| Internet-Draft | json-web-proof | July 2024 |
| Miller, et al. | Expires 23 January 2025 | [Page] |
The JOSE set of standards established JSON-based container formats for Keys, Signatures, and Encryption. They also established IANA registries to enable the algorithms and representations used for them to be extended. Since those were created, newer cryptographic algorithms that support selective disclosure and unlinkability have matured and started seeing early market adoption.¶
This document defines a new container format similar in purpose and design to JSON Web Signature (JWS) called a JSON Web Proof (JWP). Unlike JWS, which integrity-protects only a single payload, JWP can integrity-protect multiple payloads in one message. It also specifies a new presentation form that supports selective disclosure of individual payloads, enables additional proof computation, and adds a protected header to prevent replay.¶
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This Internet-Draft will expire on 23 January 2025.¶
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The JOSE specifications are very widely deployed and well supported, enabling use of cryptographic primitives with a JSON representation. JWTs [RFC7519] are one of the most common representations for identity and access claims. For instance, they are used by the OpenID Connect and Secure Telephony Identity Revisited (STIR) standards. Also, JWTs are used by W3C's Verifiable Credentials and are used in many decentralized identity systems.¶
With these new use cases, there is an increased focus on adopting privacy-protecting cryptographic primitives. While such primitives are still an active area of academic and applied research, the leading candidates introduce new patterns that are not currently supported by JOSE. These new patterns are largely focused on two areas: supporting selective disclosure when presenting information and minimizing correlation through the use of Zero-Knowledge Proofs (ZKPs) in addition to traditional signatures.¶
There are a growing number of these cryptographic primitives that support selective disclosure while protecting privacy across multiple presentations. Examples used in the context of Verifiable Credentials are:¶
All of these follow the same pattern of taking multiple claims (a.k.a., "attributes" or "messages" in the literature) and binding them together into a single issued token. These are then later securely one-way transformed into a presentation that reveals potentially only a subset of the original claims, predicate proofs about the claim values, or proofs of knowledge of the claims.¶
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.¶
The roles of "issuer", "holder", and "verifier" are used as defined by the VC Data Model [VC-DATA-MODEL-2.0]. The term "presentation" is also used as defined by this source, but the term "credential" is avoided in this specification to minimize confusion with other definitions.¶
The following terms are used throughout this series of documents:¶
binding: A mechanism, indicated in an issued JWP, for how to verify a presentation was created by the intended holder.¶
linkability: The property where multiple presentations may be correlated to a single issued JWP, either through consistency in the cryptographic integrity or due to particulars of JWP usage by an application. Such issued JWPs may be referred to as single-use, as multiple uses may leak unintended knowledge.¶
unlinkability: The property of issuance and presentation algorithms and of application usage, where one presentation can only be correlated with other presentations based on holder-disclosed information.¶
A JSON Web Proof (JWP) is very similar to a JWS [RFC7515], with the addition that it can contain multiple individual secured payloads instead of a single one. JWP-supporting algorithms are then able to separate and act on the individual payloads contained within.¶
The intent of JSON Web Proofs is to establish a common container format for multiple payloads that can be integrity-verified against a cryptographic proof value also in the container. It does not create or specify any cryptographic protocols, multi-party protocols, or detail any algorithm-specific capabilities.¶
To fully support the newer privacy primitives, JWP utilizes the three roles of issuer, holder, and verifier, as defined by the VC Data Model [VC-DATA-MODEL-2.0]. There are also two forms of a JWP: the issued form created by an issuer for a holder, and the presented form created by a holder for a verifier.¶
The four principal interactions used by JWP are issue, confirm, present, and verify.¶
A JWP is initially created by the issuer using the issue interaction. A successful result is an issued JWP that has a single issuer-protected header, one or more payloads, and an initial proof value that contains the issuing algorithm output. The holder, upon receiving an issued JWP, then uses the confirm interaction to check the integrity protection of the header and all payloads using the proof value.¶
After validation, the holder uses the present interaction to apply any selective disclosure choices, perform privacy-preserving transformations for unlinkability, and add a presentation-protected header that ensures the resulting presented JWP cannot be replayed. The verifier then uses the verify interaction to ensure the integrity protection of the protected headers and any disclosed payloads, along with verifying any additional ZKPs covering non-disclosed payloads.¶
