JSON Proof Algorithms

JSON Proof Algorithms Ping Identity

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Internet jose jose zkp jwp jws jpa The JSON Proof Algorithms (JPA) specification registers cryptographic algorithms and identifiers to be used with the JSON Web Proof and JSON Web Key (JWK) specifications. It defines IANA registries for these identifiers.

Introduction The JSON Web Proof (JWP) draft establishes a new secure container format that supports selective disclosure and unlinkability using Zero-Knowledge Proofs (ZKPs) or other cryptographic algorithms.

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 welcomed at https://github.com/json-web-proofs/json-web-proofs.

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 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 . 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.

Terminology The terms "JSON Web Signature (JWS)", "Base64url Encoding", "Header Parameter", "JOSE Header", "JWS Payload", "JWS Signature", and "JWS Protected Header" are defined by . The terms "JSON Web Proof (JWP)", "JWP Payload", "JWP Proof", and "JWP Protected Header" are defined by . These terms are defined by this specification:

Stable Key:: An asymmetric key-pair used by an issuer that is also shared via an out-of-band mechanism to a verifier to validate the signature.
Ephemeral Key:: An asymmetric key-pair that is generated for one-time use by an issuer and never stored or used again outside of the creation of a single JWP.
Presentation Key:: An asymmetric key-pair that is generated by a holder and used to ensure that a presentation is not able to be replayed by any other party.

Background JWP defines a container binding together a protected header, one or more payloads, and a cryptographic proof. It does not define any details about the interactions between an application and the cryptographic libraries that implement proof-supporting algorithms. Due to the nature of ZKPs, this specification also documents the subtle but important differences in proof algorithms versus those defined by the JSON Web Algorithms . These differences help support more advanced capabilities such as blinded signatures and predicate proofs.

Algorithm Basics The four principal interactions that every proof algorithm MUST support are [issue](#issue), [confirm](#confirm), [present](#present), and [verify](#verify).

Issue The JWP is first created as the output of a JPA's issue operation. Every algorithm MUST support a JSON issuer protected header along with one or more octet string payloads. The algorithm MAY support using additional items provided by the holder for issuance such as blinded payloads, keys for replay prevention, etc. All algorithms MUST provide integrity protection for the issuer header and all payloads and MUST specify all digest and/or hash2curve methods used.

Confirm Performed by the holder to validate that the issued JWP is correctly formed and protected. Each algorithm MAY support using additional input items options, such as those sent to the issuer for issuance. After confirmation, an algorithm MAY return a modified JWP for serialized storage without the local state (such as with blinded payloads now unblinded). The algorithm MUST fully verify the issued proof value against the issuer protected header and all payloads. If given a presented JWP instead of an issued one, the confirm process MUST return an error.

Present Used to apply any selective disclosure choices and perform any unlinkability transformations, as well as to show binding. An algorithm MAY support additional input options from the requesting party, such as for predicate proofs and verifiable computation requests. Every algorithm MUST support the ability to hide any or all payloads. It MUST always include the issuer protected header unmodified in the presentation. The algorithm MUST replace the issued proof value and generate a new presented proof value. It also MUST include a new presentation protected header that provides replay protection.

Verify Performed by the verifier to verify the protected headers along with any disclosed payloads and/or assertions about them from the proving party, while also verifying they are the same payloads and ordering as witnessed by the issuer. The algorithm MUST verify the integrity of all disclosed payloads and MUST also verify the integrity of both the issuer and presentation protected headers. If the presented proof contains any assertions about the hidden payloads, the algorithm MUST also verify all of those assertions. It MAY support additional options, such as those sent to the holder to generate the presentation. If given an issued JWP for verification, the algorithm MUST return an error.

Algorithm Specifications This section defines how to use specific algorithms for JWPs.

Single Use The Single Use (SU) algorithm is based on composing multiple traditional asymmetric signatures into a single JWP proof. It enables a very simple form of selective disclosure without requiring any advanced cryptographic techniques. It does not support unlinkability if the same JWP is presented multiple times, therefore when privacy is required the holder will need to interact with the issuer again to receive new single-use JWPs (dynamically or in batches).

