Credentials are a part of our daily lives; driver's licenses are used to assert that we are capable of operating a motor vehicle, university degrees can be used to assert our level of education, and government-issued passports enable us to travel between countries. This specification provides a mechanism to express these sorts of credentials on the Web in a way that is cryptographically secure, privacy respecting, and machine verifiable.

Comments regarding this document are welcome. Please file issues directly on GitHub, or send them to public-vc-comments@w3.org (subscribe, archives).

Introduction

Credentials are a part of our daily lives; driver's licenses are used to assert that we are capable of operating a motor vehicle, university degrees can be used to assert our level of education, and government-issued passports enable us to travel between countries. These credentials provide benefits to us when used in the physical world, but their use on the Web continues to be elusive.

Currently it is difficult to express education qualifications, healthcare data, financial account details, and other sorts of third-party verified machine-readable personal information on the Web. The difficulty of expressing digital credentials on the Web makes it challenging to receive the same benefits through the Web that physical credentials provide us in the physical world.

This specification provides a standard way to express credentials on the Web in a way that is cryptographically secure, privacy respecting, and machine verifiable.

For those unfamiliar with the concepts related to verifiable credentials, the following sections provide an overview of:

What is a Verifiable Credential?

In the physical world, a credential might consist of:

A verifiable credential can represent all of the same information that a physical credential represents. The addition of technologies, such as digital signatures, makes verifiable credentials more tamper-evident and more trustworthy than their physical counterparts. Holders of verifiable credentials can generate presentations and then share these presentations with verifiers to prove they possess verifiable credentials with certain characteristics. Both credentials and presentations can be transmitted rapidly, making them more convenient than their physical counterparts when trying to establish trust at a distance.

While this specification attempts to improve the ease of expressing digital credentials, it also attempts to balance this goal with a number of privacy-preserving goals. The persistence of digital information, and the ease with which disparate sources of digital data can be collected and correlated, comprise a privacy concern that the use of verifiable and easily machine-readable credentials threatens to make worse. This document outlines and attempts to address a number of these issues in Section . Examples of how to use this data model using privacy-enhancing technologies, such as zero-knowledge proofs, are also provided throughout this document.

Ecosystem Overview

This section describes the roles of the core actors and the relationships between them in an ecosystem where verifiable credentials are expected to be useful. A role is an abstraction that might be implemented in many different ways. The separation of roles suggests likely interfaces and protocols for standardization. The following roles are introduced in this specification:

holder
A role an entity might perform by possessing one or more verifiable credentials and generating presentations from them. Example holders include students, employees, and customers.
issuer
A role an entity might perform by creating a verifiable credential, associating it with a specific subject, and transmitting it to a holder. Example issuers include corporations, non-profit organizations, trade associations, governments, and individuals.
subject
A role an entity might perform by having one or more verifiable credentials asserted about it. Example subjects include human beings, animals, and things. Whilst in many cases the holder of a verifiable credential will be the subject, in certain cases it will not be. For example, a parent (the holder) may hold the verifiable credentials of a child (the subject), or a pet lover (the holder) may hold the verifiable credentials of its pet (the subject). The Subject-Holder Relationships section in the Advanced Concepts deals with these special cases.
verifier
A role an entity might perform by requesting and receiving a verifiable presentation that proves the holder possesses the required verifiable credentials with certain characteristics. Example verifiers include employers, security personnel, and websites.
verifiable data registry
A role a system might perform by mediating the creation and verification of identifiers, keys, and other relevant data, such as verifiable credential schemas and revocation registries, which might be required to use verifiable credentials. Some configurations might require correlatable identifiers for subjects. Example verifiable data registries include trusted databases, decentralized databases, government ID databases, and distributed ledgers. Often there is more than one type of verifiable data registry utilized in an ecosystem.
The roles and information flows forming the basis for this specification.

The ecosystem above is provided as an example to ground the rest of the concepts in this specification. Other ecosystems exist, such as protected environments or proprietary systems, where verifiable credentials also provide benefit.

Use Cases and Requirements

The Verifiable Credentials Use Cases [[VC-USECASES]] document outlines a number of key topics that readers might find useful, including:

As a result of documenting and analyzing the use cases document, a number of desirable ecosystem characteristics were identified for this specification, specifically:

  1. Verifiable Credentials represent statements made by an Issuer in a tamper evident and privacy respecting manner.
  2. Holders assemble collections of credentials from different issuers into a single artifact, a Verifiable Presentation.
  3. Issuers can issue Verifiable Credentials about any subject .
  4. Acting as Issuer, Holder, or Verifier requires neither registration nor approval by any authority.
  5. Verifiable Presentations allow any Verifier to verify the authenticity of credentials from any Issuer.
  6. Holders can receive Verifiable Credentials from anyone.
  7. Holders can interact with any Issuer and any Verifier through any User Agent.
  8. Holders can store Verifiable Credentials in any location, without affecting their verifiability and without the Issuer knowing anything about where they are stored or when they are accessed.
  9. Holders can present Verifiable Presentations to any Verifier without affecting authenticity of the claims and without revealing that action to the Issuer.
  10. A Verifier can verify Verifiable Presentations from any Holder, containing proofs of claims from any Issuer.
  11. Verification shouldn't depend on direct interactions between Issuers and Verifiers .
  12. Verification shouldn't reveal the identity of the Verifier to any Issuer.
  13. The specification must provide a means for Issuers to issue Verifiable Credentials that support selective disclosure, without requiring all conformant software to support that feature.
  14. Issuers can issue Verifiable Credentials which support selective disclosure.
  15. If a single Verifiable Credential supports selective disclosure, then Holders may present proofs of claims without revealing the entire Verifiable Credential.
  16. Presentations can either disclose the attributes of a credential, or satisfy derived predicates requested by the verifier. Derived predicates are boolean conditions like greater than, less than, equal to, is in set, and so on.
  17. Issuers can issue Verifiable Credentials that are revocable.
  18. Verifiable Credentials and Verifiable Presentations must be serializable in one or more interoperable machine-readable data formats.
  19. The data model and serialization must be extendable with minimal coordination.
  20. Revocation shouldn't reveal to the Issuer any identifying information about the Subject, the Holder, the specific Verifiable Credential, nor the Verifier.
  21. Issuers can disclose the reason for revocation.
  22. Issuers revoking Verifiable Credentials should distinguish between revocation for cryptographic integrity, e.g., the signing key has been compromised, and status change, e.g., the driver’s license is suspended.
  23. Issuers can provide a service for refreshing a Verifiable Credential.

There are other requirements listed in the Verifiable Credentials Use Cases document that may or may not be aligned with the requirements listed above. The VCWG will be ensuring alignment of the list of requirements from both documents over time and will most likely move the list of requirements to a single document.

Terminology

Core Data Model

The following sections outline core data model concepts, such as claims, credentials, and presentations, that form the foundation of this specification.

Claims

A claim is a statement about a subject. A subject is an entity about which claims can be made. Claims are expressed using subject-property-value relationships.

The basic structure of a claim.

The data model for claims described above is powerful and can be used to express a large variety of statements. For example, whether or not someone graduated from a particular university can be expressed as shown below.

A basic claim expressing that Pat is an alumni of "Example University".

Individual claims can be merged together to express a graph of information about a subject. The example below extends the previous claim by adding the claims that Pat knows Sam and that Sam is employed as a professor.

Multiple claims can be combined to express a graph of information.

To this point, the concepts of a claim and a graph of information are introduced. To be able to trust claims, more information must be added.

Credentials

A credential is a set of one or more claims made by the same entity. Credentials might also include an identifier and metadata to describe properties of the credential, such as the issuer, the expiry date and time, a representative image, a public key to use for verification purposes, the revocation mechanism, and so on. The metadata might be signed by the issuer. A verifiable credential is a set of tamper-evident claims and metadata that cryptographically prove who issued it.

Basic components of a verifiable credential.

Examples of verifiable credentials include digital employee identification cards, digital birth certificates, and digital educational certificates.

Credential identifiers are often used to identify specific instances of a credential. These identifiers can also be used for correlation. A holder wanting to minimize correlation is advised to use a selective disclosure scheme that does not reveal the credential identifier.

above shows the basic components of a verifiable credential, but abstracts the details about how claims are organized into information graphs, which are then organized into verifiable credentials. below shows a more complete depiction of a verifiable credential, which is normally composed of at least two information graphs. The first graph expresses the credential itself, which contains credential metadata and claims. The second graph expresses the digital proof, which is usually a digital signature.

Information graphs associated with a basic verifiable credential.

It is possible to have a credential, such as a marriage certificate, containing multiple claims about different subjects that are not required to be related.

Presentations

Enhancing privacy is a key design feature of this specification. Therefore, it is important for entities using this technology to be able to express only the portions of their persona that are appropriate for a given situation. The expression of a subset of one's persona is called a verifiable presentation. Examples of different personas include a person's professional persona, their online gaming persona, their family persona, or an incognito persona.

A verifiable presentation expresses data from one or more verifiable credentials, and is packaged in such a way that the authorship of the data is verifiable. If credentials are directly presented, they become a presentation. Data formats derived from credentials that are cryptographically verifiable but do not of themselves contain credentials, might also be presentations.

The data in a presentation is often about the same subject, but was issued by multiple issuers. The aggregation of this information typically expresses an aspect of a person, organization, or entity.

Basic components of a verifiable presentation.

above shows the components of a verifiable presentation, but abstracts the details about how verifiable credentials are organized into information graphs, which are then organized into verifiable presentations. below shows a more complete depiction of a verifiable presentation, which is normally composed of at least four information graphs. The first graph expresses the presentation itself, which contains presentation metadata. The verifiablePresentation property in the graph refers to one or more verifiable credentials (each a self-contained graph), which in turn contains credential metadata and claims. The third graph expresses the credential graph proof, which is usually a digital signature. The fourth graph expresses the presentation graph proof, which is usually a digital signature.

Information graphs associated with a basic verifiable presentation.

It is possible to have a presentation, such as a business persona, that draws on multiple credentials about different subjects that are often, but not required to be, related.

Lifecycle

This section elaborates on the lifecycle of credentials and presentations in the ecosystem. The lifecycle of credentials will take a common path: issuance of one or more credentials, storage of credentials, and then the composition of credentials into a presentation for verifiers.

Basic

We will illustrate the basic lifecycle with our first scenario, redeeming an alumni discount at an university. In Example 1 Pat receives an "AlumniCredential" from "Example University" that will be stored in Pat's mobile device's digital wallet, a type of credential repository.

  {
    "@context": [
      "https://w3.org/2018/credentials/v1",
      "https://example.com/examples/v1"
    ],
    "id": "http://example.edu/credentials/1872",
    "type": ["VerifiableCredential", "AlumniCredential"],
    "issuer": "https://example.edu/issuers/565049",,
    "issuanceDate": "2010-01-01T19:73:24Z",
    "credentialSubject": {
      "id": "did:example:ebfeb1f712ebc6f1c276e12ec21",
      "alumniOf": "Example University"
    },
    "proof": {
      "type": "RsaSignature2018",
      "created": "2017-06-18T21:19:10Z",
      "creator": "https://example.com/jdoe/keys/1",
      "nonce": "c0ae1c8e-c7e7-469f-b252-86e6a0e7387e",
      "signatureValue": "BavEll0/I1zpYw8XNi1bgVg/sCneO4Jugez8RwDg/+
        MCRVpjOboDoe4SxxKjkCOvKiCHGDvc4krqi6Z1n0UfqzxGfmatCuFibcC1wps
        PRdW+gGsutPTLzvueMWmFhwYmfIFpbBu95t501+rSLHIEuujM/+PXr9Cky6Ed
        +W3JT24="
    }
  }
        

In the final chapter of this lifecycle, Pat's "AlumniCredential" will be presented for a discount. The verifier, a ticket sales counter, states that any alumni of "Example University" will receive a 72% discount on football season tickets. Using a mobile device, Pat selects the Verifiable Credential received earlier that will then be composed into a presentation. That presentation is then sent to the verifier in the form of a Verifiable Presentation in order to receive the discount. We can find that presentation in Example 2.

