Internet-Draft | SPICE SD-CWT | December 2024 |
Prorock, et al. | Expires 7 June 2025 | [Page] |
- Workgroup:
- Secure Patterns for Internet CrEdentials
- Internet-Draft:
- draft-ietf-spice-sd-cwt-02
- Updates:
- RFC8392 (if approved)
- Published:
- Intended Status:
- Standards Track
- Expires:
SPICE SD-CWT
Abstract
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
This note is to be removed before publishing as an RFC.¶
The latest revision of this draft can be found at https://ietf-wg-spice.github.io/draft-ietf-spice-sd-cwt/draft-ietf-spice-sd-cwt.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-ietf-spice-sd-cwt/.¶
Discussion of this document takes place on the Secure Patterns for Internet CrEdentials Working Group mailing list (mailto:spice@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/spice/. Subscribe at https://www.ietf.org/mailman/listinfo/spice/.¶
Source for this draft and an issue tracker can be found at https://github.com/ietf-wg-spice/draft-ietf-spice-sd-cwt.¶
Status of This Memo
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶
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This Internet-Draft will expire on 7 June 2025.¶
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
1. Introduction
This document updates the CBOR Web Token (CWT) specification [RFC8392], 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 [I-D.draft-ietf-oauth-selective-disclosure-jwt], with changes to align with conventions from CBOR Object Signing and Encryption (COSE) [RFC9052]. 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 [RFC8392]. 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 Section 3 of [RFC8392].¶
1.1. High level flow
Figure 1: High level SD-CWT Issuance and Presentation Flow¶
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.¶
-
The issuer SHOULD confirm the holder controls all confirmation material before issuing credentials using the
cnf
claim.¶ -
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.¶
2. 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
This document uses terms from CWT [RFC8392], and COSE [RFC9052] [RFC9053].¶
The terms Claim Name, Claim Key and Claim Value are defined in [RFC8392].¶
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.¶
3. Overview of Selective Disclosure CWT
3.1. 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) [I-D.ietf-cbor-edn-literals]. 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.¶
{ / iss / 1 : "https://issuer.example", / sub / 2 : "WRQ2RbY5RYJCIxfDQL9agl9fFSCYVu4Xocqb6zerc1M", / exp / 4 : 1725330600, /2024-09-02T19:30:00Z/ / nbf / 5 : 1725243840, /2024-09-01T19:25:00Z/ / iat / 6 : 1725244200, /2024-09-01T19:30:00Z/ / cnf / 8 : { / cose key / 1 : { / alg: ES256 / 3: -7, / kty: EC2 / 1: 2, / crv: P-256 / -1: 1, / x / -2: b64'hVTrJ13Nb70cesZBqiyQ2SAi_Q0wJLWvGMfMYa1Sei0', / y / -3: b64'TceuLGd-ltDMgll2Vc6S1VA_VCk9h4ddHnnOR3AZQ0M' } }, /most_recent_inspection_passed/ 500: true, /inspector_license_number/ 501: "ABCD-123456", /inspection_dates/ 502 : [ 1549560720, / 2019-02-07T17:32:00 / 1612498440, / 2021-02-04T20:14:00 / 1674004740, / 2023-01-17T17:19:00 / ], /inspection_location/ 503: { "country": "us", / United States / "region": "ca", / California / "postal_code": "94188" } }¶
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.¶
3.2. 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:¶
/ cose-sign1 / 18([ / protected / << { / alg / 1 : -35, / ES384 / / typ / 16 : "application/sd+cwt", / kid / 4 : 'https://issuer.example/cwk3.cbor', / sd_alg / 18 : -16 / SHA256 / } >>, / unprotected / { / sd_claims / 17 : / these are all the disclosures / <<[ <<[ /salt/ h'8d5c15fa86265d8ff77a0e92720ca837', /claim/ 501, / inspector_license_number / /value/ "ABCD-123456" ]>>, <<[ /salt/ h'86c84b9c3614ba27073c7e5a475a2a13', /value/ 1549560720 / inspected 7-Feb-2019 / ]>>, <<[ /salt/ h'86c84b9c3614ba27073c7e5a475a2a13', /value/ 1612498440 / inspected 4-Feb-2021 / ]>>, <<[ /salt/ h'30eef86edeaa197df7bd3d17dd89cd87', /claim/ "region", /value/ "ca" /California/ ]>>, <<[ /salt/ h'284538c4a1881fac49b2edc550c1913e', /claim/ "postal_code", /value/ "94188" ]>> ]>> }, / payload / << { / iss / 1 : "https://issuer.example", / sub / 2 : "WRQ2RbY5RYJCIxfDQL9agl9fFSCYVu4Xocqb6zerc1M", / exp / 4 : 1725330600, /2024-09-02T19:30:00Z/ / nbf / 5 : 1725243840, /2024-09-01T19:25:00Z/ / iat / 6 : 1725244200, /2024-09-01T19:30:00Z/ / cnf / 8 : { / cose key / 1 : { / alg: ES256 / 3: -7, / kty: EC2 / 1: 2, / crv: P-256 / -1: 1, / x / -2: h'hVTrJ13Nb70cesZBqiyQ2SAi_Q0wJLWvGMfMYa1Sei0', / y / -3: h'TceuLGd-ltDMgll2Vc6S1VA_VCk9h4ddHnnOR3AZQ0M' } }, /most_recent_inspection_passed/ 500: true, / redacted_claim_keys / 59(0) : [ / redacted inspector_license_number / h'7e6e350907d0ba3aa7ae114f8da5b360' + h'601c0bb7995cd40049b98e4f58fb6ec0' ], /inspection_dates/ 502 : [ / redacted inspection date 7-Feb-2019 / 60(h'a0f74264a8c97655c958aff3687f1390' + h'ed0ab6f64cd78ba43c3fefee0de7b835') / redacted inspection date 4-Feb-2021 / 60(h'1e7275bcda9bc183079cd4515c5c0282' + h'a2a0e9105b660933e2e68f9a3f40974b') 1674004740, / 2023-01-17T17:19:00 / ], / inspection_location / 503 : { "country" : "us", / United States / / redacted_claim_keys / 59(0) : [ / redacted region / h'c47e3b047c1cd6d9d1e1e01514bc2ec9' + h'ed010ac9ae1c93403ec72572bb1e00e7', / redacted postal_code / h'0b616e522a05d8d134a834979710120d' + h'41ac1522b056d5f9509cf7e850047302' ] } } >>, / signature / h'3337af2e66959614' /TODO: fix / ])¶
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.¶
<<[ /salt/ h'8d5c15fa86265d8ff77a0e92720ca837', /claim/ 501, / inspector_license_number / /value/ "ABCD-123456" ]>>¶
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
).¶
7e6e350907d0ba3aa7ae114f8da5b360601c0bb7995cd40049b98e4f58fb6ec0¶
4. 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).¶
/ sd_claims / 17 : /just the disclosures chosen by the Holder/ <<[ <<[ /salt/ h'8d5c15fa86265d8ff77a0e92720ca837', /claim/ 501, / inspector_license_number / /value/ "ABCD-123456" ]>>, <<[ /salt/ h'86c84b9c3614ba27073c7e5a475a2a13', /value/ 1549560720 / inspected 7-Feb-2019 / ]>>, <<[ /salt/ h'30eef86edeaa197df7bd3d17dd89cd87', /claim/ "region", /value/ "ca" /California/ ]>> ]>>¶
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 [I-D.ietf-httpbis-unprompted-auth].¶
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 [RFC9528]).¶
/ sd_kbt / 18 : << 18([ / protected / << { / alg / 1 : -7 / ES256 /, / typ / 16 : "application/kb+cwt", / kcwt / 13 : / *** SD-CWT from Issuer goes here / / with Holder's choice of disclosures / / in the SD-CWT unprotected header *** / h'0123456789abcdef...0123' } >>, / unprotected / {}, / payload / << { / cnonce / 39 : h'8c0f5f523b95bea44a9a48c649240803', / aud / 3 : "https://verifier.example/app", / iat / 6 : 1725283443, / 2024-09-02T06:24:03Z / } >>, / signature / h'1237af2e678945' / TODO: fix / ]) >>¶
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.¶
5. Update to the CBOR Web Token Specification
The CBOR Web Token Specification (Section 1.1 of [RFC8392]), 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.¶
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.¶
6. 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
[RFC9596] with the value "application/sd-cwt" in the SD-CWT.¶
An SD-CWT is a CWT containing the "blinded claim hash" of at least one blinded claim in the CWT payload.
Optionally the salted claim values (and often claim names) for the corresponding Blinded Claim Hash are actually disclosed in the sd_claims
claim in the unprotected header of the CWT (the disclosures).¶
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.¶
6.1. 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.¶
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 ] ; a collection of Salted Disclosed Claims salted-array = [ +bstr .cbor salted ]¶
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).¶
redacted_claim_element = #6.60( bstr .size 16 )¶
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 Section 12.2.¶
Finally, an issuer may create "decoy digests" which look like a blinded claim hash but have only a salt. Decoy digests are discussed in Section 10.¶
7. 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 Section 8.¶
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 Section 10).
