Which of the following provides an efficient means of collecting verifiable historical data

This section is non-normative.

In the physical world, a credential might consist of:

• Information related to identifying the subject of the credential (for example, a photo, name, or identification number)
• Information related to the issuing authority (for example, a city government, national agency, or certification body)
• Information related to the type of credential this is (for example, a Dutch passport, an American driving license, or a health insurance card)
• Information related to specific attributes or properties being asserted by the issuing authority about the subject (for example, nationality, the classes of vehicle entitled to drive, or date of birth)
• Evidence related to how the credential was derived
• Information related to constraints on the credential (for example, expiration date, or terms of use).

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 verifiable presentations and then share these verifiable presentations with verifiers to prove they possess verifiable credentials with certain characteristics.

Both verifiable credentials and verifiable 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 7. Privacy Considerations. Examples of how to use this data model using privacy-enhancing technologies, such as zero-knowledge proofs, are also provided throughout this document.

The word "verifiable" in the terms verifiable credential and verifiable presentation refers to the characteristic of a credential or presentation as being able to be verified by a verifier, as defined in this document. Verifiability of a credential does not imply that the truth of claims encoded therein can be evaluated; however, the issuer can include values in the evidence property to help the verifier apply their business logic to determine whether the claims have sufficient veracity for their needs.

This section is non-normative.

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:

Note

Figure 1 above provides an example ecosystem in which 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.

This section is non-normative.

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

• A more thorough explanation of the roles introduced above
• The needs identified in market verticals, such as education, finance, healthcare, retail, professional licensing, and government
• Common tasks performed by the roles in the ecosystem, as well as their associated requirements
• Common sequences and flows identified by the Working Group.

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

• Credentials represent statements made by an issuer.
• Verifiable credentials represent statements made by an issuer in a tamper-evident and privacy-respecting manner.
• Holders assemble collections of credentials and/or verifiable credentials from different issuers into a single artifact, a presentation.
• Holders transform presentations into verifiable presentations to render them tamper-evident.
• Issuers can issue verifiable credentials about any subject.
• Acting as issuer, holder, or verifier requires neither registration nor approval by any authority, as the trust involved is bilateral between parties.
• Verifiable presentations allow any verifier to verify the authenticity of verifiable credentials from any issuer.
• Holders can receive verifiable credentials from anyone.
• Holders can interact with any issuer and any verifier through any user agent.
• Holders can share verifiable presentations, which can then be verified without revealing the identity of the verifier to the issuer.
• 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.
• Holders can present verifiable presentations to any verifier without affecting authenticity of the claims and without revealing that action to the issuer.
• A verifier can verify verifiable presentations from any holder, containing proofs of claims from any issuer.
• Verification should not depend on direct interactions between issuers and verifiers.
• Verification should not reveal the identity of the verifier to any issuer.
• 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.
• Issuers can issue verifiable credentials that support selective disclosure.
• If a single verifiable credential supports selective disclosure, then holders can present proofs of claims without revealing the entire verifiable credential.
• Verifiable presentations can either disclose the attributes of a verifiable credential, or satisfy derived predicates requested by the verifier. Derived predicates are Boolean conditions, such as greater than, less than, equal to, is in set, and so on.
• Issuers can issue revocable verifiable credentials.
• The processes of cryptographically protecting credentials and presentations, and verifying verifiable credentials and verifiable presentations, have to be deterministic, bi-directional, and lossless. Any verification of a verifiable credential or verifiable presentation has to be transformable to the generic data model defined in this document in a deterministic process, such that the resulting credential or presentation is semantically and syntactically equivalent to the original construct, so that it can be processed in an interoperable fashion.
• Verifiable credentials and verifiable presentations have to be serializable in one or more machine-readable data formats. The process of serialization and/or de-serialization has to be deterministic, bi-directional, and lossless. Any serialization of a verifiable credential or verifiable presentation needs to be transformable to the generic data model defined in this document in a deterministic process such that the resulting verifiable credential can be processed in an interoperable fashion. The serialized form also needs to be able to be generated from the data model without loss of data or content.
• The data model and serialization must be extendable with minimal coordination.
• Revocation by the issuer should not reveal any identifying information about the subject, the holder, the specific verifiable credential, or the verifier.
• Issuers can disclose the revocation reason.
• Issuers revoking verifiable credentials should distinguish between revocation for cryptographic integrity (for example, the signing key is compromised) versus revocation for a status change (for example, the driver’s license is suspended).
• Issuers can provide a service for refreshing a verifiable credential.