  {
    "@context": [
      "https://w3.org/2018/credentials/v1",
      "https://example.com/examples/v1"
    ],
    "id": "did:example:Fg15hCqJ19LP4X4RaiumJVb8YeZnzUjWLh74hKqBvk5B",
    "type": "VerifiablePresentation",
    "verifiableCredential": [{
      "id": "http://example.edu/credentials/1872",
      "type": ["VerifiableCredential", "AlumniCredential"],
      "issuer": "https://example.edu/issuers/565049",,
      "issuanceDate": "2010-01-01T19:73:24Z",
      "credentialSubject": {
        "id": "did:example:ebfeb1f712ebc6f1c276e12ec21",
        "alumniOf": "Example University"
      },
      "proof": {
        "type": "RsaSignature2018",
        "created": "2017-06-18T21:19:10Z",
        "creator": "https://example.com/jdoe/keys/1",
        "nonce": "c0ae1c8e-c7e7-469f-b252-86e6a0e7387e",
        "signatureValue": "BavEll0/I1zpYw8XNi1bgVg/sCneO4Jugez8RwDg/+
        MCRVpjOboDoe4SxxKjkCOvKiCHGDvc4krqi6Z1n0UfqzxGfmatCuFibcC1wps
        PRdW+gGsutPTLzvueMWmFhwYmfIFpbBu95t501+rSLHIEuujM/+PXr9Cky6Ed
        +W3JT24="
      }
    }],
    "proof": { ... }
  }
        

Trust Model

The verifiable credentials trust model is as follows:

This trust model differentiates itself from other trust models by ensuring the:

By decoupling the trust between the identity provider and the relying party a more flexible and dynamic trust model is created such that market competition and customer choice is increased.

Readers that would like to learn more about how this trust model interacts with various threat models studied by the Working Group are urged to read the Verifiable Credentials Use Cases Document [[VC-USECASES]].

The data model detailed in this specification does not imply a transitive trust model, such as that provided by more traditional Certificate Authority trust models. In the Verifiable Credentials Data Model, a verifier either directly trusts or does not trust an issuer. While it is possible to build transitive trust models using the Verifiable Credentials Data Model, implementers are urged to learn about the security weaknesses introduced by broadly delegating trust in the manner adopted by Certificate Authority systems.

A conforming document is any concrete expression of the data model that complies with the normative statements in this specification. Specifically, all relevant normative statements in , , and of this document MUST be enforced.

A conforming processor is any algorithm realized as software and/or hardware that generates or consumes a conforming document as described by this specification and also enforces all relevant normative statements in the section of this document when performing verification.

This specification makes no normative statements with regard to the conformance of roles in the ecosystem such as issuers, holders, or verifiers because the conformance of ecosystem roles are highly application, use case, and market vertical specific.

Basic Concepts

This section introduces some basic concepts for the specification, in preparation for Section later in the document.

Contexts

When two software systems need to exchange data, they must use terminology and a protocol that both systems understand. As an analogy, consider how two people communicate. Both people must use the same language and the words they use must mean the same thing to each other. This specification uses the @context property to express the context of a conversation.

@context
The value of the @context property MUST be one or more URIs, where the value of the first URI is https://www.w3.org/2018/credentials/v1. If more than one URI is provided, the URIs MUST be interpreted as an ordered set. It is RECOMMENDED that dereferencing the URIs results in a document containing machine-readable information about the context.
{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://example.com/examples/v1"
  ],
  "id": "http://example.edu/credentials/58473",
  "type": ["VerifiableCredential", "AlumniCredential"],
  "credentialSubject": {
    "id": "did:example:ebfeb1f712ebc6f1c276e12ec21",
    "alumniOf": "Example University"
  },
  "proof": { ... }
}
        

The example above uses the base context URI (https://www.w3.org/2018/credentials/v1) to establish that the conversation is about a verifiable credential. The second URI (https://example.com/examples/v1) establishes that the conversation is about examples.

This document uses the examples context URI (https://example.com/examples/v1) for the purposes of demonstrating examples. The examples context URI MUST NOT be used for any other purpose, such as in pilot or production systems.

The data available at https://www.w3.org/2018/credentials/v1 is a static document that is never updated and SHOULD be downloaded and cached. The associated human-readable vocabulary document for the Verifiable Credentials Data Model is available at https://www.w3.org/2018/credentials. This concept is further expanded on in Section .

Identifiers

When expressing statements about a specific thing, such as a person, product, or organization, it is often useful to use some kind of identifier so that others can express statements about the same thing. Identifiers that others are expected to use when expressing statements about a specific thing MUST be expressed using the id property.

Developers should remember that identifiers might be harmful in scenarios where pseudonymity is required. Developers are encouraged to read Section carefully when considering such scenarios. Where privacy is a strong consideration, the id property MAY be omitted.

id
The value of the id property MUST be a single URI. It is RECOMMENDED that dereferencing the URI results in a document containing machine-readable information about the identifier.
{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://example.com/examples/v1"
  ],
  "id": "http://example.edu/credentials/3732",
  "type": ["VerifiableCredential", "UniversityDegreeCredential"],
  "credentialSubject": {
    "id": "did:example:ebfeb1f712ebc6f1c276e12ec21",
    "degree": {
      "type": "BachelorDegree",
      "name": "Bachelor of Science in Mechanical Engineering"
    }
  },
  "proof": { ... }
}
        

The example above uses two types of identifiers. The first identifier is for the credential and uses an HTTP-based URL. The second identifier is for the subject of the credential (the thing the claims are about) and uses a decentralized identifier, also known as a DID.

As of this publication, DIDs are a new type of identifier that are not necessary for verifiable credentials to be useful. Specifically, verifiable credentials do not depend on DIDs and DIDs do not depend on verifiable credentials. However, it is expected that many verifiable credentials will use DIDs and that software libraries implementing this specification will probably need to resolve DIDs. DID-based URLs are used for expressing identifiers associated with subjects, issuers, holders, credential status lists, cryptographic keys, and other machine-readable information that is associated with a verifiable credential.

Types

Software systems that process the kinds of objects specified in this document use type information to determine whether or not a provided credential or presentation is appropriate. Type information SHOULD be expressed using the type property.

type
The value of the type property MUST be, or map to, one or more URIs. If more than one URI is provided, the URIs MUST be interpreted as an unordered set. Syntactic conveniences, such as JSON-LD terms, SHOULD be used to ease developer usage. It is RECOMMENDED that dereferencing the URIs results in a document containing machine-readable information about the type.
{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://example.com/examples/v1"
  ],
  "id": "http://example.edu/credentials/3732",
  "type": ["VerifiableCredential", "UniversityDegreeCredential"],
  "credentialSubject": {
    "id": "did:example:ebfeb1f712ebc6f1c276e12ec21",
    "degree": {
      "type": "BachelorDegree",
      "name": "Bachelor of Science in Mechanical Engineering"
    }
  },
  "proof": { ... }
}
        

With respect to this specification, the following table lists the objects that MUST have a type specified.

Object Type
Credential object VerifiableCredential and a more specific credential type. For example,
"type": ["VerifiableCredential", "UniversityDegreeCredential"]
Presentation object VerifiablePresentation and a more specific presentation type. For example,
"type": ["VerifiablePresentation", "CredentialManagerPresentation"]
credentialStatus object A valid credential status type. For example,
"type": "CredentialStatusList2017"
termsOfUse object A valid terms of use type. For example,
"type": "OdrlPolicy2017")
evidence object A valid evidence type. For example,
"type": "DocumentVerification2018"

All credentials, presentations, and encapsulated objects MUST specify, or be associated with, additional more narrow types (like UniversityDegreeCredential, for example) so software systems can process this additional information.

When processing encapsulated objects defined in this specification, (for example, objects associated with the credentialSubject object or deeply nested therein), software systems SHOULD use the type information specified in encapsulating objects higher in the hierarchy. Specifically, an encapsulating object, such as credential, SHOULD convey the associated object types so that verifiers can quickly determine the contents of an associated object based on the encapsulating object type.

For example, a credential object with the type of UniversityDegreeCredential, signals to a verifier that the object associated with the credentialSubject property contains the identifier for the:

This enables implementers to rely on values associated with the type property for verification purposes. The expectation of types and their associated properties should be documented in at least a human-readable specification, and preferably, in an additional machine-readable representation.

The type system for the Verifiable Credentials Data Model is the same as for [[JSON-LD]] and is detailed in Section 5.4: Specifying the Type and Section 8: JSON-LD Grammar. When using a JSON-LD context (see Section ), this specification aliases the @type keyword to type to make the JSON-LD documents more easily understood. While application developers and document authors do not need to understand the specifics of the JSON-LD type system, implementers of this specification who want to support interoperable extensibility, do.

Issuer

Issuer information can be expressed using the following properties:

issuer
The value of the issuer property MUST be a URI. It is RECOMMENDED that dereferencing the URI results in a document containing machine-readable information about the issuer that can be used to verify the information expressed in the credential.
issuanceDate
The value of the issuanceDate property MUST be a string value of an [[!ISO8601]] combined date and time string representing the date and time the credential was issued. Note that this date represents the earliest date when the information associated with the credentialSubject property became valid.
{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://example.com/examples/v1"
  ],
  "id": "http://example.edu/credentials/3732",
  "type": ["VerifiableCredential", "UniversityDegreeCredential"],
  "issuer": "https://example.edu/issuers/14",
  "issuanceDate": "2010-01-01T19:23:24Z",
  "credentialSubject": {
    "id": "did:example:ebfeb1f712ebc6f1c276e12ec21",
    "degree": {
      "type": "BachelorDegree",
      "name": "Bachelor of Science in Mechanical Engineering"
    }
  },
  "proof": { ... }
}
        

Proofs (Signatures)

For a credential or presentation to be verifiable, the following property MUST be present:

proof
No single proof mechanism is currently standardised or recommended for verifiable credentials. The method used for a proof MUST be included using the type property. Because the method used for a mathematical proof varies by representation language and the technology used, the set of name-value pairs that is expected as the value of the proof property will vary accordingly. For example, if digital signatures are used for the proof mechanism, the proof property is expected to have name-value pairs that include a signature, a reference to the signing entity, and a representation of the signing date. The example below uses RSA digital signatures.
{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://example.com/examples/v1"
  ],
  "id": "http://example.gov/credentials/3732",
  "type": ["VerifiableCredential", "UniversityDegreeCredential"],
  "issuer": "https://example.edu",
  "issuanceDate": "2010-01-01",
  "credentialSubject": {
    "id": "did:example:ebfeb1f712ebc6f1c276e12ec21",
    "degree": {
      "type": "BachelorDegree",
      "name": "Bachelor of Science in Mechanical Engineering"
    }
  },
  "proof": {
    "type": "RsaSignature2018",
    "created": "2018-06-18T21:19:10Z",
    "verificationMethod": "https://example.com/jdoe/keys/1",
    "nonce": "c0ae1c8e-c7e7-469f-b252-86e6a0e7387e",
    "signatureValue": "BavEll0/I1zpYw8XNi1bgVg/sCneO4Jugez8RwDg/+
      MCRVpjOboDoe4SxxKjkCOvKiCHGDvc4krqi6Z1n0UfqzxGfmatCuFibcC1wps
      PRdW+gGsutPTLzvueMWmFhwYmfIFpbBu95t501+rSLHIEuujM/+PXr9Cky6Ed
      +W3JT24="
  }
}
        

The group is currently discussing various alignments with the JOSE stack, specifically JWS and JWK.