If there are no disclosures sd_claims
is an empty array.
The payload MUST also include a key confirmation element (cnf
) [RFC8747] for the Holder's public key.¶
7.1. Issuer generation
The Issuer follows all the requirements of generating a valid CWT as updated by Section 5. 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.¶
7.2. 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
andexp
claims;¶ -
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 Section 12.2) 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 Appendix A.¶
sd-cwt-issued = #6.18([ protected: bstr .cbor sd-protected, sd-unprotected, payload: bstr .cbor sd-payload, signature: bstr ]) sd-protected = { &(typ: 16) ^ => "application/sd+cwt", &(alg: 1) ^ => int, &(sd_alg: 18) ^= int, ; -16 for sha-256 * key => any } sd-unprotected = { ? &(sd_claims: 17) ^ => bstr .cbor 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 &(cnf: 8) ^ => { * key => any }, ; key confirmation ? &(cnonce: 39) ^ => bstr, ; ? &(redacted_keys: #6.59(0)) ^ => [ * bstr ], * key => any }¶
8. 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.¶
8.1. Creating a Key Binding Token
Regardless if it discloses any claims, the Holder sends the Verifier a unique Holder key binding (SD-KBT) Section 8.1 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 sub
and iss
of an SD-KBT are implied from the cnf
claim in the included SD-CWT.
The aud
claim MUST be included and relevant to the Verifier.
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.¶
kbt-cwt = #6.18([ protected: bstr .cbor kbt-protected, kbt-unprotected, payload: bstr .cbor kbt-payload, signature: bstr ]) kbt-protected = { &(typ: 16) ^ => "application/kb+cwt", &(alg: 1) ^ => int, &(kcwt: 13) ^ => bstr .cbor 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 cnonce
is a bstr
and MUST be treated as opaque to the Holder.¶
9. 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 Section 7.2 of [RFC8392].
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 Section 7.2 of [RFC8392].¶
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:¶
{ &(digested-salted-disclosed-claim) => 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.¶
10. Decoy Digests
TODO¶
11. Credential Types
This specification defines the CWT claim vct (for verifiable credential type). The vct value MUST be a case-sensitive StringOrURI (see [RFC7519]) 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 [RFC7515].¶
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 [I-D.draft-ietf-oauth-sd-jwt-vc].¶
Profiles built on this specification are also encouraged to use more specific media types, as described in [RFC9596].¶
12. Examples
TODO - Provide more examples¶
12.1. Minimal spanning example
The following example contains claims needed to demonstrate redaction of key-value pairs and array elements.¶
12.2. Nested example
TODO¶
13. Implementation Status
Note to RFC Editor: Please remove this section as well as references to [BCP205] 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 [BCP205]. 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 [BCP205], "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".¶
13.1. 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)¶
14. Security Considerations
Security considerations from COSE {(RFC9052)} and CWT [RFC8392] apply to this specification.¶
14.1. 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.¶
15. IANA Considerations
15.1. 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).¶
15.3. 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).¶
15.4. Media Types
This section requests the registration of new media types in https://www.iana.org/assignments/media-types/media-types.xhtml.¶
15.4.1. 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 [RFC8392]¶
-
Interoperability considerations: n/a¶
-
Published specification: RFC XXXX¶
-
Applications that use this media type: TBD¶
-
Fragment identifier considerations: n/a¶
-
Additional information:¶
-
Person & email address to contact for further information: Michael Prorock, mprorock@mesur.io¶
-
Intended usage: COMMON¶
-
Restrictions on usage: none¶
-
Author: Michael Prorock, mprorock@mesur.io¶
-
Change controller: IETF¶
-
Provisional registration? No¶
15.4.2. 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 [RFC8392]¶
-
Interoperability considerations: n/a¶
-
Published specification: RFC XXXX¶
-
Applications that use this media type: TBD¶
-
Fragment identifier considerations: n/a¶
-
Additional information:¶
-
Person & email address to contact for further information: Orie Steele, orie@transmute.industries¶
-
Intended usage: COMMON¶
-
Restrictions on usage: none¶
-
Author: Orie Steele, orie@transmute.industries¶
-
Change controller: IETF¶
-
Provisional registration? No¶
16. References
16.1. Normative References
- [BCP205]
-
Best Current Practice 205, <https://www.rfc-editor.org/info/bcp205>.