JWS Algorithm The Single Use algorithm is based on using multiple signatures to cover the individual payloads, all of which are generated with the same Asymmetric JSON Web Algorithm (JWA). The internal signing algorithm to use is part of the registration for a new Single Use algorithm identifier. The chosen JWA MUST be an asymmetric signing algorithm so that each signature can be verified without sharing any private values between the parties. This ensures that the verifier cannot brute force any non-disclosed payloads based only on their disclosed individual signatures.

Holder Setup In order to support the protection of a presentation by a holder to a verifier, the holder MUST use a Presentation Key during the issuance and the presentation of every Single Use JWP. This Presentation Key MUST be generated and used for only one JWP. The issuer MUST verify that the holder has possession of this key. The holder-issuer communication to exchange this information is out of scope of this specification, but can be accomplished by the holder using this key to generate a JWS that signs a value the issuer can verify as unique.

Issuer Setup To create a Single Use JWP, the issuer first generates a unique Ephemeral Key using the selected internal algorithm. This key-pair will be used to sign each of the payloads of a single JWP and then discarded.

Signing payloads Each individual payload is signed using the selected internal algorithm using the Ephemeral Key.

Issuer Protected Header The JWK of the issuer's Ephemeral Key MUST be included in the issuer protected header with the property name of proof_jwk and contain only the REQUIRED values to represent the public key. The holder's Presentation Key JWK MUST be included in issuer protected header using the presentation_jwk parameter. The issuer protected header is signed using the given JWA and the issuer's Stable Key.

Payloads Each JWP payload is processed in order and signed using the given JWA using the issuer's Ephemeral Key.

Proof The proof value is an octet string array. The first entry is the octet string of the issuer protected header signature, with an additional entry for each payload signature.

Presentation Protected Header To generate a new presentation, the holder first creates a presentation protected header that is specific to the verifier being presented to. This header MUST contain a parameter that both the holder and verifier trust as being unique and non-replayable. Use of the nonce header parameter is RECOMMENDED for this purpose. This specification registers the nonce header parameter for the presentation protected header that contains a string value either generated by the verifier or derived from values provided by the verifier. When present, the verifier MUST ensure the nonce value matches during verification. The presentation protected header MAY contain other header parameters that are either provided by the verifier or by the holder. These presentation header parameters SHOULD NOT contain values that are common across multiple presentations and SHOULD be unique to a single presentation and verifier. In various examples in this specification, the octet string serialized presentation header is referenced as presentation_header.

Presentation

Editor's Note: The current definition here is incomplete, the holder's signature needs to also incorporate the presented proof.

The holder derives a new proof as part of presentation. The presented proof value will always contain the issuer's Stable Key signature for the issuer protected header as the first element. The second element of the presented proof is the holder's signature of the presentation protected header using the holder's presentation key. This signature is constructed using the same algorithm described in generating the issuer's signature over the issuer protected header. Signing only the presentation header with the Presentation Key is sufficient to protect the entire presentation since that key is private to the holder and only the contents of the presentation header are used for replay prevention. For each payload which is to be disclosed, the corresponding payload signature (from the issued JWP) is included in the proof. If a payload is omitted from the presented JWP, the signature value will NOT be includeed, and the presentation proof will have one less part. For example, if the second and fifth of five payloads are not disclosed, then the holder's derived proof would consist of the issuer's signature over the issuer protected header, the holder's signature over the holder's protected header, the ephemeral key signature over the first, third and fourth payloads. Since the individual signatures in the proof value are unique and remain unchanged across multiple presentations, a Single Use JWP SHOULD only be presented a single time to each verifier in order for the holder to remain unlinkable across multiple presentations.