As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.

The key words MAY, MUST, MUST NOT, RECOMMENDED, and SHOULD in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

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 Sections 4. Basic Concepts, 5. Advanced Concepts, and 6. Syntaxes of this document MUST be enforced. A serialization format for the conforming document MUST be deterministic, bi-directional, and lossless as described in Section 6. Syntaxes. The conforming document MAY be transmitted or stored in any such serialization format.

A conforming processor is any algorithm realized as software and/or hardware that generates or consumes a conforming document. Conforming processors MUST produce errors when non-conforming documents are consumed.

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.

Digital proof mechanisms, a subset of which are digital signatures, are required to ensure the protection of a verifiable credential. Having and validating proofs, which may be dependent on the syntax of the proof (for example, using the JSON Web Signature of a JSON Web Token for proofing a key holder), are an essential part of processing a verifiable credential. At the time of publication, Working Group members had implemented verifiable credentials using at least three proof mechanisms:

• JSON Web Tokens [RFC7519] secured using JSON Web Signatures [RFC7515]
• Data Integrity Proofs [DATA-INTEGRITY]
• Camenisch-Lysyanskaya Zero-Knowledge Proofs [CL-SIGNATURES].

Implementers are advised to note that not all proof mechanisms are standardized as of the publication date of this specification. The group expects some of these mechanisms, as well as new ones, to mature independently and become standardized in time. Given there are multiple valid proof mechanisms, this specification does not standardize on any single digital signature mechanism. One of the goals of this specification is to provide a data model that can be protected by a variety of current and future digital proof mechanisms. Conformance to this specification does not depend on the details of a particular proof mechanism; it requires clearly identifying the mechanism a verifiable credential uses.

This document also contains examples that contain JSON and JSON-LD content. Some of these examples contain characters that are invalid JSON, such as inline comments (//) and the use of ellipsis (...) to denote information that adds little value to the example. Implementers are cautioned to remove this content if they desire to use the information as valid JSON or JSON-LD.

This section is non-normative.

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

The data model for claims, illustrated in Figure 2 above, is powerful and can be used to express a large variety of statements. For example, whether someone graduated from a particular university can be expressed as shown in Figure 3 below.

Individual claims can be merged together to express a graph of information about a subject. The example shown in Figure 4 below extends the previous claim by adding the claims that Pat knows Sam and that Sam is employed as a professor.

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

This section is non-normative.

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.

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

Note

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.

Figure 5 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. Figure 6 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 verifiable credential itself, which contains credential metadata and claims. The second graph expresses the digital proof, which is usually a digital signature.

Note

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.

This section is non-normative.

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 verifiable credentials are presented directly, they become verifiable presentations. Data formats derived from verifiable credentials that are cryptographically verifiable, but do not of themselves contain verifiable credentials, might also be verifiable presentations.

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

Figure 7 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.

Figure 8 below shows a more complete depiction of a verifiable presentation, which is normally composed of at least four information graphs. The first of these information graphs, the Presentation Graph, expresses the verifiable presentation itself, which contains presentation metadata. The verifiableCredential property in the Presentation Graph refers to one or more verifiable credentials, each being one of the second information graphs, i.e., a self-contained Credential Graph, which in turn contains credential metadata and claims. The third information graph, the Credential Proof Graph, expresses the credential graph proof, which is usually a digital signature. The fourth information graph, the Presentation Proof Graph, expresses the presentation graph proof, which is usually a digital signature.

Note

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

This section is non-normative.

