JSON Proof Token
draft-ietf-jose-json-proof-token-00

jose                                                           J. Miller
Internet-Draft                                             Ping Identity
Intended status: Standards Track                                M. Jones
Expires: 28 October 2023                                      individual
                                                                D. Waite
                                                           Ping Identity
                                                           26 April 2023

                            JSON Proof Token
                  draft-ietf-jose-json-proof-token-00

Abstract

   JSON Proof Token (JPT) is a compact, URL-safe, privacy-preserving
   representation of claims to be transferred between three parties.
   The claims in a JPT are encoded as base64url-encoded JSON objects
   that are used as the payloads of a JSON Web Proof (JWP)
   (https://www.ietf.org/archive/id/draft-ietf-jose-json-web-proof-
   00.html) structure, enabling them to be digitally signed and
   selectively disclosed.  JPTs also support reusability and
   unlinkability when using Zero-Knowledge Proofs (ZKPs).

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 28 October 2023.

Copyright Notice

   Copyright (c) 2023 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.

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   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Design Considerations . . . . . . . . . . . . . . . . . . . .   3
     4.1.  Unlinkability . . . . . . . . . . . . . . . . . . . . . .   3
     4.2.  Selective Disclosure  . . . . . . . . . . . . . . . . . .   4
     4.3.  Familiarity . . . . . . . . . . . . . . . . . . . . . . .   4
     4.4.  Proofs  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Claim Names . . . . . . . . . . . . . . . . . . . . . . . . .   4
   6.  Claims  . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   7.  Payloads  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     7.1.  Disclosed . . . . . . . . . . . . . . . . . . . . . . . .   6
     7.2.  Proof Methods . . . . . . . . . . . . . . . . . . . . . .   6
   8.  Example JPT . . . . . . . . . . . . . . . . . . . . . . . . .   6
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   11. Informative References  . . . . . . . . . . . . . . . . . . .   6
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   7
   Appendix B.  Document History . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

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), but
   with additional support for selective disclosure and unlinkability.
   JPTs encode claim values to be transmitted as payloads of a JSON Web
   Proof (JWP) (https://www.ietf.org/archive/id/draft-ietf-jose-json-
   web-proof-00.html).  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.

   |  Editor's Note: This draft is still early and incomplete, there
   |  will be significant changes to the algorithms as currently defined
   |  here.  Please do not use any of these definitions or examples for
   |  anything except personal experimentation and learning.
   |  Contributions and feedback are welcome at https://github.com/json-
   |  web-proofs/json-web-proofs (https://github.com/json-web-proofs/
   |  json-web-proofs).

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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.

4.  Design Considerations

   The rationale behind the design for JSON Proof Tokens is important
   when considering how it is structured.  These sections detail the
   underlying reasoning for the approach defined by JPTs.

4.1.  Unlinkability

   Supporting unlinkability is perhaps the most challenging design
   constraint for JPTs.  Even the smallest oversight can introduce a
   subtle vector for relying parties to collude and correlate one or
   more subjects across their usage.

   The principal tools to prevent this are data minimization and
   uniformity.  The data included in a JPT SHOULD be minimized to remove
   potential correlation points.  The data SHOULD contain only values
   that are able to be selectively disclosed with consent or transformed
   by the proof algorithm when presented.

   Any other data that is repeated across multiple JPTs is externalized
   so that it is uniform across every issuance.  This includes
   preventing the usage of optional headers, dynamic mapping of claims
   to payloads, changes to how many payloads are included, and the
   ordering of the payloads.

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4.2.  Selective Disclosure

   While JWPs provide the underling structure for easily supporting
   selective disclosure, JPTs must go a step further to ensure that
   applications can effectively provide choice and consent on exactly
   what is being disclosed.  Software using JWPs must know the mappings
   from payloads to claims.  JPTs do not support disclosing claims that
   are intended to be private from the issuer to the relying party.  All
   disclosed payloads MUST be mapped to claims and made accessible to
   the application.

4.3.  Familiarity

   JPTs are intended to be as close to a JWT as possible in order to
   provide the simplest transition for any JWT-based system to add
   support for a JPT.

   Although there are some stark differences in the lifecycle of a JPT,
   from the application's perspective, the interface to a JPT can be
   made fairly similar: a JSON object containing a mix of required and
   optional claims with well-understood values.

   The most significant divergence required by JPTs is that of
   supporting values that may be disclosed or may instead only be a
   proof about the value.  Applications are required to interact with
   the JPT on a payload-by-payload basis instead of just verifying a JWT
   and then being able to interact with the JSON body directly.

4.4.  Proofs

   In order to generate a variety of efficient ZKPs of knowledge, range,
   membership, or other predicates, it is essential that each individual
   payload is only a single claim value.  This greatly simplifies the
   task of linking a derived proof of a given claim to the specific
   payload that was also signed by the issuer.  While JPTs support
   claims that have complex object or array compound values, they also
   allow for simple claim values such as JSON strings, numbers, and
   booleans that can be used directly in generating predicate proofs.

5.  Claim Names

   It is suggested that the claim names used with JPTs come from those
   in the IANA JSON Web Token Claims Registry, when those fit the
   application's needs.

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6.  Claims

   Using a JSON Proof Token requires combining information from two
   sources: the claim names and the payloads.  The simplest solution is
   to list the claim names in an ordered array that aligns with the
   included payloads.  This claims array can be conveniently included in
   the JWP Protected Header using the claims key.

   When the claims array is stored in the header, any variations of it
   are disclosed to the verifier and can be used to correlate and link
   usages.  Given the privacy design considerations around linkability
   it is recommended that the claims are defined external to an
   individual JPT and either referenced or known by the application
   context.

   In order to facilitate this external definition of the claim names,
   an additional cid key is defined with a required digest value
   calculated as defined here.  This cid can be used similar to a kid in
   order to ensure 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.

   All payloads are claim values and MUST be the base64url encoding of
   the UTF-8 representation of a JSON value.

   The following is an example JWP Protected Header that includes a
   claims array:

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

7.  Payloads

   |  Editor's Note: This section is significantly incomplete, use it
   |  only as an indicator of the intended direction.

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   Application resolves each claim as required when processing the JPT.
   Resolution can result in one of three things: 1.  A disclosed JSON
   value 2.  A custom proof method 3.  A null value

7.1.  Disclosed

   Always an octet string of valid JSON text.

7.2.  Proof Methods

   *  proof methods can be returned instead of a disclosed payload

   *  these are generated by the algorithm from information in the JWP's
      proof value

   *  a proof method may be custom based on the capabilities of the
      algorithm

   *  define common proof method types available?

      -  range

      -  membership

      -  time

      -  knowledge

      -  linking

8.  Example JPT

   See the JSON Web Proof draft appendix.

9.  Security Considerations

   *  Protected Header Minimization

10.  IANA Considerations

   This document has no IANA actions.

11.  Informative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

Appendix A.  Acknowledgements

   TBD

Appendix B.  Document History

   [[ To be removed from the final specification ]]

   -00

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

Authors' Addresses

   Jeremie Miller
   Ping Identity
   Email: jmiller@pingidentity.com

   Michael B. Jones
   individual
   Email: michael_b_jones@hotmail.com
   URI:   https://self-issued.info/

   David Waite
   Ping Identity
   Email: dwaite+jwp@pingidentity.com

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