Verification The verifier MUST verify the issuer protected header octets against the first part in the proof using the issuer's Stable Key. It MUST also verify the presentation protected header octets against the second part in the proof value using the holder's Presentation Key, as provided in the presentation_jwk claim in the issuer protected header. With the headers verified, the issuer's Ephemeral Key as given in the issuer protected header proof_jwk parameter can then be used to verify each of the disclosed payload signatures.

JPA Registration The proposed JWP alg value is of the format "SU-" appended with the relevant JWS alg value for the chosen public and ephemeral key-pair algorithm, for example "SU-ES256".

BBS The BBS Signature Scheme is under active development within the CRFG. This algorithm supports both selective disclosure and unlinkability, enabling the holder to generate multiple presentations from one issued JWP without a verifier being able to correlate those presentations together based on the proof.

JPA Algorithms The BBS alg parameter value in the issuance protected header corresponds to a ciphersuite identifier of BBS_BLS12381G1_XMD:SHA-256_SSWU_RO_. The BBS-PROOF alg parameter value in the presentation protected header corresponds to the same ciphersuite, but used in presentation form.

Key Format The key used for the BBS algorithm is an elliptic curve-based key pair, specifically against the G_2 subgroup of a pairing friendly curve. Additional details on key generation can be found in The JWK form of this key is an OKP type with a curve of BLS12381G2, with x being the base64url-encoded form of the output of point_to_octets_E2. The use of this curve is described in . There is no additional holder key necessary for presentation proofs.

Issuance Issuance is performed using the Sign operation from . This operation utilizes the issuer's BLS12-381 G2 key pair as SK and PK, along with desired protected header and payloads as the octets header and the octets array messages. The octets resulting from this operation form a single octet string in the issuance proof array, to be used along with the protected header and payloads to serialize the JWP.

Issuance Proof Verification Holder verification of the signature on issuance form is performed using the Verify operation from . This operation utilizes the issuer's public key as PK, the proof as signature, the protected header octets as header and the array of payload octets as messages.

Presentation Derivation of a presentation is done by the holder using the ProofGen operation from . This operation utilizes the issuer's public key as PK, the issuer protected header as header, the issuance proof as signature, the issuance payloads as messages, and the holder's presentation protected header as ph. The operation also takes a vector of indexes into messages, describing which payloads the holder wishes to disclose. All payloads are required for proof generation, but only these indicated payloads will be required to be disclosed for later proof verification. The output of this operation is the presentation proof, as a single octet string. Presentation serialization leverages the two protected headers and presentation proof, along with the disclosed payloads. Non-disclosed payloads are represented with the absent value of null in JSON serialization and a zero-length string in compact serialization.

Presentation Verification Verification of a presentation is done by the verifier using the ProofVerify operation from . This operation utilizes the issuer's public key as PK, the issuer protected header as header, the issuance proof as signature, the holder's presentation protected header as ph, and the payloads as disclosed_messages. In addition, the disclosed_indexes scalar array is calculated from the payloads provided. Values disclosed in the presented payloads have a zero-based index in this array, while the indices of absent payloads are omitted.

Message Authentication Code The Message Authentication Code (MAC) JPA uses a MAC to both generate ephemeral keys and compute authentication codes to protect the issuer header and each payload individually. Like the the Single Use algorithm family, it also does not support unlinkability if the same JWP is presented multiple times. and requires an individually issued JWP for each presentation in order to fully protect privacy. When compared to the JWS approach, using a MAC requires less computation but can result in potentially larger presentation proof values. The design is intentionally minimal and only involves using a single standardized MAC method instead of a mix of MAC/hash methods or a custom hash-based construct. It is able to use any published cryptographic MAC method such as HMAC or KMAC. It uses traditional public-key based signatures to verify the authenticity of the issuer and holder.

Holder Setup Prior to the issuer creating a new JWP, it must have presentation binding information provided by the holder. This enables the holder to perform replay prevention while presenting the JWP. The presentation key used by the holder must be transferred to the issuer and verified, likely through a challenge and self-signing mechanism. If the holder requires unlinkability, it must also generate a new key that is verified and bound to each new JWP. How these holder presentation keys are transferred and verified is out of scope of this specification. Protocols such as OpenID Connect can be used to accomplish this. What is required by this definition is that the holder's presentation key MUST be included in the issuer's protected header using the pjwk parameter with a JWK as the value.