While issue and confirm only occur when a JWP is initially created by the issuer, the present and verify steps may be safely repeated by a holder on an issued JWP. He resulting presented JWP is only unlinkable when supported by the underlying algorithm.¶
Algorithm definitions that support JWPs are in separate companion specifications - just as the JSON Web Algorithms [RFC7518] specification does for JWS and JWE [RFC7516]. The JSON Proof Algorithms (JPA) [I-D.ietf-jose-json-proof-algorithms] specification defines how an initial set of algorithms are used with JWP.¶
The members of the JSON object(s) representing the JWP Header describe the proof applied to the Protected Header and the Payload and optionally, additional properties of the JWP. The Header Parameter names within the JWP Header MUST be unique; JWP parsers MUST either reject JWPs with duplicate Header Parameter names or use a JSON parser that returns only the lexically last duplicate member name, as specified in Section 15.12 ("The JSON Object") of ECMAScript 5.1 [ECMAScript].¶
Implementations are required to understand the specific Header Parameters defined by this specification that are designated as "MUST be understood" and process them in the manner defined in this specification. All other Header Parameters defined by this specification that are not so designated MUST be ignored when not understood. Unless listed as a critical Header Parameter, per Section 4.1.4, all Header Parameters not defined by this specification MUST be ignored when not understood.¶
There are three classes of Header Parameter names: Registered Header Parameter names, Public Header Parameter names, and Private Header Parameter names.¶
These requirements are intentionally parallel to those in Section 4 of [RFC7515].¶
The following Header Parameter names for use in JWPs are registered in the IANA "JSON Web Proof Header Parameters" registry established by Section 8.1, with meanings as defined in the subsections below.¶
As indicated by the common registry, Header Parameters used in the Issued Form (see Section 5.1) and the Presented Form Section 5.2 share a common Header Parameter space; when a parameter is used by both forms, its usage must be compatible between them.¶
These Header Parameters are intentionally parallel to those in Section 4.1 of [RFC7515].¶
The alg (algorithm) Header Parameter identifies the cryptographic algorithm
used to secure the JWP.
The JWP Proof value is not valid if the alg value does not represent
a supported algorithm or if there is not a key for use with that algorithm
associated with the party that secured the content.
alg values should either be registered in
the IANA "JSON Web Proof Algorithms" registry
established by [I-D.ietf-jose-json-proof-algorithms]
or be a value that contains a Collision-Resistant Name.
The alg value is a case-sensitive ASCII string containing a StringOrURI value.
This Header Parameter MUST be present
and MUST be understood and processed by implementations.¶
A list of defined alg values for this use can be found
in the IANA "JSON Web Proof Algorithms" registry
established by [I-D.ietf-jose-json-proof-algorithms];
the initial contents of this registry are registered
by [I-D.ietf-jose-json-proof-algorithms].¶
The kid (key ID) Header Parameter
is a hint indicating which key was used to secure the JWP.
This parameter allows originators to explicitly signal a change of
key to recipients.
The structure of the kid value is unspecified.
Its value MUST be a case-sensitive string.
Use of this Header Parameter is OPTIONAL.¶
When used with a JWK,
the kid value is used to match a JWK kid parameter value.¶
The typ (type) Header Parameter is used by JWP applications to declare the
media type (#IANA.MediaTypes) of this complete JWP.
This is intended for use by the application when
more than one kind of object could be present in
an application data structure that can contain a JWP;
the application can use this value to disambiguate among
the different kinds of objects that might be present.
It will typically not be used by applications when
the kind of object is already known.
This parameter is ignored by JWP implementations;
any processing of this parameter is performed by the JWP application.
Use of this Header Parameter is OPTIONAL.¶
Per [RFC2045], all media type values, subtype values, and parameter names are case insensitive. However, parameter values are case sensitive unless otherwise specified for the specific parameter.¶
To keep messages compact in common situations,
it is RECOMMENDED that producers omit an "application/" prefix
of a media type value in a typ
Header Parameter when no other '/' appears in the media type value.
A recipient using the media type value MUST treat it as if
"application/" were prepended to any typ value not containing a '/'.
For instance, a typ value of example SHOULD be used to represent
the application/example media type,
whereas the media type application/example;part="1/2" cannot
be shortened to example;part="1/2".¶
The typ value jwp can be used by applications
to indicate that this object is a JWP using the JWP Compact Serialization.
The typ value jwp+json can be used by applications
to indicate that this object is a JWP using the JWP JSON Serialization.
Other type values can also be used by applications.¶
It is RECOMMENDED that the typ Header Parameter be used for explicit typing,
in parallel to the recommendations in Section 3.11 of [RFC8725].¶
The crit (critical) Header Parameter indicates that extensions to
this specification and/or [I-D.ietf-jose-json-proof-algorithms]
are being used that MUST be understood and processed.