The previous sections introduced the concepts of claims, verifiable credentials, and verifiable presentations using graphical depictions. This section provides a concrete set of simple but complete lifecycle examples of the data model expressed in one of the concrete syntaxes supported by this specification. The lifecycle of credentials and presentations in the Verifiable Credentials Ecosystem often take a common path:

1. Issuance of one or more verifiable credentials.
2. Storage of verifiable credentials in a credential repository (such as a digital wallet).
3. Composition of verifiable credentials into a verifiable presentation for verifiers.
4. Verification of the verifiable presentation by the verifier.

To illustrate this lifecycle, we will use the example of redeeming an alumni discount from a university. In the example below, Pat receives an alumni verifiable credential from a university, and Pat stores the verifiable credential in a digital wallet.

Example 1

: A simple example of a verifiable credential

Pat then attempts to redeem the alumni discount. The verifier, a ticket sales system, states that any alumni of "Example University" receives a discount on season tickets to sporting events. Using a mobile device, Pat starts the process of purchasing a season ticket. A step in this process requests an alumni verifiable credential, and this request is routed to Pat's digital wallet. The digital wallet asks Pat if they would like to provide a previously issued verifiable credential. Pat selects the alumni verifiable credential, which is then composed into a verifiable presentation. The verifiable presentation is sent to the verifier and verified.

Example 2

: A simple example of a verifiable presentation

Note

Implementers that are interested in understanding more about the proof mechanism used above can learn more in Section 4.7 Proofs (Signatures) and by reading the following specifications: Data Integrity [DATA-INTEGRITY], Linked Data Cryptographic Suites Registry [LDP-REGISTRY], and JSON Web Signature (JWS) Unencoded Payload Option [RFC7797]. A list of proof mechanisms is available in the Verifiable Credentials Extension Registry [VC-EXTENSION-REGISTRY].

When two software systems need to exchange data, they need to use terminology 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 might be referred to as the context of a conversation.

Verifiable credentials and verifiable presentations have many attributes and values that are identified by URIs [RFC3986]. However, those URIs can be long and not very human-friendly. In such cases, short-form human-friendly aliases can be more helpful. This specification uses the @context property to map such short-form aliases to the URIs required by specific verifiable credentials and verifiable presentations.

Note

In JSON-LD, the @context property can also be used to communicate other details, such as datatype information, language information, transformation rules, and so on, which are beyond the needs of this specification, but might be useful in the future or to related work. For more information, see Section 3.1: The Context of the [JSON-LD] specification.

Verifiable credentials and verifiable presentations MUST include a @context property.

Note

Though this specification requires that a @context property be present, it is not required that the value of the @context property be processed using JSON-LD. This is to support processing using plain JSON libraries, such as those that might be used when the verifiable credential is encoded as a JWT. All libraries or processors MUST ensure that the order of the values in the @context property is what is expected for the specific application. Libraries or processors that support JSON-LD can process the @context property using full JSON-LD processing as expected.

{ "@context": [ "//www.w3.org/2018/credentials/v1", "//www.w3.org/2018/credentials/examples/v1" ], "id": "//example.edu/credentials/58473", "type": ["VerifiableCredential", "AlumniCredential"], "issuer": "//example.edu/issuers/565049", "issuanceDate": "2010-01-01T00:00:00Z", "credentialSubject": { "id": "did:example:ebfeb1f712ebc6f1c276e12ec21", "alumniOf": { "id": "did:example:c276e12ec21ebfeb1f712ebc6f1", "name": [{ "value": "Example University", "lang": "en" }, { "value": "Exemple d'Université", "lang": "fr" }] } }, "proof": { } }

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

Note

This document uses the example context URI (//www.w3.org/2018/credentials/examples/v1) for the purpose of demonstrating examples. Implementations are expected to not use this URI for any other purpose, such as in pilot or production systems.

The data available at //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 //www.w3.org/2018/credentials/. This concept is further expanded on in Section 5.3 Extensibility.

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. This specification defines the optional id property for such identifiers. The id property is intended to unambiguously refer to an object, such as a person, product, or organization. Using the id property allows for the expression of statements about specific things in the verifiable credential.

