JSON Proof Token and CBOR Proof Token

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Table of Contents

1. Introduction

JSON Proof Token (JPT) is a compact claims representation format intended to be used in the same ways as a JSON Web Token (JWT) [RFC7519], but with additional support for selective disclosure and unlinkability. JPTs encode claim values to be transmitted as payloads of a JSON Web Proof (JWP) [I-D.ietf-jose-json-web-proof]. JPTs are always represented using the JWP Compact Serialization. The corresponding claim names are not transmitted in the payloads and are stored in a separate structure that can be externalized and shared across multiple JPTs.¶

Likewise, CBOR Proof Token (CPT) is a similar compact claims representation format intended to be used in the same ways as a CBOR Web Token (CWT) [RFC8392], but with the same support for selective disclosure and unlinkability. CPTs are represented using the JWP CBOR Serialization. The corresponding claim names are not transmitted in the payloads and are stored in a separate structure that can be externalized and shared across multiple CPTs.¶

2. Conventions and Definitions

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.¶

3. Background

JWP defines a container binding together a protected header, one or more payloads, and a cryptographic proof. It does not define how claims are organized into payloads and what formats they are in. JPTs are intended to be as close to a JWT as possible, while also supporting the selective disclosure and unlinkability of JWPs. Likewise, CPTs are intended to be as close to a CWT as possible, while also supporting the selective disclosure and unlinkability of JWPs.¶

4. Design Considerations

The rationale behind the design for JSON Proof Tokens and CBOR Proof Tokens is important when considering how they are structured. These sections detail the underlying reasoning informing their design.¶

5. Claim Names

It is RECOMMENDED that the claim names used with JPTs come from those in the IANA JSON Web Token Claims Registry [IANA.JWT] established by [RFC7519], when those fit the application's needs. Likewise, it is RECOMMENDED that the claim names used with CPTs come from those in the IANA CBOR Web Token Claims Registry [IANA.CWT] established by [RFC8392], when those fit the application's needs.¶

6. Claims Header Parameter

A JSON Proof Token or CBOR Proof Token assigns each playload a claim name. Payloads MUST each have a negotiated and understood claim name within the application context. The simplest solution to establish payload claim names is as an ordered array that aligns with the included payloads. This claims array can be conveniently included in the Claims header parameter.¶

The claims Header Parameter is an array listing the Claim Names corresponding to the JWP payloads, in the same order as the payloads. Each array value is a Claim Name, as defined in [RFC7519] or [RFC8392]. Use of this Header Parameter is OPTIONAL.¶

All JPT payloads that are claim values MUST be the base64url encoding of the UTF-8 representation of a JSON value. That said, predicate proofs derived from payload values are not represented as claims; they are contained in the presentation proof using algorithm-specific representations. All CPT payloads that are claim values MUST be a CBOR value. Likewise, CPT predicate proofs derived from payload values are not represented as claims; they are contained in the presentation proof using algorithm-specific representations.¶

The following is an example JWP Issuer Protected Header that includes a claims property:¶

{
  "kid": "HjfcpyjuZQ-O8Ye2hQnNbT9RbbnrobptdnExR0DUjU8",
  "alg": "BBS",
  "claims": [
    "iat",
    "exp",
    "family_name",
    "given_name",
    "email",
    "address",
    "age_over_21"
  ]
}

In this example, the "iat" and "exp" would be JSON-formatted numbers, "family_name", "given_name" and "email" would be JSON strings (in quotes), "address" would be a JSON object, and "age_over_21" would be either true or false.¶

7. Claims ID ("cid") JWP Header Parameter

A Claims ID ("cid") value can be used as an identifier for a set of claim names without explicitly listing them. Its use is similar to the Key ID ("kid") header parameter.¶

The structure of the cid value is unspecified. For JPTs, its value MUST be a case-sensitive string. For CPTs, its value MUST be a binary string. Use of this JWP Header Parameter is OPTIONAL.¶

The cid can be used similarly to a kid in order to ensure that is it possible to externally resolve and then verify that the correct list of claim names is being used when processing the payloads containing the claim values.¶

If there is an associated JWK containing the signing key information, the claims key is also registered there as a convenient location for the claim names. Likewise, if there is an associated COSE_Key containing the signing key information, the claims key is also registered there as a convenient location for the claim names.¶

When the claims array is transferred as a property in the Issuer Protected Header, any variations of that array between JWP will be visible to the verifier, and can leak information about the subject or provide an additional vector for linkability. Given the privacy design considerations around linkability, it is RECOMMENDED that the claims are defined external to an individual JPT or CPT and either referenced or known by the application context.¶

The following is an example JWP Protected Header that includes a cid:¶

{
  "kid": "HjfcpyjuZQ-O8Ye2hQnNbT9RbbnrobptdnExR0DUjU8",
  "alg": "BBS",
  "cid": "guA8PAI14Gkn4273f1rR606yMbRMFg4y"
}

8. Presented Claims and Proofs

Each claim in the issued form of the JPT or CPT results in one of three things in the presented form of the JPT or CPT: 1. A disclosed JSON or CBOR value. 1. An indicator that the value was not disclosed. 1. An algorithm-specific proof method.¶

9. Example JPTs

See the examples in Appendix A.1 of [I-D.ietf-jose-json-proof-algorithms].¶

10. Security Considerations

• Protected Header Minimization¶

11. IANA Considerations

Appendix A. Acknowledgements

This work was incubated in the DIF Applied Cryptography Working Group.¶

We would like to thank Brent Zundel for his valuable contributions to this specification.¶

Appendix B. Document History

[[ To be removed from the final specification ]]¶

-08¶

• Defined CBOR Proof Token (CPT).¶
• Registered application/jpt and application/cpt media types and CPT CoAP Content-Format.¶
• Made some additional references normative.¶

-07¶

• Changing primary editor¶
• Move claims definition from JWP, to live beside cid¶
• Update cid registry entry to assign CBOR label¶

-06¶

• Update reference to new repository home¶
• Fixed #99: Discussed issued and presented forms of JPTs.¶

-05¶

• Define and register Claims ID JWP Header Parameter.¶

-04¶

• Refactoring figures and examples to be built from a common set across all three documents¶

-03¶

• Improvements resulting from a full proofreading.¶
• Added examples of JSON object and JSON boolean claims.¶

-02¶

• Update example to use the current BBS algorithm¶

-01¶

• Correct cross-references within group.¶

-00¶

• Created initial working group draft based on draft-jmiller-jose-json-proof-token-01¶

Authors' Addresses