Its value is an array listing the Header Parameter names
present in the JWP Header that use those extensions.
If any of the listed extension Header Parameters are not
understood and supported by the recipient, then the JWP is invalid.
Producers MUST NOT include Header Parameter names defined by
this specification or [I-D.ietf-jose-json-proof-algorithms] for use with JWP,
duplicate names, or
names that do not occur as Header Parameter names within the JWP Header
in the crit list.
Producers MUST NOT use the empty list [] as the crit value.
Recipients MAY consider the JWP to be invalid if the critical list
contains any Header Parameter names defined by
this specification or [I-D.ietf-jose-json-proof-algorithms] for use with JWP
or if any other constraints on its use are violated.
When used, this Header Parameter MUST be integrity protected;
therefore, it MUST occur only within the JWP Protected Header.
Use of this Header Parameter is OPTIONAL.
This Header Parameter MUST be understood and processed by implementations.¶
The proof_jwk (Proof JWK) represents the public key used by the issuer
for proof of possession.
This key is represented as a JSON Web Key public key value.
It MUST contain only public key parameters and
SHOULD contain only the minimum JWK parameters necessary to represent the key;
other JWK parameters included can be checked for consistency and honored, or they can be ignored.
This Header Parameter MUST be present in the JWP issuer header parameters
and MUST be understood and processed by implementations.¶
The presentation_jwk (Presentation JWK) represents the public key used by the holder
for proof of possession.
This key is represented as a JSON Web Key public key value.
It MUST contain only public key parameters and
SHOULD contain only the minimum JWK parameters necessary to represent the key;
other JWK parameters included can be checked for consistency and honored, or they can be ignored.
This Header Parameter MUST be present in the JWP issuer header parameters
and MUST be understood and processed by implementations.¶
The iss (issuer) Header Parameter identifies the principal that issued the JWP.
The processing of this claim is generally application specific.
The iss value is a case-sensitive string
containing a StringOrURI value.
Its definition is intentionally parallel to the iss claim defined in [RFC7519].
Use of this Header Parameter is OPTIONAL.¶
The aud (audience) Header Parameter
identifies the recipients that the JWP is intended for.
Each principal intended to process the JWP MUST identify itself
with a value in the audience Header Parameter. If the principal
processing the Header Parameter does not identify itself with a
value in the aud Header Parameter when this Header Parameter is present,
then the JWP MUST be rejected.
In the general case,
the aud value is an array of
case-sensitive strings, each containing a StringOrURI value.
In the special case when the JWP has one audience,
the aud value MAY be a single
case-sensitive string containing a StringOrURI value.
The interpretation of audience values is generally application specific.
Its definition is intentionally parallel to the aud claim defined in [RFC7519].
Use of this Header Parameter is OPTIONAL.¶
The nonce (nonce) Header Parameter is a case-sensitive string value
used to associate protocol state with a JWP.
This can be used, for instance, to mitigate replay attacks.
The use of nonce values is generally protocol specific.
Its definition is intentionally parallel to the nonce claim
registered in the IANA "JSON Web Token Claims" registry (#IANA.JWT.Claims).
Use of this Header Parameter is OPTIONAL.¶
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].