If the id property is present:

• The id property MUST express an identifier that others are expected to use when expressing statements about a specific thing identified by that identifier.
• The id property MUST NOT have more than one value.
• The value of the id property MUST be a URI.

Note

Developers should remember that identifiers might be harmful in scenarios where pseudonymity is required. Developers are encouraged to read Section 7.3 Identifier-Based Correlation carefully when considering such scenarios. There are also other types of correlation mechanisms documented in Section 7. Privacy Considerations that create privacy concerns. Where privacy is a strong consideration, the id property MAY be omitted.

CredentialVerifiable Credential (with proof)Verifiable Credential (as JWT)

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

Software systems that process the kinds of objects specified in this document use type information to determine whether or not a provided verifiable credential or verifiable presentation is appropriate. This specification defines a type property for the expression of type information.

Verifiable credentials and verifiable presentations MUST have a type property. That is, any credential or presentation that does not have type property is not verifiable, so is neither a verifiable credential nor a verifiable presentation.

CredentialVerifiable Credential (with proof)Verifiable Credential (as JWT)

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

Note

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 5.3 Extensibility), 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.

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 a 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:

• Subject in the id property.
• Type of degree in the type property.
• Title of the degree in the name property.

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.

Note

The type system used in the data model described in this specification allows for multiple ways to associate types with data. Implementers and authors are urged to read the section on typing in the Verifiable Credentials Implementation Guidelines [VC-IMP-GUIDE].

A verifiable credential contains claims about one or more subjects. This specification defines a credentialSubject property for the expression of claims about one or more subjects.

A verifiable credential MUST have a credentialSubject property.

Example 6

: Usage of the credentialSubject property

CredentialVerifiable Credential (with proof)Verifiable Credential (as JWT)

It is possible to express information related to multiple subjects in a verifiable credential. The example below specifies two subjects who are spouses. Note the use of array notation to associate multiple subjects with the credentialSubject property.

Example 7

: Specifying multiple subjects in a verifiable credential

This specification defines a property for expressing the issuer of a verifiable credential.

A verifiable credential MUST have an issuer property.

CredentialVerifiable Credential (with proof)Verifiable Credential (as JWT)

It is also possible to express additional information about the issuer by associating an object with the issuer property:

CredentialVerifiable Credential (with proof)Verifiable Credential (as JWT)

Note

The value of the issuer property can also be a JWK (for example, "//example.com/keys/foo.jwk") or a DID (for example, "did:example:abfe13f712120431c276e12ecab").

This specification defines the issuanceDate property for expressing the date and time when a credential becomes valid.

CredentialVerifiable Credential (with proof)Verifiable Credential (as JWT)

Note

It is expected that the next version of this specification will add the validFrom property and will deprecate the issuanceDate property in favor of a new issued property. The range of values for both properties are expected to remain as [XMLSCHEMA11-2] combined date-time strings. Implementers are advised that the validFrom and issued properties are reserved and use for any other purpose is discouraged.

At least one proof mechanism, and the details necessary to evaluate that proof, MUST be expressed for a credential or presentation to be a verifiable credential or verifiable presentation; that is, to be verifiable.

This specification identifies two classes of proof mechanisms: external proofs and embedded proofs. An external proof is one that wraps an expression of this data model, such as a JSON Web Token, which is elaborated on in Section 6.3.1 JSON Web Token. An embedded proof is a mechanism where the proof is included in the data, such as a Linked Data Signature, which is elaborated upon in Section 6.3.2 Data Integrity Proofs.

When embedding a proof, the proof property MUST be used.

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.

Example 11

: Usage of the proof property on a verifiable credential

Note

As discussed in Section 1.4 Conformance, there are multiple viable proof mechanisms, and this specification does not standardize nor recommend any single proof mechanism for use with verifiable credentials. For more information about the proof mechanism, see the following specifications: Data Integrity [DATA-INTEGRITY], Linked Data Cryptographic Suites Registries [LDP-REGISTRY], and JSON Web Signature (JWS) Unencoded Payload Option [RFC7797]. A list of proof mechanisms is available in the Verifiable Credentials Extension Registry [VC-EXTENSION-REGISTRY].

