-cwt-01.

Acknowledgments The authors would like to thank those that have worked on similar items for providing selective disclosure mechanisms in JSON, especially: Brent Zundel, Roy Williams, Tobias Looker, Kristina Yasuda, Daniel Fett, Brian Campbell, Oliver Terbu, and Michael Jones.