At the time of writing, this BCP comprises the following:Sheffer, Y. and A. Farrel, "Improving Awareness of Running Code: The Implementation Status Section", BCP 205, RFC 7942, DOI 10.17487/RFC7942, , <https://www.rfc-editor.org/info/rfc7942>. - [I-D.ietf-cbor-edn-literals]
- Bormann, C., "CBOR Extended Diagnostic Notation (EDN)", Work in Progress, Internet-Draft, draft-ietf-cbor-edn-literals-13, , <https://datatracker.ietf.org/doc/html/draft-ietf-cbor-edn-literals-13>.
- [RFC2119]
- Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/rfc/rfc2119>.
- [RFC7515]
- Jones, M., Bradley, J., and N. Sakimura, "JSON Web Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, , <https://www.rfc-editor.org/rfc/rfc7515>.
- [RFC7519]
- Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token (JWT)", RFC 7519, DOI 10.17487/RFC7519, , <https://www.rfc-editor.org/rfc/rfc7519>.
- [RFC8174]
- Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/rfc/rfc8174>.
- [RFC8392]
- Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig, "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392, , <https://www.rfc-editor.org/rfc/rfc8392>.
- [RFC8747]
- Jones, M., Seitz, L., Selander, G., Erdtman, S., and H. Tschofenig, "Proof-of-Possession Key Semantics for CBOR Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, , <https://www.rfc-editor.org/rfc/rfc8747>.
- [RFC8949]
- Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", STD 94, RFC 8949, DOI 10.17487/RFC8949, , <https://www.rfc-editor.org/rfc/rfc8949>.
- [RFC9052]
- Schaad, J., "CBOR Object Signing and Encryption (COSE): Structures and Process", STD 96, RFC 9052, DOI 10.17487/RFC9052, , <https://www.rfc-editor.org/rfc/rfc9052>.
- [RFC9053]
- Schaad, J., "CBOR Object Signing and Encryption (COSE): Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053, , <https://www.rfc-editor.org/rfc/rfc9053>.
- [RFC9528]
- Selander, G., Preuß Mattsson, J., and F. Palombini, "Ephemeral Diffie-Hellman Over COSE (EDHOC)", RFC 9528, DOI 10.17487/RFC9528, , <https://www.rfc-editor.org/rfc/rfc9528>.
- [RFC9596]
- Jones, M.B. and O. Steele, "CBOR Object Signing and Encryption (COSE) "typ" (type) Header Parameter", RFC 9596, DOI 10.17487/RFC9596, , <https://www.rfc-editor.org/rfc/rfc9596>.
16.2. Informative References
- [I-D.draft-ietf-oauth-sd-jwt-vc]
- Terbu, O., Fett, D., and B. Campbell, "SD-JWT-based Verifiable Credentials (SD-JWT VC)", Work in Progress, Internet-Draft, draft-ietf-oauth-sd-jwt-vc-08, , <https://datatracker.ietf.org/doc/html/draft-ietf-oauth-sd-jwt-vc-08>.
- [I-D.draft-ietf-oauth-selective-disclosure-jwt]
- Fett, D., Yasuda, K., and B. Campbell, "Selective Disclosure for JWTs (SD-JWT)", Work in Progress, Internet-Draft, draft-ietf-oauth-selective-disclosure-jwt-14, , <https://datatracker.ietf.org/doc/html/draft-ietf-oauth-selective-disclosure-jwt-14>.
- [I-D.ietf-httpbis-unprompted-auth]
- Schinazi, D., Oliver, D., and J. Hoyland, "The Concealed HTTP Authentication Scheme", Work in Progress, Internet-Draft, draft-ietf-httpbis-unprompted-auth-12, , <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-unprompted-auth-12>.