This specification defines the expirationDate property for the expression of credential expiration information.

CredentialVerifiable Credential (with proof)Verifiable Credential (as JWT)

Note

It is expected that the next version of this specification will add the validUntil property in a way that deprecates, but preserves backwards compatibility with the expirationDate property. Implementers are advised that the validUntil property is reserved and its use for any other purpose is discouraged.

This specification defines the following credentialStatus property for the discovery of information about the current status of a verifiable credential, such as whether it is suspended or revoked.

• id property, which MUST be a URI.
• type property, which expresses the credential status type (also referred to as the credential status method). It is expected that the value will provide enough information to determine the current status of the credential and that machine readable information needs to be retrievable from the URI. For example, the object could contain a link to an external document noting whether or not the credential is 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.

CredentialVerifiable Credential (with proof)Verifiable Credential (as JWT)

Defining the data model, formats, and protocols for status schemes are out of scope for this specification. A Verifiable Credential Extension Registry [VC-EXTENSION-REGISTRY] exists that contains available status schemes for implementers who want to implement verifiable credential status checking.

Presentations MAY be used to combine and present credentials. They can be packaged in such a way that the authorship of the data is verifiable. The data in a presentation is often all about the same subject, but there is no limit to the number of subjects or issuers in the data. The aggregation of information from multiple verifiable credentials is a typical use of verifiable presentations.

A verifiable presentation is typically composed of the following properties:

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

{ "@context": [ "//www.w3.org/2018/credentials/v1", "//www.w3.org/2018/credentials/examples/v1" ], "id": "urn:uuid:3978344f-8596-4c3a-a978-8fcaba3903c5", "type": ["VerifiablePresentation", "CredentialManagerPresentation"], "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 Data Integrity [DATA-INTEGRITY] specification. An example of a verifiable presentation using the JWT proof mechanism is given in section 6.3.1 JSON Web Token.

This section is non-normative.

Section 1.2 Ecosystem Overview provided an overview of the verifiable credential ecosystem. This section provides more detail about how the ecosystem is envisaged to operate.

The roles and information flows in the verifiable credential ecosystem are as follows:

• An issuer issues a verifiable credential to a holder. Issuance always occurs before any other actions involving a credential.
• A holder might transfer one or more of its verifiable credentials to another holder.
• A holder presents one or more of its verifiable credentials to a verifier, optionally inside a verifiable presentation.
• A verifier verifies the authenticity of the presented verifiable presentation and verifiable credentials. This should include checking the credential status for revocation of the verifiable credentials.
• An issuer might revoke a verifiable credential.
• A holder might delete a verifiable credential.

Note

The order of the actions above is not fixed, and some actions might be taken more than once. Such action-recurrence might be immediate or at any later point.

The most common sequence of actions is envisioned to be:

1. An issuer issues to a holder.
2. The holder presents to a verifier.
3. The verifier verifies.

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

This specification also does not define an authorization framework nor the decisions that a verifier might make after verifying a verifiable credential or verifiable presentation, taking into account the holder, the issuers of the verifiable credentials, the contents of the verifiable credentials, and its own policies.

In particular, Sections 5.6 Terms of Use and C. Subject-Holder Relationships specify how a verifier can determine:

• Whether the holder is a subject of a verifiable credential.
• The relationship between the subject and the holder.
• Whether the original holder passed a verifiable credential to a subsequent holder.
• Any restrictions using the verifiable credentials by the holder or verifier.

This section is non-normative.