Expiration

Expiration information for a credential MAY be provided by adding the following property:

expirationDate
If specified, the value of the expirationDate property MUST be a string value of an [[!ISO8601]] combined date and time string representing the date and time the credential ceases to be valid.
{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://example.com/examples/v1"
  ],
  "id": "http://example.edu/credentials/3732",
  "type": ["VerifiableCredential", "UniversityDegreeCredential"],
  "issuer": "https://example.edu/issuers/14",
  "issuanceDate": "2010-01-01T19:23:24Z",
  "expirationDate": "2020-01-01T19:23:24Z",
  "credentialSubject": {
    "id": "did:example:ebfeb1f712ebc6f1c276e12ec21",
    "degree": {
      "type": "BachelorDegree",
      "name": "Bachelor of Science in Mechanical Engineering"
    }
  },
  "proof": { ... }
}
        

Status

Information about the current status of a verifiable credential, such as whether it is suspended or revoked, MAY be provided using the credentialStatus property.

credentialStatus
If specified, the value of the credentialStatus property MUST include the:
  • id property, which MUST be a URL.
  • type property, which expresses the credential status type (also referred to as the credential status scheme), which MUST provide enough information to determine the current status of the credential (for example, suspended or revoked).

The precise contents of the credential status information is determined by the specific credentialStatus type definition, and varies depending on factors such as whether it is simple to implement or if it is privacy-enhancing.

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://example.com/examples/v1"
  ],
  "id": "http://example.edu/credentials/3732",
  "type": ["VerifiableCredential", "UniversityDegreeCredential"],
  "issuer": "https://example.edu/issuers/14",
  "issuanceDate": "2010-01-01T19:23:24Z",
  "credentialSubject": {
    "id": "did:example:ebfeb1f712ebc6f1c276e12ec21",
    "degree": {
      "type": "BachelorDegree",
      "name": "Bachelor of Science in Mechanical Engineering"
    }
  },
  "credentialStatus": {
    "id": "https://example.edu/status/24",
    "type": "CredentialStatusList2017"
  },
  "proof": { ... }
}
        

Defining the data model, formats, and protocols for status schemes are out of scope for this specification. A status scheme registry [[VC-STATUS-REGISTRY]] exists for implementers who want to implement credential status checking.

Presentations

Verifiable presentations MAY be used to combine and present verifiable credentials.

If verifiable presentations are used, they MUST be constructed from verifiable credentials themselves, or of data derived from verifiable credentials in a cryptographically verifiable format.

The example below shows a verifiable presentation that embeds verifiable credentials.

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://example.com/examples/v1"
  ],
  "id": "urn:uuid:3978344f-8596-4c3a-a978-8fcaba3903c5",
  "type": ["VerifiablePresentation"],
  "verifiableCredential": [{ ... }],
  "proof": [{ ... }]
}
        

The contents of the verifiableCredential property shown above are verifiable credentials as described by this specification. The contents of the proof property are proofs as described by the Linked Data Proofs [[LD-PROOFS]] specification. The id property is optional and MAY be used to provide a unique identifier for the presentation. The value associated with the id property MUST be a URI.

Some zero-knowledge cryptography schemes might enable holders to indirectly prove they hold a credential without revealing the credential itself. In these schemes, the original attribute, such as date of birth, might be translated into another value, such as "over the age of 15", which is cryptographically asserted such that a verifier can trust the value if they trust the issuer.

We are currently working on an example of a ZKP-style verifiable presentation that contains derived data instead of directly embedded verifiable credentials.

If a presentation supports derived predicates, a claim about birthdate in a credential can become the basis of proof that at a given point in time, the age of a subject did or will fall within a specified range. Selective disclosure schemes using zero-knowledge proofs can use claims expressed in this model to prove additional statements about those claims. For example, a claim specifying a subject's date of birth can be used as a predicate to prove the subject's age is within a given range, and therefore prove the subject qualifies for age-related discounts, without actually revealing the subject's birthdate. The holder has the flexibility to use the claim in any way that is applicable to the desired presentation.

A basic claim expressing that Pat's date of birth is 2010-01-01T19:23:24Z. Date encoding would be determined by the schema.

Advanced Concepts

Building on the concepts introduced in Section , this section explores more complex topics about verifiable credentials.

Extensibility

One of the goals of the Verifiable Credentials Data Model is to enable permissionless innovation. To achieve this, the data model needs to be extensible in a number of different ways. The data model is required to:

The requirement to support multiple types of machine-readable data formats e.g., JWT is currently not supported.

This approach to data modelling is often called an open world assumption, meaning that any entity can say anything about any other entity. While this approach seems to conflict with building simple and predictable software systems, balancing extensibility with program correctness is always more challenging with an open world assumption than with closed software systems.

The rest of this section describes, through a series of examples, how both extensibility and program correctness are achieved.

Let us assume that we start with the verifiable credential shown below.

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://example.com/examples/v1"
  ],
  "id": "http://example.com/credentials/4643",
  "type": ["VerifiableCredential"],
  "issuer": "https://example.com/issuers/14",
  "issuanceDate": "2018-02-24T05:28:04Z",
  "credentialSubject": {
    "id": "did:example:abcdef1234567",
    "name": "Jane Doe"
  },
  "proof": { ... }
}
        

This verifiable credential states that the entity associated with did:example:abcdef1234567 has a name with a value of Jane Doe.

Now let us assume that a developer wants to extend the verifiable credential to store two additional pieces of information: an internal corporate reference number, and Jane's favorite food.

The first thing to do is to create a JSON-LD context containing two new terms, as shown below.

{
  "@context": {
    "referenceNumber": "https://example.com/vocab#referenceNumber",
    "favoriteFood": "https://example.com/vocab#favoriteFood"
  }
}
        

After this JSON-LD context is created, the developer publishes it somewhere so it is accessible to verifiers who will be processing the verifiable credential. Assuming the above JSON-LD context is published at https://example.com/contexts/mycontext.jsonld, we can extend this example by including the context and adding the new properties and credential type to the verifiable credential.

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://example.com/contexts/mycontext.jsonld"
  ],
  "id": "http://example.com/credentials/4643",
  "type": ["VerifiableCredential", "CustomExt12"],
  "issuer": "https://example.com/issuers/14",
  "issuanceDate": "2018-02-24T05:28:04Z",
  "referenceNumber": 83294847,
  "credentialSubject": {
    "id": "did:example:abcdef1234567",
    "name": "Jane Doe",
    "favoriteFood": "Papaya"
  },
  "proof": { ... }
}
        

This example demonstrates extending the Verifiable Credentials Data Model in a permissionless and decentralized way. The mechanism shown also ensures that verifiable credentials created in this way provide a mechanism to prevent namespace conflicts and semantic ambiguity.

A dynamic extensibility model such as this does increase the implementation burden. Software written for such a system has to determine whether verifiable credentials with extensions are acceptable based on the risk profile of the application. Some applications might accept only certain extensions while highly secure environments might not accept any extensions. These decisions are up to the developers of these applications and are specifically not the domain of this specification.

Developers are urged to ensure that extension JSON-LD contexts are highly available. Implementations that cannot fetch a context will produce an error. Strategies for ensuring that extension JSON-LD contexts are always available include using content-addressed URLs for contexts, bundling context documents with implementations, or enabling aggressive caching of contexts.

Semantic Interoperability

This specification endeavors to enable both the JSON and JSON-LD syntaxes to be semantically compatible with one another without the JSON implementations needing to process the documents as JSON-LD. To achieve this, the specification imposes the following additional restrictions on both syntaxes:

  • JSON-based processors MUST process the @context property, ensuring the expected values exist in the expected order for the credential type being processed. It is advised that the expected order of values of the @context property should be defined by at least a human-readable extension specification and preferably by a machine-readable specification as well.
  • JSON-LD-based processors MUST produce an error when a JSON-LD context redefines any term in the active context.

To avoid the possibility of accidentally overriding terms, developers are urged to scope their extensions. For example, the following extension scopes the new favoriteFood term so that it can only be used within the credentialSubject property.

{
  "@context": {
    "referenceNumber": "https://example.com/vocab#referenceNumber",
    "credentialSubject": {
      "@id": "https://www.w3.org/2018/credentials#credentialSubject",
      "@context": {
        "favoriteFood": "https://example.com/vocab#favoriteFood"
      }
    }
  }
}
          

Data Schemas

Data schemas are useful when enforcing a specific structure on a given collection of data. There are at least two types of data schemas that this specification considers:

The expression of a data schema can be performed using the following property:

credentialSchema
The value of the credentialSchema property MUST be one or more data schemas that provide verifiers with enough information to determine if the provided data conforms to the provided schema. Each credentialSchema MUST specify its type (for example, JsonSchemaValidator2018), and an id property that MUST be a URI identifying the schema file. The precise contents of each data schema is determined by the specific type definition.
{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://example.org/examples/v1"
  ],
  "id": "http://example.edu/credentials/3732",
  "type": ["VerifiableCredential", "UniversityDegreeCredential"],
  "issuer": "https://example.edu/issuers/14",
  "issuanceDate": "2010-01-01T19:23:24Z",
  "credentialSubject": {
    "id": "did:example:ebfeb1f712ebc6f1c276e12ec21",
    "degree": {
      "type": "BachelorDegree",
      "name": "Bachelor of Science in Mechanical Engineering"
    }
  },
  "credentialSchema": {
    "id": "https://example.org/examples/degree.json",
    "type": "JsonSchemaValidator2018"
  },
  "proof": { ... }
}
        

In the example above, the issuer is specifying a credentialSchema, which points to a [[?JSON-SCHEMA]] file that can be used by a verifier to determine if the verifiable credential is well formed.

The group is still exploring how to utilize this feature for ZKPs and will most likely adjust this section based on implementer feedback.

Data schemas can also be used to specify mappings to other binary formats, such as those used to perform zero-knowledge proofs.

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://example.org/examples/v1"
  ],
  "id": "http://example.edu/credentials/3732",
  "type": ["VerifiableCredential", "UniversityDegreeCredential"],
  "issuer": "https://example.edu/issuers/14",
  "issuanceDate": "2010-01-01T19:23:24Z",
  "credentialSubject": {
    "id": "did:example:ebfeb1f712ebc6f1c276e12ec21",
    "degree": {
      "type": "BachelorDegree",
      "name": "Bachelor of Science in Mechanical Engineering"
    }
  },
  "credentialSchema": {
    "id": "https://example.org/examples/degree.zkp",
    "type": "ZkpExampleSchema2018"
  },
  "proof": { ... }
}
        

In the example above, the issuer is specifying a credentialSchema, which points to a zero-knowledge packed binary data format that is capable of transforming the input data into a format, which can then be used by a verifier to determine if the proof provided with the verifiable credential is valid.