Issuer Setup To use the MAC algorithm, the issuer must have a stable public key pair to perform signing. To start the issuance process, a single 32-byte random Shared Secret must first be generated. This value will be shared privately to the holder as part of the issuer's JWP proof value. The Shared Secret is used by both the issuer and holder as the MAC method's key to generate a new set of unique ephemeral keys. These keys are then used as the input to generate a MAC that protects each payload.

Issuer Protected Header The holder's presentation key JWK MUST be included in the issuer protected header using the pjwk parameter. The issuer MUST validate that the holder has possession of this key through a trusted mechanism such as verifying the signature of a unique nonce value from the holder.

Combined MAC Representation The combined MAC representation is a single octet string representing the MAC values of the issuer protected header, along with each payload provided by the issuer. This representation is signed by the issuer, but not shared - parties will recreate this octet string and verify the signature to verify the integrity of supplied issuer protected header and the integrity of any disclosed payloads. The issuer protected header is included in this value as a MAC created using the fixed key "issuer_header" in UTF-8 encoded octets. The value is the issuer header JSON as a UTF-8 encoded octet string. A unique key is generated for each payload using a MAC, with the Shared Secret as the key and a value of "payload_X" as UTF-8 encoded octets, where "X" is replaced by the zero-based array index of the payload, for example "payload_0", "payload_1", etc. Each payload then itself has a corresponding MAC, using the above per-payload key and the payload octet string. The combined MAC representation is the octet string formed by the the concatentation of the issuer protected header MAC output, along with each payload MAC output.

Issuer Proof The issuer proof consists of two octet strings. The first octet string is the issuer signature over the combined MAC representation. The issuer signs the JWS using its stable public key, and a fixed header containing the alg associated with signing algorithm in use. jws_header = '{"alg":"ES256"}' The signature value of the JWS is extracted and base64url-decoded into an octet string. The second octet string is the Shared Secret used to generate the per-payload keys for the combined representation.

Presentation Protected Header See the JWS Presentation Protected Header section.

Presentation Proof

Editor's Note: The current definition here is incomplete, the holder's signature needs to also incorporate the presented proof.

The first value in the presentation proof is the presentation signature. This is a signature over the presentation protected header, using the key specified by the pjwk parameter in the issuer protected header. The second value is the issuer signature over the Combined MAC Representation provided with the issued form. The remaining values are used by the verifier to reconstruct the combined MAC representation without access to the Shared Secret. There is one value corresponding to each payload, whether it has been disclosed or not. If a payload is disclosed, the unique per-payload key derived from the shared secret is used as the payload's entry in the proof array. If a payload is not disclosed, the payload's MAC in the combined MAC representation is used as the payload's entry in the proof array.

Verification of the Presentation Proof The verifier must recreate the Combined MAC Representation from the presentation proof to verify integrity over the disclosed information. The issuer protected header MAC is recreated using the same mechanism described above. For each payload in the presentation:

If the payload is disclosed, then the presentation proof contains the unique per-payload key. The corresponding payload MAC can be computed by performing the MAC operation with this key and the corresponding payload.
If the payload is not disclosed, then the presentation proof contains the payload MAC, which can be used directly

The concatenation of the octets of the issuer protected header MAC and each payload MAC forms the Combined MAC Representation. The issuer signature in the proof is then verified by converting these values to a JWS as described above, and verifying that JWS.