Use of this Header Parameter is OPTIONAL.¶
Additional Header Parameter names can be defined by those using JWPs. However, in order to prevent collisions, any new Header Parameter name should either be registered in the IANA "JSON Web Proof Header Parameters" registry established by Section 8.1 or be a Public Name (a value that contains a Collision-Resistant Name). In each case, the definer of the name or value needs to take reasonable precautions to make sure they are in control of the part of the namespace they use to define the Header Parameter name.¶
New Header Parameters should be introduced sparingly, as they can result in non-interoperable JWPs.¶
A producer and consumer of a JWP may agree to use Header Parameter names that are Private Names (names that are not Registered Header Parameter names Section 4.1 or Public Header Parameter names Section 4.2.) Unlike Public Header Parameter names, Private Header Parameter names are subject to collision and should be used with caution.¶
A JWP is always in one of two forms: the issued form or the presented form. A structural difference between the two forms is the number of protected headers. An issued JWP has only one issuer protected header, while a presented JWP will have both the issuer protected header and an additional presentation protected header. Each protected header is a JSON object that is serialized as a UTF-8 encoded octet string.¶
All JWP forms have one or more payloads; each payload is an octet string. The payloads are arranged in an array for which the ordering is preserved in all serializations.¶
The JWP proof value is one or more octet strings that are only meant to be generated from and processed by the underlying JPA. Internally, the proof value may contain one or more cryptographic statements that are used to check the integrity protection of the header(s) and all payloads. Each of these statements may be a ZKP or a traditional cryptographic signature. The algorithm is responsible for how these statements are serialized into a single proof value.¶
When a JWP is first created, it is always in the issued form. It will contain the issuer protected header along with all of the payloads.¶
The issued form can only be confirmed by a holder as being correctly formed and protected. It is NOT to be verified directly or presented as-is to a verifier. The holder SHOULD treat an issued JWP as private and use appropriately protected storage.¶
The issuer protected header applies to all of the payloads equally. It is recommended that any payload-specific information not be included in this header and instead be handled outside of the cryptographic envelope. This is to minimize any correlatable signals in the metadata, to reduce a verifier's ability to group different presentations based on small header variations from the same issuer. The protected header is always disclosed, whereas payloads can be selectively disclosed.¶
Every issuer protected header MUST have an alg value that identifies a valid JSON Proof Algorithm (JPA).¶
For example:¶
{
"alg": "BBS"
}
¶
Payloads are represented and processed as individual octet strings and arranged in an ordered array when there are multiple payloads. All application context of the placement and encoding of each payload value is out of scope of this specification and SHOULD be well defined and documented by the application or other specifications.¶
JPAs MAY provide software interfaces that perform the encoding of individual payloads which accept native inputs such as numbers, sets, or elliptic curve points. This enables the algorithm to support advanced features such as blinded values and predicate proofs. These interfaces would generate the octet string encoded payload value as well as include protection of that payload in the combined proof value.¶
The issuer proof is one or more binary octet strings that are opaque to applications. Individual proof-supporting algorithms are responsible for the contents and security of the proof value, along with any required internal structures.¶
The issuer proof is used by the holder to perform validation, checking that the issuer header and all payloads are properly encoded and protected by the given proof.¶
When an issued JWP is presented, it undergoes a transformation that adds a presentation protected header. It may also have one or more payloads hidden, disclosing only a subset of the original issued payloads. The proof value will always be updated to add integrity protection of the presentation header along with the necessary cryptographic statements to verify the presented JWP.¶
When supported by the underling JPA, a single issued JWP can be used to safely generate multiple presented JWPs without becoming correlatable.¶
A JWP may also be single use, where an issued JWP can only be used once to generate a presented form. In this case, any additional presentations would be inherently correlatable. These are still useful for applications needing only selective disclosure or where new unique issued JWPs can be retrieved easily.¶
The presented form of a JWP MUST contain a presentation protected header. It is added by the holder and MUST be integrity protected by the underling JPA.¶
This header is used to ensure that a presented JWP cannot be replayed and is cryptographically bound to the verifier it was presented to.¶
While there are not any required values in the presentation header, it MUST contain one or more header values that uniquely identify the presented JWP to both the holder and verifier. For example, header values that would satisfy this requirement include nonce and aud.¶
Any one or more payloads may be non-disclosed in a presented JWP. When a payload is not disclosed, the position of other payloads does not change; the resulting array will simply be sparse and only contain the disclosed payloads.¶
The disclosed payloads will always be in the same array positions to preserve any index-based references by the application between the issued and presented forms of the JWP. How the sparse array is represented is specific to the serialization used.¶
Algorithms MAY support including a proof about a payload in the presentation. Applications then treat that proven payload the same as any other non-disclosed payload and do not include it in the presented array of payloads. Rather, proofs about payloads, such as "age >= 21", are included in the presentation proof.¶