Refreshing

It is useful for systems to enable the manual or automatic refresh of a expired verifiable credential. This specification enables an issuer to include a link to a refresh service.

The issuer can include the refresh service as an element inside the verifiable credential if it is intended for either the verifier or the holder (or both), or inside the verifiable presentation if it is intended for the holder only.

Including the refresh reference inside the verifiable presentation enables the holder to refresh the credential details before creating a presentation to share with a verifier, while including the refresh reference inside the verifiable credential enables both the holder and the verifier to perform future updates of the credential.

A refresh service can be expressed using the following property:

refreshService
The value of the refreshService property MUST be one or more refresh services that provides enough information to the recipient's software such that the recipient can refresh the credential. Each refreshService value MUST specify its type (for example, ManualRefreshService2018), and its URL as the value of id. The precise content of each refresh service is determined by the specific refreshService type definition.
{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://example.org/examples/v1"
  ],
  "id": "http://example.edu/credentials/3732",
  "type": ["VerifiableCredential", "UniversityDegreeCredential"],
  "issuer": "https://example.edu/issuers/14",
  "issuanceDate": "2010-01-01T19:23:24Z",
  "credentialSubject": {
    "id": "did:example:ebfeb1f712ebc6f1c276e12ec21",
    "degree": {
      "type": "BachelorDegree",
      "name": "Bachelor of Science in Mechanical Engineering"
    }
  },
  "refreshService": {
    "id": "https://example.edu/refresh/3732"
    "type": "ManualRefreshService2018",
  },
  "proof": { ... }
}
        

In the example above, the issuer specifies a manual refreshService that can be used by directing the holder or the verifierto https://example.edu/refresh/3732.

Mode of Operation

Section provided an overview of the verifiable credential ecosystem. This section provides more detail about how the ecosystem is envisaged to operate, as follows:

The order of the following steps is not fixed and some steps might be repeated.

  1. The verifier defines its policies for accepting verifiable credentials from holders for its supported actions.
  2. The issuer issues a verifiable credential to the holder.
  3. The holder might pass on its verifiable credentials to another holder.
  4. The final holder presents its verifiable credentials to the verifier in a verifiable presentation, requesting a supported action.
  5. The verifier verifies the authenticity of the verifiable presentation and verifiable credentials.
  6. The verifier verifies that the holder posses the verifiable credentials.
  7. The verifier decides whether to accept the verifiable credentials for the requested action, taking into account its policy, the holder, and the contents of the verifiable credentials.
  8. The verifier either performs the requested action or rejects the request.

This specification does not define any protocol for transfering verifiable credentials or verifiable presentations, but assuming other specifications do specify how they are transferred between entities, then this Verifiable Credential Data Model is:

In particular, Sections and specify how a verifier can determine:

Terms of Use

Terms of use can be utilized by an issuer or a holder to communicate the terms under which a verifiable credential or verifiable presentation was issued. The issuer places their terms of use inside the credential. The holder places their terms of use inside a presentation. The value of this property tells the verifier what actions it either 1) must perform (an Obligation), 2) must not perform (a Prohibition), or 3) may perform (a Permission) if it is to accept the verifiable credential or verifiable presentation.

Further study is required to determine how a subject who is not a holder places terms of use on their verifiable credentials. One way could be for the subject to request the issuer to place the terms of use inside the issued verifiable credentials. Another way could be for the subject to delegate a verifiable credential to a holder and placing terms of use restrictions on the delegated verifiable credential.

Terms of use MAY be expressed using the following property:

termsOfUse
If specified, the value of the termsOfUse property MUST specify one or more terms of use policies under which the creator issued the credential or presentation. If the recipient (a holder or verifier) is not willing to adhere to the specified terms of use, then they do so on their own responsibility and might incur legal liability if they violate the stated terms of use. Each termsOfUse value MUST specify its type, for example, IssuerPolicy, and optionally, its instance id. The precise contents of each term of use is determined by the specific termsOfUse type definition.

The group is currently exploring a variety of ways of expressing the terms of use associated with a Verifiable Credential, namely, the Open Digital Rights Language. A related initiative is the one at Customer Commons.

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://example.org/examples/v1"
  ],
  "id": "http://example.edu/credentials/3732",
  "type": ["VerifiableCredential", "UniversityDegreeCredential"],
  "issuer": "https://example.edu/issuers/14",
  "issuanceDate": "2010-01-01T19:23:24Z",
  "credentialSubject": {
    "id": "did:example:ebfeb1f712ebc6f1c276e12ec21",
    "degree": {
      "type": "BachelorDegree",
      "name": "Bachelor of Science in Mechanical Engineering"
    }
  },
  "termsOfUse": [{
    "type": "IssuerPolicy",
    "id": "http://example.com/policies/credential/4",
    "profile": "http://example.com/profiles/credential",
    "prohibition": [{
      "assigner": "https://example.edu/issuers/14",
      "assignee": "AllVerifiers",
      "target": "http://example.edu/credentials/3732",
      "action": ["Archival"]
    }]
  },
  "proof": { ... }
}
        

In the example above, the issuer (assigner) is prohibiting verifiers (assignee) from storing the data in an archive.

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://example.org/examples/v1"
  ],
  "id": "did:example:ebfeb1f712ebc6f1c276e12ec21",
  "type": "VerifiablePresentation",
  "verifiableCredential": [{
    "id": "http://example.edu/credentials/3732",
    "type": ["VerifiableCredential", "UniversityDegreeCredential"],
    "issuer": "https://example.edu/issuers/14",
    "issuanceDate": "2010-01-01T19:23:24Z",
    "credentialSubject": {
      "id": "did:example:ebfeb1f712ebc6f1c276e12ec21",
      "degree": {
      "type": "BachelorDegree",
      "name": "Bachelor of Science in Mechanical Engineering"
    }
    },
    "proof": { ... }
  }],
  "termsOfUse": [{
    "type": "HolderPolicy",
    "id": "http://example.com/policies/credential/6",
    "profile": "http://example.com/profiles/credential",
    "prohibition": [{
      "assigner": "did:example:ebfeb1f712ebc6f1c276e12ec21",
      "assignee": "https://wineonline.example.org/",
      "target": "http://example.edu/credentials/3732",
      "action": ["3rdPartyCorrelation"]
    }]
  },
  "proof": [ ... ]
}
        

In the example above, the holder (assigner), who is also the subject, expressed a Term of Use prohibiting the verifier (assignee) (https://wineonline.example.org) from using the information provided to correlate the holder / subject using a third-party service. If the verifier were to use a third-party service for correlation, they would violate the terms under which the holder created the presentation.

Evidence

Evidence may be included by an issuer to provide the verifier additional supporting information in a verifiable credential. This could be used by the verifier to establish the confidence with which it relies on the claims in the verifiable credential.

For example, an issuer might check physical documentation provided by the subject or might perform a set of background checks before issuing the credential. In certain scenarios, this information is useful to the verifier when determining the risk associated with relying on a given credential.

Evidence information can be expressed using the following property:

evidence
The value of the evidence property MUST be one or more evidence schemes providing enough information to a verifier to determine whether the evidence gathered by the issuer meets its requirements for issuing a credential. The precise content of each evidence scheme is determined by the specific evidence type definition.

The group is currently determining whether or not they should publish a very simple scheme for evidence as a part of this specification.

The group is currently discussing how attachments and references to non-credential data are supported by the specification.

The group is currently discussing how attachments and references to credentials are supported by the specification.

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://example.org/examples/v1"
  ],
  "id": "http://example.edu/credentials/3732",
  "type": ["VerifiableCredential", "UniversityDegreeCredential"],
  "issuer": "https://example.edu/issuers/14",
  "issuanceDate": "2010-01-01T19:23:24Z",
  "credentialSubject": {
    "id": "did:example:ebfeb1f712ebc6f1c276e12ec21",
    "degree": {
      "type": "BachelorDegree",
      "name": "Bachelor of Science in Mechanical Engineering"
    }
  },
  "evidence": [{
    "id": "https://example.edu/evidence/f2aeec97-fc0d-42bf-8ca7-0548192d4231",
    "type": ["DocumentVerification"],
    "verifier": "https://example.edu/issuers/14",
    "evidenceDocument": "DriversLicense",
    "subjectPresence": "Physical",
    "documentPresence": "Physical"
  },{
    "id": "https://example.edu/evidence/f2aeec97-fc0d-42bf-8ca7-0548192dxyzab",
    "type": ["SupportingActivity"],
    "verifier": "https://example.edu/issuers/14",
    "evidenceDocument": "Fluid Dynamics Focus",
    "subjectPresence": "Digital",
    "documentPresence": "Digital"
  }],
  "proof": { ... }
}
        

The evidence property provides different and complimentary information to the proof property. The evidence property is used to express supporting information, such as documentary evidence, related to the integrity of the credential. In contrast, the proof property is used to express machine-verifiable mathematical proofs related to the authenticity of the issuer and integrity of the credential.

Zero-Knowledge Proofs

A zero-knowledge proof is a cryptographic method where an entity can prove to another entity that they know a certain value without disclosing the actual value. A real world example is proving that an accredited university has granted a degree to you without revealing your identity or any other personally identifiable information contained on the degree.

One of the goals of this specification is to provide a data model that enables multiple different types of zero-knowledge proof mechanisms to be supported. The examples below highlight how the data model may be utilized to issue, present, and verify zero-knowledge based verifiable credentials.

The first stage of utilizing zero-knowledge based verifiable credentials is for the issuer to issue a verifiable credential in a manner that enables the holder to present the information to a verifier in a privacy-enhancing manner. There are two requirements that are required of most verifiable credentials when they are to be used in zero-knowledge systems:

The following example describes one way that a verifiable credential can express the information needed by a Camenisch-Lysyanskaya Zero-Knowledge Proof system, also known as CL Signatures.

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://example.com/examples/v1"
  ],
  "type": ["VerifiableCredential", "UniversityDegreeCredential"],
  "credentialSchema": {
    "id": "did:example:schema:3jf74yr7dsjw83jf",
    "type": "CLCredentialDefinition2019"
  },
  "issuer": "https://example.edu/issuers/14",
  "credentialSubject": {
    "givenName": "Jane",
    "familyName": "Doe",
    "degree": {
      "type": "BachelorDegree",
      "name": "Bachelor of Science in Mechanical Engineering",
      "college": "College of Engineering"
    }
  },
  "proof": {
    "type": "CLSignature2019",
    "created": "2018-06-17T10:03:48Z",
    "verificationMethod": "did:example:ebfeb1f712ebc6f1c276e12ec21/keys/234",
    "nonce": "bb864289-cec1-4222-afd7-d314239be92e",
    "proofValue: "zqmRkM7gkq5LFYKjixMdqM7gkq5LFYyWbNsVpsq...7Ey3",
  }
}
        

The example above provides the credential definition by using the credentialSchema property and a specific proof that is usable in the Camenisch-Lysyanskaya Zero-Knowledge Proof system.