Appendix B. Comparison to SD-JWT
SD-CWT is modeled after SD-JWT, with adjustments to align with conventions in CBOR and COSE.¶
B.1. Media Types
The COSE equivalent of application/sd-jwt
is application/sd+cwt
.¶
THe COSE equivalent of application/kb+jwt
is application/kb+cwt
.¶
B.2. 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.¶
B.3. Issuance
The issuance process for SD-CWT is similar to SD-JWT, with the exception that a confirmation claim is REQUIRED.¶
B.4. 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.¶
B.5. 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.¶
Appendix C. Keys used in the examples
C.1. Subject / Holder
Holder key pair in JWK format¶
{ "kty": "EC", "alg": "ES256", "kid": "WRQ2RbY5RYJCIxfDQL9agl9fFSCYVu4Xocqb6zerc1M", "crv": "P-256", "x": "hVTrJ13Nb70cesZBqiyQ2SAi_Q0wJLWvGMfMYa1Sei0", "y": "TceuLGd-ltDMgll2Vc6S1VA_VCk9h4ddHnnOR3AZQ0M", "d": "V1moblm7OwAt3kZ9pLUvPQbmws1DlFbPBIW5uGQpTOU" }¶
Input to Holder public JWK thumbprint (ignore line breaks)¶
{"crv":"P-256","kty":"EC","x":"hVTrJ13Nb70cesZBqiyQ2SAi_Q0wJLWvGMfMYa1S ei0","y":"TceuLGd-ltDMgll2Vc6S1VA_VCk9h4ddHnnOR3AZQ0M"}¶
SHA-256 of the Holder public JWK input string (in hex)¶
59143645b6394582422317c340bf5a825f5f15209856ee17a1ca9beb37ab7353¶
Holder public JWK thumbprint¶
WRQ2RbY5RYJCIxfDQL9agl9fFSCYVu4Xocqb6zerc1M¶
Holder public key in PEM format¶
-----BEGIN PUBLIC KEY----- MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEhVTrJ13Nb70cesZBqiyQ2SAi/Q0w JLWvGMfMYa1Sei1Nx64sZ36W0MyCWXZVzpLVUD9UKT2Hh10eec5HcBlDQw== -----END PUBLIC KEY-----¶
Hodler private key in PEM format¶
-----BEGIN PRIVATE KEY----- MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQgV1moblm7OwAt3kZ9 pLUvPQbmws1DlFbPBIW5uGQpTOWhRANCAASFVOsnXc1vvRx6xkGqLJDZICL9DTAk ta8Yx8xhrVJ6LU3HrixnfpbQzIJZdlXOktVQP1QpPYeHXR55zkdwGUND -----END PRIVATE KEY-----¶
C.2. Issuer
Issuer key pair in JWK format¶
{ "kty": "EC", "alg": "ES384", "kid": "https://issuer.example/cwk3.cbor", "crv": "P-384", "x":"wxeYsMeIX6NSj7-HfltMOm3GelpdxrMHtyjDclkm8qvl-0lkzZHjlIpUk_brtsu_", "y":"j2x-x2FpHK03TE2qk4dFPxgFjs5Y6wqOhKBVox-3-SFLJ1CVIsFZ52T4cR4RYJVU", "d":"ccVNIiGTfqYS2xIh8NPd93HJOBxOO-QdWqConWhfCc_vdMS78QR4P9V-h6sifQdM" }¶
Input to Issuer JWK thumbprint (ignore line breaks)¶
{"crv":"P-384","kty":"EC","x":"wxeYsMeIX6NSj7-HfltMOm3GelpdxrMHtyjDclkm 8qvl-0lkzZHjlIpUk_brtsu_","y":"j2x-x2FpHK03TE2qk4dFPxgFjs5Y6wqOhKBVox-3 -SFLJ1CVIsFZ52T4cR4RYJVU"}¶
SHA-256 of the Issuer JWK input string (in hex)¶
18d4ddb7065d945357e3972dee76af4eddc7c285fb42efcfa900c6a4f8437850¶
Issuer JWK thumbprint¶
GNTdtwZdlFNX45ct7navTt3HwoX7Qu_PqQDGpPhDeFA¶
Issuer public key in PEM format¶
-----BEGIN PUBLIC KEY----- MHYwEAYHKoZIzj0CAQYFK4EEACIDYgAEwxeYsMeIX6NSj7+HfltMOm3GelpdxrMH tyjDclkm8qvl+0lkzZHjlIpUk/brtsu/j2x+x2FpHK03TE2qk4dFPxgFjs5Y6wqO hKBVox+3+SFLJ1CVIsFZ52T4cR4RYJVU -----END PUBLIC KEY-----¶
Issuer private key in PEM format¶
-----BEGIN PRIVATE KEY----- MIG2AgEAMBAGByqGSM49AgEGBSuBBAAiBIGeMIGbAgEBBDBxxU0iIZN+phLbEiHw 0933cck4HE475B1aoKidaF8Jz+90xLvxBHg/1X6HqyJ9B0yhZANiAATDF5iwx4hf o1KPv4d+W0w6bcZ6Wl3Gswe3KMNyWSbyq+X7SWTNkeOUilST9uu2y7+PbH7HYWkc rTdMTaqTh0U/GAWOzljrCo6EoFWjH7f5IUsnUJUiwVnnZPhxHhFglVQ= -----END PRIVATE KEY-----¶
Appendix D. Document History
Note: RFC Editor, please remove this entire section on publication.¶
D.1. 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 newsd_alg
COSE protected header. No moresd_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)¶
D.2. 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¶
D.3. 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.¶