JPA Registration Proposed JWP alg value is of the format "MAC-" appended with a unique identifier for the set of MAC and signing algorithms used. Below are the initial registrations:

MAC-H256 uses HMAC SHA-256 as the MAC and ECDSA using P-256 and SHA-256 for the signatures
MAC-H384 uses HMAC SHA-384 as the MAC and ECDSA using P-384 and SHA-384 for the signatures
MAC-H512 uses HMAC SHA-512 as the MAC and ECDSA using P-521 and SHA-512 for the signatures
MAC-K25519 uses KMAC SHAKE128 as the MAC and EdDSA using Curve25519 for the signatures
MAC-K448 uses KMAC SHAKE256 as the MAC and EdDSA using Curve448 for the signatures
MAC-H256K uses HMAC SHA-256 as the MAC and ECDSA using secp256k1 and SHA-256 for the signatures

Security Considerations

Editor's Note: This will follow once the algorithms defined here have become more stable.

Data minimization of the proof value
Unlinkability of the protected header contents

IANA Considerations The following registration procedure is used for all the registries established by this specification. Values are registered on a Specification Required 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 algorithm: 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 include 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.

JSON Web Proof Algorithms Registry This specification establishes the IANA "JSON Web Proof Algorithms" registry for values of the JWP alg (algorithm) parameter in JWP Header Parameters. The registry records the algorithm name, the algorithm description, the algorithm usage locations, the implementation requirements, the change controller, and a reference to the specification that defines it. The same algorithm name can be registered multiple times, provided that the sets of usage locations are disjoint. It is suggested that the length of the key be included in the algorithm name when multiple variations of algorithms are being registered that use keys of different lengths and the key lengths for each need to be fixed (for instance, because they will be created by key derivation functions). This allows readers of the JSON text to more easily make security decisions. The Designated Experts should perform reasonable due diligence that algorithms being registered either are currently considered cryptographically credible or are being registered as Deprecated or Prohibited. The implementation requirements of an algorithm may be changed over time as the cryptographic landscape evolves, for instance, to change the status of an algorithm to Deprecated or to change the status of an algorithm from Optional to Recommended+ or Required. Changes of implementation requirements are only permitted on a Specification Required basis after review by the Designated Experts, with the new specification defining the revised implementation requirements level.

Registration Template

Algorithm Name: The name requested (e.g., "SU-ES256"). This name is a case-sensitive ASCII string. Names may not match other registered names in a case-insensitive manner unless the Designated Experts state that there is a compelling reason to allow an exception.
Algorithm Description: Brief description of the algorithm (e.g., "Single-Use JWP using ES256").
Algorithm Usage Location(s): The algorithm usage locations, which should be one or more of the values Issued or Presented. Other values may be used with the approval of a Designated Expert.
JWP Implementation Requirements: The algorithm implementation requirements for JWP, which must be one the words Required, Recommended, Optional, Deprecated, or Prohibited. Optionally, the word can be followed by a "+" or "-". The use of "+" indicates that the requirement strength is likely to be increased in a future version of the specification. The use of "-" indicates that the requirement strength is likely to be decreased in a future version of the specification. Any identifiers registered for non-authenticated encryption algorithms or other algorithms that are otherwise unsuitable for direct use as JWP algorithms must be registered as "Prohibited".
Change Controller: For Standards Track RFCs, list the "IETF". For others, give the name of the responsible party. Other details (e.g., postal address, email address, home page URI) may also be included.
Specification Document(s): Reference to the document or documents that specify the parameter, preferably including URIs that can be used to retrieve copies of the documents. An indication of the relevant sections may also be included but is not required.
Algorithm Analysis Documents(s): References to a publication or publications in well-known cryptographic conferences, by national standards bodies, or by other authoritative sources analyzing the cryptographic soundness of the algorithm to be registered. The Designated Experts may require convincing evidence of the cryptographic soundness of a new algorithm to be provided with the registration request unless the algorithm is being registered as Deprecated or Prohibited. Having gone through working group and IETF review, the initial registrations made by this document are exempt from the need to provide this information.