The next example utilizes the verifiable credential above to generate a new derived verifiable credential with a privacy-preserving proof. The derived verifiable credential is then placed in a verifiable presentation that further proves that the entire assertion is valid. There are three requirements of most verifiable presentations when they are to be used in zero-knowledge systems:

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://example.com/examples/v1"
  ],
  "type": "VerifiablePresentation",
  "verifiableCredential": [{
    "@context": [
      "https://www.w3.org/2018/credentials/v1",
      "https://example.com/examples/v1"
    ],
    "type": ["VerifiableCredential", "UniversityDegreeCredential"],
    "credentialSchema": {
      "id": "did:example:schema:3jf74yr7dsjw83jf",
      "type": "CLCredentialDefinition2019"
    },
    "issuer": "https://example.edu/issuers/14",
    "credentialSubject": {
      "degree": {
        "type": "BachelorDegree",
        "college": "College of Engineering"
      }
    },
    "proof": {
      "type": "CLDerivedCredentialProof2019",
      "proofValue": "zM7gkqMqmRkdqM7...kq5"
    }
  }],
  "proof": {
    "type": "CLPresentationProof2019",
    "proofValue": "zWbNskM7...psq"
  }
}
        

Important details regarding the format for the credential definition and of the proofs are omitted on purpose because they are outside of the scope of this document. The purpose of this section is to guide implementers wanting to extend verifiable credentials and verifiable presentations to support zero-knowledge proof systems.

Disputes

There are at least two different cases to consider for an entity wanting to dispute a credential issued by an issuer:

Only the subject of a verifiable credential is entitled to issue a DisputeCredential. A DisputeCredential issued by anyone other than the subject should be disregarded by the verifier, unless the verifier has some other way of determining the truth of the dispute. This is to prevent denial of service attacks whereby an attacker falsely disputes a true claim.

The mechanism for issuing a DisputeCredential is the same as for a regular credential except that the credentialSubject identifier in the DisputeCredential property is the identifier of the disputed credential.

For example, if a credential with an identifier of https://example.org/credentials/245 is disputed, an entity can issue one of the credentials shown below. In the first example, the subject might present this to the verifier along with the disputed credential. In the second example, the entity might publish the DisputeCredential in a public venue to make it known that the credential is disputed.

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://example.com/examples/v1"
  ],
  "id": "http://example.com/credentials/123",
  "type": ["VerifiableCredential", "DisputeCredential"],
  "credentialSubject": {
    "id": "http://example.com/credentials/245",
    "currentStatus": "Disputed",
    "statusReason": "Address is out of date"
  },
  "issuer": "https://example.com/people#me",
  "issuanceDate": "2017-12-05T14:27:42Z",
  "proof": { ... }
}
        
{
  "@context": "https://w3id.org/credentials/v1",
  "id": "http://example.com/credentials/321",
  "type": ["VerifiableCredential", "DisputeCredential"],
  "credentialSubject": {
    "id": "http://example.com/credentials/245",
    "currentStatus": "Disputed",
    "statusReason": "Credential contains disputed statements",
    "disputedClaim": {
      "id": "did:example:ebfeb1f712ebc6f1c276e12ec21",
      "address": "Is Wrong"
    }
  },
  "issuer": "https://example.com/people#me",
  "issuanceDate": "2017-12-05T14:27:42Z",
  "proof": { ... }
}
        

In the above credential the issuer is claiming that the address in the disputed credential is wrong, for example, the latter might wrongly be claiming to have the same address as that of the issuer.

If a credential does not have an identifier, a content-addressed identifier can be used to identify the disputed credential. Similarly, content-addressed identifiers can be used to uniquely identify individual claims.

The group is currently exploring whether the specification of a vocabulary term to express content-based identifiers for claims is within scope as well as the specific vocabulary terms for disputed claims.

Authorization

Verifiable credentials are intended as a means of reliably identifying subjects. While it is recognized that Role Based Access Controls (RBAC) and Attribute Based Access Controls (ABAC) rely on this identification as a means of authorizing subjects to access resources, this specification does not provide a complete solution for RBAC or ABAC - authorization is not an appropriate use for this specification without an accompanying authorization framework.

The Working Group did consider authorization use cases during the creation of this specification and is pursuing that work as an architectural layer built on top of this specification.

Syntactic Sugar

In general, the data model and syntaxes described in this document are designed such that developers can copy and paste examples to incorporate verifiable credentials into their software systems. The design goal of this approach is to provide a low barrier to entry while still ensuring global interoperability between a heterogeneous set of software systems. This section describes some of these approaches, which will most likely go unnoticed by most developers, but whose details will be of interest to implementers. The most noteworthy syntactic sugars used throughout the ecosystem are as follows:

Syntaxes

Many of the concepts in this document were introduced by example using the JSON syntax. This section formalizes how the data model (described in Sections , , and ) is realized in JSON and JSON-LD. Although syntactic mappings are provided for these two syntaxes only, applications and services can use any other data representation syntax, such as XML, YAML, or CBOR, that is capable of expressing the data model.

JSON

The data model as described in Section can be encoded in Javascript Object Notation (JSON) [[!RFC8259]] by mapping property values to JSON types as follows:

JSON-LD

[[!JSON-LD]] is a JSON-based format used to serialize Linked Data. The syntax is designed to easily integrate into deployed systems already using JSON, and provides a smooth upgrade path from JSON to JSON-LD. It is primarily intended to be a way to use Linked Data in Web-based programming environments, to build interoperable Web services, and to store Linked Data in JSON-based storage engines.

JSON-LD is useful when extending the data model described in this specification. Instances of the data model are encoded in JSON-LD in the same way they are encoded in JSON (Section ), with the addition of the @context property. The JSON-LD Context is described in detail in the [[!JSON-LD]] specification and its use is elaborated on in Section .

Multiple contexts MAY be used or combined to express any arbitrary information about credentials in idiomatic JSON. If an application is processing a verifiable credential or verifiable presentation, and a @context property is not present at the top-level of the JSON-LD document, then a @context property with a value of https://www.w3.org/2018/credentials/v1 MUST be assumed. The JSON-LD Context available at https://www.w3.org/2018/credentials/v1 is a static document that is never updated and can therefore be downloaded and cached client side. The associated vocabulary document for the Verifiable Credentials Data Model is available at https://www.w3.org/2018/credentials.

JSON Web Token

JSON Web Token (JWT) [[!RFC7519]] is still a widely used means to express claims to be transferred between two parties. Providing a representation of the Verifiable Credentials Data Model for JWT allows existing systems and libraries to participate in the ecosystem described in Section . A JWT encodes a set of claims as a JSON object that is contained in a JSON Web Signature (JWS) [[!RFC7515]] or JWE [[!RFC7516]]. For this specification, the use of JWE is out of scope.

Relation to the Verifiable Credentials Data Model

This specification defines encoding rules of the Verifiable Credential Data Model onto JWT and JWS. It further defines processing rules how and when to make use of specific JWT registered claim names and specific JWS registered header parameter names to allow systems based on JWT to comply with this specification. If these specific claim names and header parameters are present, their respective counterpart in the standard verifiable credential and verifiable presentation MAY be omitted to avoid duplication.

IANA Considerations

This specification introduces two new registered claim names, which contain those parts of the standard verifiable credentials and verifiable presentation where no explicit encoding rules for JWT exist. These objects are enclosed in the JWT payload as follows:

JWT and JWS Considerations

JWT Encoding

To encode a verifiable credential as a JWT, specific properties introduced by this specification MUST be encoded as standard JOSE header parameters, JWT registered claim names, or contained in the JWS signature part. If no explicit rule is specified, properties are encoded in the same way as with a standard verifiable credential, and are added to the vc property of the JWT. The following paragraphs describe these encoding rules.

If the JWS is present, the digital signature either refers to the issuer of the verifiable credential, or in the case of a verifiable presentation, the holder of the verifiable credential. The JWS proves that the issuer of the JWT signed the contained JWT payload and therefore, the proof property can be omitted.

If no JWS is present, a proof property MUST be provided. The proof property can be used to represent more complex proofs, for example if the creator is different from the issuer, or a proof not based on digital signatures, such as Proof of Work. The issuer MAY include both a JWS and a proof property. For backward compatibility reasons, the issuer MUST use JWS to represent proofs based on a digital signature.

The following defines rules for JOSE headers in the context of this specification:

  • alg MUST be used for RSA and ECDSA-based digital signatures. If only the proof attribute is used, the alg header MUST be set to none.
  • kid MAY be used if there are multiple keys associated with the issuer of the JWT. The key discovery is out of the scope of this specification. For example, the kid can refer to a key in a DID document, or can be the identifier of a key inside a JWKS.
  • typ, if present, MUST be set to JWT.

For backward compatibility with JWT processors, the following JWT registered claim names MUST be used instead of, or in addition to, their respective standard verifiable credential counterparts:

Other JOSE header parameters and claim names not specified herein can be used if their use is not explicitly discouraged. Additional claims MUST be added to the credentialSubject property of the JWT.

This version of the specification defines no JWT specific encoding rules for the concepts outlined in Section Advanced Concepts (for example, refreshService, termsOfUse, and evidence). These concepts can be encoded as they are without any transformation, and can be added to the vc property of the JWT.

JWT Decoding

To decode a JWT to a standard verifiable credential, the following transformation MUST be performed:

  1. Create a JSON object.
  2. If no @context is specified under vc, add the default @context to the new JSON object.
  3. Add the content from the vc property to the new JSON object.
  4. Transform the remaining JWT specific headers and claims, and add the results to the new JSON object.

To transform the JWT specific headers and claims, the following MUST be done. If:

  • exp is present, the UNIX timestamp MUST be converted to an [[!ISO8601]] combined date and time string, and MUST be used to set the value of the expirationDate property of credentialSubject of the new JSON object.
  • iss is present, the value MUST be used to set the issuer property of the new JSON object.
  • iat is present, the UNIX timestamp MUST be converted to an [[!ISO8601]] combined date and time string, and MUST be used to set the value of the issuanceDate property of the new JSON object.
  • jti is present, the value MUST be used to set the value of the id property of credentialSubject of the new JSON object.
  • sub is present, the value MUST be used to set the value of the id property of the new JSON object.
{
    "alg": "RS256",
    "typ": "JWT",
    "kid": "did:example:abfe13f712120431c276e12ecab#keys-1"
}
          

In the example above, the verifiable credential uses a proof based on JWS digital signatures, and the corresponding verification key can be obtained using the kid header parameter.

{
  "sub": "did:example:ebfeb1f712ebc6f1c276e12ec21",
  "jti": "http://example.edu/credentials/3732",
  "iss": "did:example:abfe13f712120431c276e12ecab",
  "iat": "1541493724",
  "exp": "1573029723",
  "nonce": "660!6345FSer",
  "vc": {
    "@context": [
      "https://www.w3.org/2018/credentials/v1",
      "https://example.com/examples/v1"
    ],
    "type": ["VerifiableCredential", "UniversityDegreeCredential"],
    "credentialSubject": {
      "degree": {
        "type": "BachelorDegree",
        "name": "Bachelor of Science in Mechanical Engineering"
      }
    }
  }
}
          

In the example above, vc does not contain the id property because the JWT encoding uses the jti attribute to represent a unique identifier. The sub attribute encodes the information represented by the id property of credentialSubject.

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{
  "alg": "RS256",
  "typ": "JWT",
  "kid": "did:example:ebfeb1f712ebc6f1c276e12ec21#keys-1"
}
          

In the example above, the verifiable presentation uses a proof based on JWS digital signatures, and the corresponding verification key can be obtained using the kid header parameter.