The presentation proof is one or more binary octet strings that are opaque to applications. Individual proof-supporting algorithms are responsible for the contents and security of the proof value, along with any required internal structures.¶
The proof of a presented JWP will always be different than the issued proof. At a minimum, it MUST be updated to include protection of the added presentation header.¶
Algorithms SHOULD generate an un-correlatable presentation proof in order to support multiple presentations from a single issued JWP.¶
The algorithm is responsible for representing selective disclosure of payloads in a presented proof. If multiple octet strings are insufficient for representing a proof as defined by an algorithm, the algorithm is responsible for defining how such information is represented within one or more octet strings.¶
Each disclosed payload MUST be base64url encoded when preparing it to be serialized. The headers and proof are also individually base64url encoded.¶
Like JWS, JWP supports both a Compact Serialization and a JSON Serialization.¶
The individually encoded payloads are concatenated with the ~ character to form an ordered delimited array. Any non-disclosed payloads are left blank, resulting in sequential ~~ characters such that all payload positions are preserved.¶
A payload which is disclosed but which contains no data (i.e. a zero-length octet string) is encoded as a single _ character of data, which is not a valid result from base64url-encoding a value.¶
Additionally, an algorithm MAY supply multiple octet strings for a proof. These are concatenated with the ~ character to form an ordered delimited array.¶
The headers, concatenated payloads, and proof value are then concatenated with a . character to form the final compact serialization. The issued form will only contain one header and always have three . separated parts. The presented form will always have four . separated parts, the issued header, followed by the protected header, then the payloads and the proof.¶
Figure: Compact Serialization of Presentation¶
Non-disclosed payloads in the JSON serialization are represented with a null value in the payloads array. A zero-length payload is represented as a zero-length base64url encoded sequence, the empty string "".¶
Proofs are represented as an array of one or more encoded octet strings.¶
This example flattened JSON serialization shows the presentation form with both the issuer and presentation headers, and with the first and third payloads hidden.¶
Figure: JSON Serialization of Presentation¶
Notes to be expanded:¶
The following registration procedure is used for all the registries established by this specification.¶
Values are registered on a Specification Required [RFC5226] basis after a three-week review period on the jose-reg-review@ietf.org mailing list, on the advice of one or more Designated Experts. However, to allow for the allocation of values prior to publication, the Designated Experts may approve registration once they are satisfied that such a specification will be published.¶
Registration requests sent to the mailing list for review should use an appropriate subject (e.g., "Request to register JWP header parameter: example").¶
Within the review period, the Designated Experts will either approve or deny the registration request, communicating this decision to the review list and IANA. Denials should include an explanation and, if applicable, suggestions as to how to make the request successful. Registration requests that are undetermined for a period longer than 21 days can be brought to the IESG's attention (using the iesg@ietf.org mailing list) for resolution.¶
Criteria that should be applied by the Designated Experts includes determining whether the proposed registration duplicates existing functionality, whether it is likely to be of general applicability or useful only for a single application, and whether the registration description is clear.¶
IANA must only accept registry updates from the Designated Experts and should direct all requests for registration to the review mailing list.¶
It is suggested that multiple Designated Experts be appointed who are able to represent the perspectives of different applications using this specification, in order to enable broadly informed review of registration decisions. In cases where a registration decision could be perceived as creating a conflict of interest for a particular Expert, that Expert should defer to the judgment of the other Experts.¶
This specification establishes the IANA "JSON Web Proof Header Parameters" registry for Header Parameter names. The registry records the Header Parameter name and a reference to the specification that defines it. The same Header Parameter name can be registered multiple times, provided that the parameter usage is compatible between the specifications. Different registrations of the same Header Parameter name will typically use different Header Parameter Usage Locations values.¶
Issued or Presented. Other values may be used with the approval of a Designated Expert.¶
This section registers the Header Parameter names defined in Section 4.1 in this registry.¶
This section registers the application/jwp
media type [RFC2046]
in the IANA "Media Types" registry (#IANA.MediaTypes)
in the manner described in [RFC6838],
which can be used to indicate that the content is
a JWP using the JWP Compact Serialization.
This section also registers the application/jwp+json
media type in the IANA "Media Types" registry,
which can be used to indicate that the content is
a JWP using the JWP JSON Serialization.¶
Additional information:¶
Additional information:¶
This section registers the +jwp
structured syntax suffix [RFC6838]
in the IANA "Structured Syntax Suffix" registry (#IANA.StructuredSuffix)
in the manner described in [RFC6838],
which can be used to indicate that the media type is encoded as a JWP.¶
The syntax and semantics for fragment identifiers for a specific "xxx/yyy+jwp" SHOULD be processed as follows:¶
For cases defined in +jwp, where the fragment identifier resolves per the +jwp rules, then process as specified in +jwp.¶
For cases defined in +jwp, where the fragment identifier does not resolve per the +jwp rules, then process as specified in "xxx/yyy+jwp".¶
For cases not defined in +jwp, then process as specified in "xxx/yyy+jwp".¶
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.¶
[[ To be removed from the final specification ]]¶
-05¶
+jwp structured syntax suffix.¶
-04¶
-03¶
~ characters in the compact serialization.¶
-02¶
-01¶
-00¶