Initial Registry Contents

Algorithm Name: SU-ES256
Algorithm Description: Single-Use JWP using ES256
Algorithm Usage Location(s): Issued, Presented
JWP Implementation Requirements: Recommended
Change Controller: IETF
Specification Document(s): of this specification
Algorithm Analysis Documents(s): n/a
Algorithm Name: SU-ES384
Algorithm Description: Single-Use JWP using ES384
Algorithm Usage Location(s): Issued, Presented
JWP Implementation Requirements: Optional
Change Controller: IETF
Specification Document(s): of this specification
Algorithm Analysis Documents(s): n/a
Algorithm Name: SU-ES512
Algorithm Description: Single-Use JWP using ES512
Algorithm Usage Location(s): Issued, Presented
JWP Implementation Requirements: Optional
Change Controller: IETF
Specification Document(s): of this specification
Algorithm Analysis Documents(s): n/a
Algorithm Name: BBS
Algorithm Description: Corresponds to a ciphersuite identifier of BBS_BLS12381G1_XMD:SHA-256_SSWU_RO_H2G_HM2S_
Algorithm Usage Location(s): Issued
JWP Implementation Requirements: Required
Change Controller: IETF
Specification Document(s): of this specification
Algorithm Analysis Documents(s): n/a
Algorithm Name: BBS-PROOF
Algorithm Description: Corresponds to a ciphersuite identifier of BBS_BLS12381G1_XMD:SHA-256_SSWU_RO_H2G_HM2S_
Algorithm Usage Location(s): Presented
JWP Implementation Requirements: Required
Change Controller: IETF
Specification Document(s): of this specification
Algorithm Analysis Documents(s): n/a
Algorithm Name: MAC-H256
Algorithm Description: MAC-H256 uses HMAC SHA-256 as the MAC and ECDSA using P-256 and SHA-256 for the signatures
Algorithm Usage Location(s): Issued, Presented
JWP Implementation Requirements: Optional
Change Controller: IETF
Specification Document(s): of this specification
Algorithm Analysis Documents(s): n/a
Algorithm Name: MAC-H384
Algorithm Description: MAC-H384 uses HMAC SHA-384 as the MAC and ECDSA using P-384 and SHA-384 for the signatures
Algorithm Usage Location(s): Issued, Presented
JWP Implementation Requirements: Optional
Change Controller: IETF
Specification Document(s): of this specification
Algorithm Analysis Documents(s): n/a
Algorithm Name: MAC-H512
Algorithm Description: MAC-H512 uses HMAC SHA-512 as the MAC and ECDSA using P-521 and SHA-512 for the signatures
Algorithm Usage Location(s): Issued, Presented
JWP Implementation Requirements: Optional
Change Controller: IETF
Specification Document(s): of this specification
Algorithm Analysis Documents(s): n/a
Algorithm Name: MAC-K25519
Algorithm Description: MAC-K25519 uses KMAC SHAKE128 as the MAC and EdDSA using Curve25519 for the signatures
Algorithm Usage Location(s): Issued, Presented
JWP Implementation Requirements: Optional
Change Controller: IETF
Specification Document(s): of this specification
Algorithm Analysis Documents(s): n/a
Algorithm Name: MAC-K448
Algorithm Description: MAC-K448 uses KMAC SHAKE256 as the MAC and EdDSA using Curve448 for the signatures
Algorithm Usage Location(s): Issued, Presented
JWP Implementation Requirements: Optional
Change Controller: IETF
Specification Document(s): of this specification
Algorithm Analysis Documents(s): n/a
Algorithm Name: MAC-H256K
Algorithm Description: MAC-H256K uses HMAC SHA-256 as the MAC and ECDSA using secp256k1 and SHA-256 for the signatures
Algorithm Usage Location(s): Issued, Presented
JWP Implementation Requirements: Optional
Change Controller: IETF
Specification Document(s): of this specification
Algorithm Analysis Documents(s): n/a

JSON Web Proof Header Parameters Registration This section registers the following JWP Header Parameters in the IANA "JSON Web Proof Header Parameters" registry established by .