{
  "iss": "did:example:ebfeb1f712ebc6f1c276e12ec21",
  "jti": "urn:uuid:3978344f-8596-4c3a-a978-8fcaba3903c5",
  "aud": "did:example:4a57546973436f6f6c4a4a57573",
  "iat": "1541493724",
  "exp": "1573029723",
  "nonce": "343s$FSFDa-",
  "vp": {
    "@context": [
      "https://www.w3.org/2018/credentials/v1",
      "https://example.com/examples/v1"
    ],
    "type": ["VerifiablePresentation"],
    // base64url-encoded JWT as string
    "verifiableCredential": ["..."]
  }
}
          

In the example above, vp does not contain the id property because the JWT encoding uses the jti attribute to represent a unique identifier. verifiableCredential contains an string array of verifiable credentials using JWT compact serialization.

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A number of the concerns have been raised around security, composability, reusability, and extensibility with respect to the use of JWTs for verifiable credentials. These concerns will be documented in time in at least the Verifiable Claims Model and Security Considerations section of this document.

Verification

This section describes a number of checks required to verify a verifiable credential. Some checks are mandatory, while other checks are applicable to only some verifiable credentials.

The group is currently discussing whether a mechanism should be provided that enables linkages to JSON Schema or other optional verification mechanisms.

Syntax

The document is syntactically valid. For example, syntactically valid JSON or JSON-LD.

Credential

Required properties MUST be present. For example, for a verifiable credential, the type and proof properties are required.

Property values MUST match the expectations described in this specification. For example, the document type property for a verifiable credential MUST contain the VerifiableCredential class.

Issuer

The value associated with the issuer property MUST identify an issuer that is known to and trusted by the verifier.

Relevant metadata about the issuer property MUST be available to the verifier. For example, an issuer can publish information containing the public keys it uses to digitally sign verifiable credentials that it issued. This metadata is relevant when checking the proofs on the verifiable credential.

Subject

The value associated with the id property for each credentialSubject MUST identify a subject to the verifier. For example, if a subject is identified and the verifier has public key metadata used for authentication purposes that is related to the subject, the verifier MAY be able to authenticate the subject using a signature generated by the subject contained in the verifiable presentation.

The group is currently discussing how authentication and WebAuthn may work.

The id property is optional. Verifiers MAY use other properties in a credential to uniquely identify a subject.

Proofs (Signatures)

The cryptographic mechanism used to prove that the information in a verifiable credential or a verifiable presentation has not been tampered with is called a proof. There are many types of cryptographic proofs including, but not limited to, digital signatures, zero-knowledge proofs, Proofs of Work, and Proofs of Stake. In general, when verifying proofs implementations MUST ensure that:

Some proofs are digital signatures. In general, when verifying digital signatures, implementations MUST ensure that:

The digital signature provides a number of protections, other than tamper resistance, that are not immediately obvious. For example, a Linked Data Signature created property establishes a date and time before which the credential should not be considered verified. The creator property enables the ability to dynamically discover information about the entity who created the data to ensure that the public key is not revoked or expired. The proofPurpose property ensures the reason the entity created the signature, such as for authentication or creating a verifiable credential, is clear and protected in the signature.

Issuance

The issuanceDate MUST be within an expected range for the verifier. For example, a verifier can ensure that the issuance date of a verifiable credential is not in the future.

Expiration

The expirationDate MUST be within an expected range for the verifier. For example, a verifier can ensure that the expiration date of a verifiable credential is not in the past.

Status

If the credentialStatus property is available, the status of the claim MUST be in good standing according to the credentialStatus type definition for the credential. For example, a verifier can ensure that the status has not been withdrawn for cause by the issuer.

Fitness for Purpose

The custom properties in the credential are appropriate for the verifier's purpose. For example if a verifier needs to determine whether a subject is older than 21 years of age, they might rely on claims of a specific birthdate, or abstract properties such as ageOver.

The issuer is trusted by the verifier to make the claims at hand. For example, a franchised fast food restaurant location trusts discount coupon claims made by the corporate headquarters of the franchise. Policy information expressed by the issuer in the verifiable credential should be respected by holders and verifiers unless they accept the liability of ignoring the policy.

Accessibility Considerations

There are a number of accessibility considerations implementers should be aware of when processing data described in this specification. As with any web standards or protocols implementation, ignoring accessibility issues makes this information unusable to a large subset of the population. It is important to follow accessibility guidelines and standards, such as [[WCAG21]], to ensure all people, regardless of ability, can make use of this data. This is especially important when establishing systems utilizing cryptography, which historically, have created problems for assistive technologies.

This section details the general accessibility considerations to take into account when utilizing this data model.

Data First Approaches

Many physical credentials in use today, such as government identification cards, have poor accessibility characteristics, including, but not limited to, small print, reliance on small and high-resolution images, and no affordances for people with vision impairments.

When utilizing this data model to create verifiable credentials, it is suggested that data model designers use a data first approach. For example, given the choice of using data or a graphical image to depict a credential, designers should express every element of the image, such as the name of an institution or the professional credential, in a machine-readable way instead of just relying on the image to convey this information. Using a data first approach is preferred because it provides the foundational elements of building different interfaces for people with varying abilities.

Privacy Considerations

This section details the general privacy considerations and specific privacy implications of deploying the Verifiable Credentials Data Model into production environments.

Spectrum of Privacy

It is important to recognize there is a spectrum of privacy ranging from pseudonymous to strongly identified. Depending on the use case, people have different comfort levels about what information they are willing to provide and what information can be derived from what is provided.

Privacy spectrum ranging from pseudonymous to fully identified.

For example, most people probably want to remain anonymous when purchasing alcohol because the regulatory check required is solely based on whether a person is above a specific age. Alternatively, for medical prescriptions written by a doctor for a patient, the pharmacy fulfilling the prescription is required to more strongly identify the medical professional and the patient. Therefore there is not one approach to privacy that works for all use cases. Privacy solutions are use case specific.

Even for those wanting to remain anonymous when purchasing alcohol, photo identification might still be required to provide appropriate assurance to the merchant. The merchant might not need to know your name or other details (other than that you are over a specific age), but in many cases just proof of age might still be insufficient to meet regulations.

The Verifiable Credentials Data Model strives to support the full privacy spectrum and does not take philosophical positions on the correct level of anonymity for any specific transaction. The following sections provide guidance for implementers who want to avoid specific scenarios that are hostile to privacy.

Personally Identifiable Information

Data associated with verifiable credentials stored in the credential.credentialSubject field is susceptible to privacy violations when shared with verifiers. Personally identifying data, such as a government-issued identifier, shipping address, and full name, can be easily used to determine, track, and correlate an entity. Even information that does not seem personally identifiable, such as the combination of a birthdate and a postal code, has very powerful correlation and de-anonymizing capabilities.

Implementers are strongly advised to warn holders when they share data with these kinds of characteristics. Issuers are strongly advised to provide privacy-protecting credentials when possible. For example, issuing ageOver credentials instead of date of birth credentials when a verifier wants to determine if an entity is over the age of 18.

Identifier-Based Correlation

Subjects of verifiable credentials are identified using the credential.credentialSubject.id field. The identifiers used to identify a subject create a greater risk of correlation when the identifiers are long-lived or used across more than one web domain.

Similarly, disclosing the credential identifier (credential.id) leads to situations where multiple verifiers, or an issuer and a verifier, can collude to correlate the holder. If holders want to reduce correlation, they should use credential schemes that allow hiding the identifier during presentation. Such schemes expect the holder to generate the identifier and might even allow hiding the identifier from the issuer, while still keeping the identifier embedded and signed in the credential.

If strong anti-correlation properties are a requirement in a verifiable credentials system, it is strongly advised that identifiers are either:

Signature-Based Correlation

The contents of verifiable credentials are secured using the credential.proof field. The properties in this field create a greater risk of correlation when the same values are used across more than one session or domain and the value does not change. Examples include the creator, created, domain (for very specific domains), nonce, and signatureValue fields.

If strong anti-correlation properties are required, it is advised that signature values and metadata are regenerated each time using technologies like third-party pairwise signatures, zero-knowledge proofs, or group signatures.

Even when using anti-correlation signatures, information might still be contained in a credential that defeats the anti-correlation properties of the cryptography used.

Long-Lived Identifier-Based Correlation

Verifiable credentials might contain long-lived identifiers that could be used to correlate individuals. These types of identifiers include subject identifiers, email addresses, government-issued identifiers, organization-issued identifiers, addresses, healthcare vitals, credential-specific JSON-LD contexts, and many other sorts of long-lived identifiers.

Organizations providing software to holders should strive to identify fields in credentials containing information that could be used to correlate individuals and warn holders when this information is shared.

Device Fingerprinting

There are mechanisms external to verifiable credentials that are used to track and correlate individuals on the Internet and the Web. Some of these mechanisms include Internet protocol (IP) address tracking, web browser fingerprinting, evercookies, advertising network trackers, mobile network position information, and in-application Global Positioning System (GPS) APIs. Using verifiable credentials cannot prevent the use of these other tracking technologies. Also, when these technologies are used in conjunction with verifiable credentials, new correlatable information could be discovered. For example, a birthday coupled with a GPS position can be used to strongly correlate an individual across multiple websites.

It is recommended that privacy-respecting systems prevent the use of these other tracking technologies when verifiable credentials are being used. In some cases, tracking technologies might need to be disabled on devices that transmit verifiable credentials on behalf of a holder.

Favor Abstract Claims

To enable recipients of verifiable credentials to use them in a variety of circumstances without revealing more personally identifiable information than necessary for transactions, issuers should consider limiting the information published in a credential to a minimal set needed for the expected purposes. One way to avoid placing personally identifiable information in a credential is to use an abstract property that meets the needs of verifiers without providing specific information about a subject.

For example, this document uses the ageOver property instead of a specific birthdate, which constitutes much stronger personally identifiable information. If retailers in a specific market commonly require purchasers to be older than a certain age, an issuer trusted in that market might choose to offer a credential claiming that subjects have met that requirement instead of offering credentials containing claims about specific birthdates. This enables individual customers to make purchases without revealing specific personally identifiable information.

The Principle of Data Minimization

Privacy violations occur when information divulged in one context leaks into another. Accepted best practice for preventing such violations is to limit the information requested, and received, to the absolute minimum necessary. This data minimization approach is required by regulation in multiple jurisdictions, including the Health Insurance Portability and Accountability Act (HIPAA) in the United States and the General Data Protection Regulation (GDPR) in the European Union.

With verifiable credentials, data minimization for issuers means limiting the content of a credential to the minimum required by potential verifiers for expected use. For verifiers, data minimization means limiting the scope of the information requested or required for accessing services.

For example, a driver's license containing a driver's ID number, height, weight, birthday, and home address is a credential containing more information than is necessary to establish that the person is above a certain age.

It is considered best practice for issuers to atomize information or use a signature scheme that allows for selective disclosure. For example, an issuer of driver's licenses could issue a credential containing every attribute that appears on a driver's license, as well as a set of credentials where every credential contains only a single attribute such as a person's birthday. It could also issue more abstract credentials (for example, a credential containing only an ageOver attribute). One possible adaptation would be for issuers to provide secure HTTP endpoints for retrieving single-use bearer credentials that promote the pseudonymous usage of credentials. Implementers that find this impractical or unsafe, should consider using selective disclosure schemes that eliminate dependence on issuers at proving time and reduce temporal correlation risk from issuers.

Verifiers are urged to only request information that is absolutely necessary for a specific transaction to occur. This is important for at least two reasons. It:

While it is possible to practice the Principle of Minimum Disclosure, it might be impossible to avoid the strong identification of an individual for specific use cases during a single session or over multiple sessions. The authors of this document cannot stress how difficult it is to meet this principle in real-world scenarios.

Bearer Credentials

A bearer credential is a privacy-enhancing piece of information, such as a concert ticket, which entitles the holder of the credential to a specific resource without divulging sensitive information about the holder.

Verifiable credentials that are bearer credentials are made possible by not specifying the subject identifier, expressed using the id property, which is nested in the credentialSubject property. For example, the following verifiable credential is a bearer credential:

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://example.com/examples/v1"
  ],
  "id": "http://example.edu/credentials/temporary/28934792387492384",
  "type": ["VerifiableCredential", "UniversityDegreeCredential"],
  "issuer": "https://example.edu/issuers/14",
  "issuanceDate": "2017-10-22T12:23:48Z",
  "credentialSubject": {
    // note that the 'id' property is not specified for bearer credentials
    "degree": {
      "type": "BachelorDegree",
      "name": "Bachelor of Science in Mechanical Engineering"
    }
  },
  "proof": { ... }
}
      

While bearer credentials can be privacy-enhancing, they must be carefully crafted so as not accidentally divulge more information than the holder of the credential expects. For example, repeated use of the same bearer credential across multiple sites enables these sites to potentially collude to unduly track or correlate the holder. Likewise, information that might seem non-identifying, such as a birthdate and postal code, can be used to statistically identify an individual when used together in the same credential or session.

Issuers of bearer credentials should ensure that the bearer credentials provide privacy-enhancing benefits that:

Holders should be warned by their software if bearer credentials containing sensitive information are issued or requested, or if there is a correlation risk when combining two or more bearer credentials across one or more sessions. While it might be impossible to detect all correlation risks, some might certainly be detectable.

Verifiers should not request bearer credentials that can be used to unduly correlate the holder.

Validity Checks

When processing verifiable credentials, verifiers typically perform many of the checks listed in Section as well as a variety of business-process-specific checks. For example, validity checks might include checking:

The process of performing these checks might result in information leakage that leads to a privacy violation of the holder. For example, a simple operation like checking a revocation list can notify the issuer that a specific business is likely interacting with the holder. This could enable issuers to collude and correlate individuals without their knowledge.

Issuers are urged to not use mechanisms, such as credential revocation lists that are unique per credential, during the verification process that could lead to privacy violations. Organizations providing software to holders should warn when credentials include information that could lead to privacy violations during the verification process. Verifiers should consider rejecting credentials that produce privacy violations or that enable bad privacy practices.

Storage Providers and Data Mining

When a holder receives a credential from an issuer, the credential needs to be stored somewhere (for example, in a credential repository). Holders are warned that the information in a verifiable credential is sensitive in nature and highly individualized, making it a high value target for data mining. Services that advertise free storage of verifiable credentials might in fact be mining personal data and selling it to organizations wanting to build individualized profiles on people and organizations.

Holders need to be aware of the terms of service for their credential repository, specifically the correlation and data mining protections in place for those who store their verifiable credentials with the service provider.

The following are some effective mitigations for data mining and profiling:

Aggregation of Credentials

Holding two pieces of information about the same subject almost always reveals more about the subject than just the sum of two pieces of information, even when the information is delivered through different channels. The aggregation of credentials is a privacy risk and all participants in the ecosystem need to be aware of the risks of data aggregation.

For example, if two bearer credentials, one for an email address and then one stating the holder is over the age of 21, are provided across multiple sessions, the verifier of the information now has a unique identifier plus age-related information for the individual. It is now easy to create and build a profile for the holder such that more and more information is leaked over time. Aggregation of credentials can also be performed across multiple sites in collusion with each other, leading to privacy violations.

From a technological perspective, preventing the aggregation of information is a very difficult privacy problem to address. While new cryptographic techniques, such as zero-knowledge proofs, are being proposed as solutions to the problem of aggregation and correlation, the existence of long-lived identifiers and browser tracking techniques defeats even the most modern cryptographic techniques.

The solution to the privacy implications of correlation or aggregation tend not to be technological in nature, but policy driven instead. Therefore, if a holder does not want information about them to be aggregated, they must express this in the verifiable presentations they transmit.

Usage Patterns

Despite the best efforts to assure privacy, actually using verifiable credentials can potentially lead to de-anonymization and a loss of privacy. This correlation can occur when:

In part, it is possible to mitigate this de-anonymization and loss of privacy by:

It is understood that these mitigation techniques are not always practical or even compatible with necessary usage. Sometimes correlation is a requirement.

For example, in some prescription drug monitoring programs, usage monitoring is a requirement. Enforcement entities need to be able to confirm that individuals are not cheating the system to get multiple prescriptions for controlled substances. This statutory or regulatory need to correlate usage overrides individual privacy concerns.

Verifiable credentials will also be used to intentionally correlate individuals across services, for example, when using a common persona to log in to multiple services, so all activity on each of those services is intentionally linked to the same individual. This is not a privacy issue as long as each of those services uses the correlation in the expected manner.

Privacy risks of credential usage occur when unintended or unexpected correlation arises from the presentation of verifiable credentials.

Sharing Information with the Wrong Party

When a holder chooses to share information with a verifier, it might be the case that the verifier is acting in bad faith and requests information that could be used to harm the holder. For example, a verifier might ask for a bank account number, which could then be used with other information to defraud the holder or the bank.

Issuers should strive to tokenize as much information as possible such that if a holder accidentally transmits credentials to the wrong verifier, the situation is not catastrophic.

For example, instead of including a bank account number for the purpose of checking an individual's bank balance, provide a token that enables the verifier to check if the balance is above a certain amount. In this case, the bank could issue a verifiable credential containing a balance checking token to a holder. The holder would then include the verifiable credential in a verifiable presentation and bind the token to a credit checking agency using a digital signature. The verifier could then wrap the verifiable presentation in their digital signature, and hand it back to the issuer to dynamically check the account balance.

Using this approach, even if a holder shares the account balance token with the wrong party, an attacker cannot discover the bank account number, nor the exact value in the account. And given the validity period for the counter-signature, does not gain access to the token for more than a few minutes.

Frequency of Claim Issuance

As detailed in Section , usage patterns can be correlated into certain types of behavior. Part of this correlation is mitigated when a holder uses a verifiable credential without the knowledge of the issuer. Issuers can defeat this protection however, by making their credentials short lived and renewal automatic.

For example, an ageOver credential is useful for gaining access to a bar. If an issuer issues such a credential with a very short expiration date and an automatic renewal mechanism, then the issuer could possibly correlate the behavior of the holder in a way that negatively impacts the holder.

Organizations providing software to holders should warn them if they repeatedly use credentials with short lifespans, which could result in behavior correlation. Issuers should avoid issuing credentials in a way that enables them to correlate usage patterns.

Prefer Single-Use Credentials

An ideal privacy-respecting system would require only the information necessary for interaction with the verifier to be disclosed by the holder. The verifier would then record that the disclosure requirement was met and forget any sensitive information that was disclosed. In many cases, competing priorities, such as regulatory burden, prevent this ideal system from being employed. In other cases, long-lived identifiers prevent single use. The design of any verifiable credentials ecosystem, however, should strive to be as privacy-respecting as possible by preferring single-use credentials whenever possible.

Using single-use credentials provides several benefits. The first benefit is to verifiers who can be sure that the data in a credential is fresh. The second benefit is to holders, who know that if there are no long-lived identifiers in the credential, the credential itself cannot be used to track or correlate them online. Finally, there is nothing for attackers to steal, making the entire ecosystem safer to operate within.

Security Considerations

There are a number of security considerations that issuers, holders, and verifiers should be aware of when processing data described by this specification. Ignoring or not understanding the implications of this section can result in security vulnerabilities.

While this section attempts to highlight a broad set of security considerations, it is not a complete list. Implementers are urged to seek the advice of security and cryptography professionals when implementing mission critical systems using the technology outlined in this specification.

Cryptography Suites and Libraries

Some aspects of the data model described in this specification can be protected through the use of cryptography. Implementers should be aware of the underlying cryptography suites and libraries used to implement the creation and verification of digital signatures and mathematical proofs used by their systems when processing credentials and presentations. Software developers with extensive experience implementing or auditing systems that use cryptography must be employed to ensure systems are properly designed. Proper red teaming is also suggested to remove bias from security reviews.

Cryptography suites and libraries have a shelf life and eventually fall to new attacks and technology advances. Production quality systems must take this into account and ensure mechanisms exist to easily and proactively upgrade expired or broken cryptography suites and libraries. Procedures should also exist to invalidate and replace existing credentials in the event of a cryptography suite or library failure. Regular monitoring of systems to ensure proper upgrades are made in a timely manner are also important to ensure the long term viability of systems processing verifiable credentials.

Content Integrity Protection

Verifiable credentials often contain URLs to data that resides outside of the credential itself. Content that exists outside a verifiable credential, such as links to images, JSON-LD Contexts, and other machine-readable data, are often not protected against tampering because the data resides outside of the protection of the proof on the verifiable credential. For example, the following highlighted links are not content-integrity protected and probably should be:

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://example.com/examples/v1"
  ],
  "id": "http://example.edu/credentials/58473",
  "type": ["VerifiableCredential", "AlumniCredential"],
  "credentialSubject": {
    "id": "did:example:ebfeb1f712ebc6f1c276e12ec21",
    "image": "https://example.edu/images/58473",
    "alumniOf": "Example University"
  },
  "proof": { ... }
}
      

While this specification does not recommend any particular content integrity protection, document authors that would like to ensure that links to content are integrity protected are advised to use URL schemes that enforce content integrity. Two such schemes are the Hashlink specification and the InterPlanetary File System (IPFS). The example below transforms the example above and adds content integrity protection to the JSON-LD Contexts using the Hashlink specification, and content integrity protection to the image by using an IPFS link:

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1?hl=z3aq31uzgnZBuWNzUB",
    "https://example.com/examples/v1?hl=z8guWNzUBnZBu3aq31"
  ],
  "id": "http://example.edu/credentials/58473",
  "type": ["VerifiableCredential", "AlumniCredential"],
  "credentialSubject": {
    "id": "did:example:ebfeb1f712ebc6f1c276e12ec21",
    "image": "ipfs:/ipfs/QmXfrS3pHerg44zzK6QKQj6JDk8H6cMtQS7pdXbohwNQfK/image",
    "alumniOf": "Example University"
  },
  "proof": { ... }
}
      

It is debatable whether or not the JSON-LD Contexts above need protection as it is expected that production implementations will ship with static copies of important JSON-LD Contexts.

While the example above is one way to achieve content integrity protection, there are other solutions that may be better suited for certain applications. Implementers are urged to understand how links to external machine-readable content that are not content-integrity protected may result in successful attacks against their applications.

Unsigned Claims

This specification allows credentials to be produced that do not contain signatures or proofs of any kind. These types of credentials are often useful for intermediate storage, or self-asserted information, which is analogous to filling out a form on a web page. Implementers should note that these types of credentials are not verifiable because the authorship is either not known or cannot be trusted.

Token Binding

A verifier might need to ensure it is the intended recipient of a verifiable presentation and not the target of a man-in-the-middle attack. Any protocol using the Verifiable Credentials Data Model that requires protection against these kinds of attacks needs to perform some sort of token binding, such as The Token Binding Protocol v1.0, which ties the request for a verifiable presentation with the response. Any protocol that does not perform token binding is susceptible to man-in-the-middle attacks.