Registry Contents

Proof JWK Header Parameter

Header Parameter Name: proof_jwk
Header Parameter Description: Issuer's Ephemeral Key
Header Parameter Usage Location(s): Issued
Change Controller: IETF
Specification Document(s): of this specification

Presentation JWK Header Parameter

Header Parameter Name: presentation_jwk
Header Parameter Description: Holder's Presentation Key
Header Parameter Usage Location(s): Issued
Change Controller: IETF
Specification Document(s): of this specification

References Normative References Informative References Verifiable Credentials Data Model 2.0 Digital Bazaar OpenLink Software W3C Invited Expert Block

Example JWPs The following examples use algorithms defined in JSON Proof Algorithms and also contain the keys used, so that implementations can validate these samples.

Example Single-Use JWP This example uses the Single-Use Algorithm as defined in JSON Proof Algorithms to create a JSON Proof Token. It demonstrates how to apply selective disclosure using an array of traditional JWS-based signatures. Unlinkability is only achieved by using each JWP one time, as multiple uses are inherently linkable via the traditional ECDSA signature embedded in the proof. To begin, we need two asymmetric keys for Single Use: one that represents the JPT Issuer's stable key and the other is an ephemeral key generated by the Issuer just for this JWP. This is the Issuer's stable private key used in this example in the JWK format:

Issuer Private Key (es256) This is the ephemeral private key used in this example in the JWK format:

Issuer Ephemeral Private Key (es256) This is the Holder's presentation private key used in this example in the JWK format:

Holder Presentation Private Key The JWP Protected Header declares that the data structure is a JPT and the JWP Proof Input is secured using the Single-Use ECDSA algorithm with the P-256 curve and SHA-256 digest. It also includes the ephemeral public key, the Holder's presentation public key and list of claims used for this JPT.

Issuer Protected header (es256) The JWP Protected Header JSON is serialized (without the above whitespace added for readability) and uses UTF-8 encoding to convert into an octet string. This gives:

Encoded Issuer Protected Header (es256, base64url-encoded) The Single Use algorithm utilizes multiple individual JWS Signatures. Each signature value is generated by creating a JWS with a single Protected Header with the associated alg value. In this example, the fixed header used for each JWS is the serialized JSON Object {"alg":"ES256"}. This protected header will be used to generate a signature over each corresponding payload in the JWP. The corresponding octet value in the proof is the octet string (base64url-decoded) value of the signature. The final proof value from the Issuer is an array with the octets of the header signature, followed by entries for each payload signature. The resulting JSON serialized JPT using the above examples is:

Issued JWP in JSON Serialization (es256) The compact serialization of the same JPT is:

Issued JWP in Compact Serialization (es256) To present this JPT, we first use the following presentation header with a nonce (provided by the Verifier):

Presentation Header This header is serialized without whitespace and UTF-8 encoded into an octet string. This gives:

Presentation Header (base64url-encoded) When signed with the holder's presentation key, the resulting signature are:

Figure: Holder Proof-of-Possession (base64url-encoded)

Then by applying selective disclosure of only the given name and age claims (family name and email hidden), we get the following presented JPT:

Figure: Final Presentation in JSON Serialization

And also in compact serialization:

Figure: Final Presentation in Compact Serialization

Example BBS JWP The following example uses the BBS algorithm. This is the Issuer's stable private key in the JWK format:

BBS private key in JWK format There is no additional holder key necessary for presentation proofs. For the following protected header and array of payloads:

Example issuer protected header

Example issuer payloads (as members of a JSON array) These components are signed using the private issuer key previously given, which is then representable in the following serializations:

Issued JWP (JSON serialization)

Issued JWP (compact serialization) For a presentation with the following presentation header:

Holder Presentation Header The holder decides to share all information other than the email address, and generates a proof. That proof is represented in the following serializations:

Presentation JWP (JSON serialization)

Presentation JWP (compact serialization)

Example MAC JWP The following example uses the MAC-H256 algorithm. This is the Issuer's stable private key in the JWK format:

Issuer private key This is the Issuer's ephemerally generated shared secret:

Shared Secret This is the Holder's presentation private key in the JWK format:

Holder private key For the following protected header and array of payloads:

Example issuer protected header

Example issuer payloads (as members of a JSON array) The first MAC is generated using the key issuer_header and a value of the issuer protected header as a UTF-8 encoded octet string. This results in the following MAC:

Issuer MAC of protected header (base64url-encoded) The issuer generates an array of derived keys with one for each payload by using the shared secret as the key, and the index of the payload (as payload_{n} in UTF-8 encoded octets) as the input in a HMAC operation. This results in the following set of derived keys:

Derived payload keys (base64url-encoded) A MAC is generated for each payload using the corresponding derived payload key. This results in the following set of MAC values:

Payload MAC values (base64url-encoded) The issuer protected header MAC and the payload MAC octet strings are concatenated into a single value known as the combined MAC representation. This representation is signed with the issuer's private key. The proof consists of two octet string values: the signature over the combined MAC representation, and the shared secret.

Issued Proof (base64url-encoded) The final issued JWP in JSON serialization is:

Issued JWP (in JSON serialization) The same JWP in compact serialization:

Issued JWP (in compact serialization) Next, we show the presentation of the JWP with selective disclosure. For presentation with the following presentation protected header:

Presentation Protected Header The holder will take the issuer proof (including shared secret) and derive the same individual payload MAC values (above). In this case, the holder has decided not to disclose the last three claims provided by the issuer (corresponding to email, address, and age_over_21) For the disclosed payloads, the holder will provide the corresponding derived key. For the non-disclosed payloads, the holder will provide the corresponding MAC value. The final presented proof value is an array of octet strings. The contents are presentation header signature, followed by the issuer signature, then the value disclosed by the holder for each payload. This results in the following proof:

Presentation proof (base64url-encoded) The final presented JWP in JSON serialization is:

Presented JWP (in JSON serialization) The same JWP in compact serialization:

Presented JWP (in compact serialization)

Acknowledgements This work was incubated in the DIF Applied Cryptography Working Group. We would like to thank Alberto Solavagione for his valuable contributions to this specification. The BBS examples were generated using the library at https://github.com/mattrglobal/pairing_crypto .

Document History [[ To be removed from the final specification ]] -05

Update of appendix describing MAC-H256 to now also be generated by the build system from a common set of code and templates.
Update single use algorithm to use an array of octet values rather than requiring splitting an octet buffer into parts during generation of a presentation and during verification.
Update BBS algorithm description and examples to clarify the proof is an array with a single octet string.
Update MAC algorithm to use an array of octet values for the proof, rather than requiring splitting an octet buffer into parts.
Add new section on the Combined MAC Representation to clarify operations are serving to recreate this octet string value.
Correct reference to the latest BBS draft.
SU and MAC families now use raw JWA rather than JWS and synthesized headers
Change algorithms to not use base64url-encoding internally. Algorithms are meant to operate on octets, while base64url-encoding is used to represent those octets in JSON and compact serializations.

-04

Refactoring figures and examples to be built from a common set across all three documents
Move single-use example appendix from JWP to JPA
Change algorithm from BBS-DRAFT-5 to BBS, and from BBS-PROOF-DRAFT-5 to BBS-PROOF
Update BBS ciphersuite ID to BBS_BLS12381G1_XMD:SHA-256_SSWU_RO_
Update to draft 5 BLS key representations

-03

Improvements resulting from a full proofreading.
Populated IANA Considerations section.
Updated to use BBS draft -05.
Updated examples.

-02

Add new BBS-DRAFT-3 and BBS-PROOF-DRAFT-3 algorithms based on draft-irtf-cfrg-bbs-signatures-03.
Remove prior BBS-X algorithm based on a particular implementation of earlier drafts.

-01

Correct cross-references within group
Describe issuer_header and presentation_header
Update BBS references to CFRG drafts
Rework reference to HMAC ( RFC2104 )
Remove ZKSnark placeholder

-00

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