Bundling Dependent Claims

It is considered best practice for issuers to atomize information in a credential, or use a signature scheme that allows for selective disclosure. In the case of atomization, if it is not done securely by the issuer, the holder might bundle together different credentials in a way that was not intended by the issuer.

For example a university might issue two credentials to a person, each containing two properties, for example, "Staff Member" in the "Department of Computing" and "Post Graduate Student" in the "Department of Economics". If these credentials are atomized into separate properties, then the university would issue four credentials to the person, each containing one of the following properties, "Staff Member", "Post Graduate Student", "Department of Computing", and "Department of Economics". The holder could then transfer the "Staff Member" and "Department of Economics" credentials to a verifier, which together would comprise a false claim.

Highly Dynamic Information

When verifiable credentials are issued for highly dynamic information, implementers should ensure the expiration times are set appropriately. Expiration periods longer than the timeframe where the credential is valid might create exploitable security vulnerabilities. Expiration periods shorter than the timeframe where the information expressed by the credential is valid creates a burden on holders and verifiers. It is therefore important to set validity periods for credentials that are appropriate to the use case and the expected lifetime for the information contained in the credential.

Device Theft and Impersonation

When verifiable credentials are stored on a device and that device is lost or stolen, it might be possible for an attacker to gain access to systems using the victim's verifiable credentials. Ways to mitigate this type of attack include:

Subject-Holder Relationships

This section describes the possible relationships between a subject and a holder and how the Verifiable Credentials Data Model expresses these relationships. The following diagram illustrates the different types of relationships covered in the rest of this section.

Subject-Holder Relationships in Verifiable Credentials.

The following sections describe how each of these relationships are handled in the data model.

Subject is the Holder

The most common relationship is when a subject is the holder. In this case, a verifier can easily deduce that a subject is the holder if the verifiable presentation is digitally signed by the holder and all contained verifiable credentials are about a subject that can be identified to be the same as the holder.

If only the credentialSubject is allowed to insert a verifiable credential into a verifiable presentation, the issuer MAY insert the subjectOnly property into the verifiable credential, as described below.

This feature is at risk and is likely to be removed due to lack of consensus.

subjectOnly Property

The subjectOnly property states that a verifiable credential MUST only be encapsulated into a verifiable presentation whose proof was issued by the credentialSubject. A verifiable presentation that contains a verifiable credential containing the subjectOnly property, whose proof creator is not the credentialSubject, is invalid.

{
  "id": "http://example.edu/credentials/3732",
  "type": ["VerifiableCredential", "ProofOfAgeCredential"],
  "issuer": "https://example.edu/issuers/14",
  "issuanceDate": "2010-01-01T19:23:24Z",
  "claim": {
    "credentialSubject": "did:example:ebfeb1f712ebc6f1c276e12ec21",
    "ageOver": 21
    },
  "SubjectOnly": "True",
  "proof": { ..
  "verificationMethod": "did:example:ebfeb1f712ebc6f1c276e12ec21",
  ... }
}
          

Credential Uniquely Identifies a Subject

In this case, the credentialSubject property might contain multiple properties each providing an aspect of the identity of the subject, which together, unambiguously identify the subject. Some use cases might not require the holder to be identified at all, such as checking to see if a doctor (the subject) is board-certified. Other use cases might require the verifier to use out-of-band knowledge to determine the relationship between the subject and the holder.

{
  "@context": ["https://www.w3.org/2018/credentials/v1", "https://schema.org/"]
  "id": "http://example.edu/credentials/332",
  "type": ["VerifiableCredential", "IdentityCredential"],
  "issuer": "https://example.edu/issuers/4",
  "issuanceDate": "2017-02-24",
  "credentialSubject": {
    "name": "J. Doe",
    "address": {
      "streetAddress": "10 Rue de Chose",
      "postalCode": "98052",
      "addressLocality": "Paris",
      "addressCountry": "FR"
    },
    "birthDate": "1989-03-15"
    ...
  },
  "proof": { ... }
}
        

The example above uniquely identifies the subject using the name, address, and birthdate of the individual.

Subject Passes the Verifiable Credential to a Holder

Usually verifiable credentials are presented to verifiers by the subject. However in some cases, the subject might need to pass the whole or part of a verifiable credential to another holder. For example, if a patient (the subject) is too ill to take a prescription (the verifiable credential) to the pharmacist (the verifier), a friend might take the prescription in to pick up the medication.

The data model allows for this, by the subject issuing a new verifiable credential and giving it to the new holder, who can then present both verifiable credentials to the verifier. However, the content of this second verifiable credential is likely to be application specific, so this specification cannot standardize the contents of this second verifiable credential. Nevertheless, a non-normative example is provided in Appendix .

Holder Acts on Behalf of the Subject

The Verifiable Credentials Data Model supports the holder acting on behalf of the subject in at least the following ways. The:

The mechanisms listed above describe the relationship between the holder and the subject and helps the verifier decide whether the relationship is sufficiently expressed for a given use case.

The additional mechanisms the issuer or the verifier uses to verify the relationship between the subject and the holder are outside the scope of this specification.

It is for further study which, if any, of these mechanisms is to be standardised by the working group.

{
  "id": "http://example.edu/credentials/3732",
  "type": ["VerifiableCredential", "AgeCredential", "RelationshipCredential"],
  "issuer": "https://example.edu/issuers/14",
  "issuanceDate": "2010-01-01T19:23:24Z",
  "credentialSubject": {
    "id": "did:example:ebfeb1f712ebc6f1c276e12ec21",
    "ageUnder": 16,
    "parent": {
      "id": "did:example:ebfeb1c276e12ec211f712ebc6f",
      "type": "Mother"
    }
  },
  "proof": { ... } // the proof is generated by the DMV
}
        

In the example above, the issuer expresses the relationship between the child and the parent such that a verifier would most likely accept the credential if it is provided by the child or the parent.

{
  "id": "http://example.edu/credentials/3732",
  "type": ["VerifiableCredential", "RelationshipCredential"],
  "issuer": "https://example.edu/issuers/14",
  "issuanceDate": "2010-01-01T19:23:24Z",
  "credentialSubject": {
    "id": "did:example:ebfeb1c276e12ec211f712ebc6f",
    "child": {
      "id": "did:example:ebfeb1f712ebc6f1c276e12ec21",
      "type": "Child"
    }
  },
  "proof": { ... } // the proof is generated by the DMV
}
        

In the example above, the issuer expresses the relationship between the child and the parent in a separate credential such that a verifier would most likely accept any of the child's credentials if they are provided by the child or if the credential above is provided with any of the child's credentials.

{
  "id": "http://example.org/credentials/23894",
  "type": ["VerifiableCredential", "RelationshipCredential"],
  "issuer": "http://example.org/credentials/23894",
  "issuanceDate": "2010-01-01T19:23:24Z",
  "credentialSubject": {
    "id": "did:example:ebfeb1f712ebc6f1c276e12ec21",
    "parent": {
      "id": "did:example:ebfeb1c276e12ec211f712ebc6f",
      "type": "Mother"
    }
  },
  "proof": { ... } // the proof is generated by the child
}
        

In the example above, the child expresses the relationship between the child and the parent in a separate credential such that a verifier would most likely accept any of the child's credentials if the credential above is provided.

Similarly, the strategies described in the examples above can be used for many other types of use cases, including power of attorney, pet ownership, and patient prescription pickup.

Subject Passes a Verifiable Credential to Someone Else

When a subject passes a verifiable credential to another holder, the subject might issue a new verifiable credential to the holder in which the:

The holder can now create a verifiable presentation containing these two verifiable credentials so that the verifier can verify that the subject gave the original verifiable credential to the holder.

{
  "id": "did:example:76e12ec21ebhyu1f712ebc6f1z2",
  "type": ["VerifiablePresentation"],
  "credential": [
    {
      "id": "http://example.gov/credentials/3732",
      "type": ["VerifiableCredential", "PrescriptionCredential"],
      "issuer": "https://example.edu",
      "issuanceDate": "2010-01-01",
      "claim": {
        "id": "did:example:ebfeb1f712ebc6f1c276e12ec21",
        "prescription": {....}
      },
      "revocation": {
        "id": "http://example.gov/revocations/738",
        "type": "SimpleRevocationList2017"
      },
      "proof": {....}
    },
    {
      "id": "https://example.com/VC/123456789",
      "type": ["VerifiableCredential", "PrescriptionCredential"],
      "issuer": "did:example:ebfeb1f712ebc6f1c276e12ec21",
      "issuanceDate": "2010-01-03",
      "claim": {
        "id": "did:example:76e12ec21ebhyu1f712ebc6f1z2",
        "prescription": {....}
      },
      "proof": {
        "type": "RsaSignature2018",
        "created": "2018-06-17T10:03:48Z",
        "verificationMethod": "did:example:ebfeb1f712ebc6f1c276e12ec21/keys/234",
        "nonce": "d61c4599-0cc2-4479-9efc-c63add3a43b2",
        "signatureValue": "pYw8XNi1..Cky6Ed="
      }
    }
  ],
  "proof": [{
    "type": "RsaSignature2018",
    "created": "2018-06-18T21:19:10Z",
    "verificationMethod": "did:example:76e12ec21ebhyu1f712ebc6f1z2/keys/2",
    "nonce": "c0ae1c8e-c7e7-469f-b252-86e6a0e7387e",
    "signatureValue": "BavEll0/I1..W3JT24="
  }]
}
      

In the above example, a patient (the original subject) passed a prescription (the original verifiable credential) to a friend, and issued a new verifiable credential to the friend, in which the friend is the subject, the original subject is the issuer, and the credential is a copy of the original prescription.

Issuer Authorises Holder

When an issuer wants to authorize a holder to possess a credential that describes a subject who is not the holder, and the holder has no known relationship with the subject, then the issuer inserts the relationship of the holder to itself into the credential for the subject.

Verifiable credentials are not an authorization framework and therefore delegation is out of scope for this specification. However, it is understood that verifiable credentials are likely to be used to build authorization and delegation systems. The following is one approach that may be appropriate for some use cases.

When an issuer wants to authorise a holder to possess a credential that describes a subject who is not the holder, and the holder has no known relationship with the subject, then the issuer might insert the relationship of the holder to itself into the subject's credential.


{
  "id": "http://example.edu/credentials/3732",
  "type": ["VerifiableCredential", "NameAndAddress"],
  "issuer": "https://example.edu/issuers/14",
  "holder": {
    "type": "LawEnforcement",
    "id": "did:example:ebfeb1276e12ec21f712ebc6f1c"
  },
  "issuanceDate": "2010-01-01",
  "credentialSubject": {
    "id": "did:example:ebfeb1f712ebc6f1c276e12ec21",
    "name": "Mr John Doe",
    "address": "10 Some Street, Anytown, ThisLocal, Country X"
  },
  "proof": {
    "type": "RsaSignature2018",
    "created": "2018-06-17T10:03:48Z",
    "verificationMethod": "https://example.edu/issuers/14/keys/234",
    "nonce": "d61c4599-0cc2-4479-9efc-c63add3a43b2",
    "signatureValue": "pY9...Cky6Ed = "
  }
}
        

Holder Acts on Behalf of the Verifier, or has no Relationship with the Subject, Issuer, or Verifier

The Verifiable Credentials Data Model currently does not support either of these scenarios. It is for further study how they might be supported.