The verifiable credentials trust model is as follows:

• The verifier trusts the issuer to issue the credential that it received. To establish this trust, a credential is expected to either:
  • Include a proof establishing that the issuer generated the credential (that is, it is a verifiable credential), or
  • Have been transmitted in a way clearly establishing that the issuer generated the verifiable credential and that the verifiable credential was not tampered with in transit or storage. This trust could be weakened depending on the risk assessment of the verifier.
• All entities trust the verifiable data registry to be tamper-evident and to be a correct record of which data is controlled by which entities.
• The holder and verifier trust the issuer to issue true (that is, not false) credentials about the subject, and to revoke them quickly when appropriate.
• The holder trusts the repository to store credentials securely, to not release them to anyone other than the holder, and to not corrupt or lose them while they are in its care.

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

• Issuer and the verifier do not need to trust the repository
• Issuer does not need to know or trust the verifier.

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.

For more information about how this trust model interacts with various threat models studied by the Working Group, see the Verifiable Credentials Use Cases document [VC-USE-CASES].

Note

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.

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:

• Model complex multi-entity relationships through the use of a graph-based data model.
• Extend the machine-readable vocabularies used to describe information in the data model, without the use of a centralized system for doing so, through the use of [LINKED-DATA].
• Support multiple types of cryptographic proof formats through the use of Data Integrity Proofs [DATA-INTEGRITY] and a variety of signature suites listed in the Linked Data Cryptographic Suites Registry [LDP-REGISTRY]
• Provide all of the extensibility mechanisms outlined above in a data format that is popular with software developers and web page authors, and is enabled through the use of [JSON-LD].

This approach to data modeling 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 we start with the verifiable credential shown below.

CredentialVerifiable Credential (with proof)Verifiable Credential (as JWT)

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 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": "//example.com/vocab#referenceNumber", "favoriteFood": "//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 //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.

Example 17

: A verifiable credential with a custom extension

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.

Implementers are advised to pay close attention to the extension points in this specification, such as in Sections 4.7 Proofs (Signatures), 4.9 Status, 5.4 Data Schemas,5.5 Refreshing, 5.6 Terms of Use, and 5.7 Evidence. While this specification does not define concrete implementations for those extension points, the Verifiable Credentials Extension Registry [VC-EXTENSION-REGISTRY] provides an unofficial, curated list of extensions that developers can use from these extension points.

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:

• Data verification schemas, which are used to verify that the structure and contents of a credential or verifiable credential conform to a published schema.
• Data encoding schemas, which are used to map the contents of a verifiable credential to an alternative representation format, such as a binary format used in a zero-knowledge proof.

It is important to understand that data schemas serve a different purpose from the @context property, which neither enforces data structure or data syntax, nor enables the definition of arbitrary encodings to alternate representation formats.

This specification defines the following property for the expression of a data schema, which can be included by an issuer in the verifiable credentials that it issues:

Note

The credentialSchema property provides an opportunity to annotate type definitions or lock them to specific versions of the vocabulary. Authors of verifiable credentials can include a static version of their vocabulary using credentialSchema that is locked to some content integrity protection mechanism. The credentialSchema property also makes it possible to perform syntactic checking on the credential and to use verification mechanisms such as JSON Schema [JSON-SCHEMA-2018] validation.

Example 18

: Usage of the credentialSchema property to perform JSON schema validation

CredentialVerifiable Credential (with proof)Verifiable Credential (as JWT)

In the example above, the issuer is specifying a credentialSchema, which points to a [JSON-SCHEMA-2018] file that can be used by a verifier to determine if the verifiable credential is well formed.

Data schemas can also be used to specify mappings to other binary formats, such as those used to perform zero-knowledge proofs. For more information on using the credentialSchema property with zero-knowledge proofs, see Section 5.8 Zero-Knowledge Proofs.

Example 19

: Usage of the credentialSchema property to perform zero-knowledge validation

In the example above, the issuer is specifying a credentialSchema pointing 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.

It is useful for systems to enable the manual or automatic refresh of an expired verifiable credential. For more information about expired verifiable credentials, see Section 4.8 Expiration. This specification defines a refreshService property, which 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. In the latter case, this enables the holder to refresh the verifiable credential before creating a verifiable presentation to share with a verifier. In the former case, including the refresh service inside the verifiable credential enables either the holder or the verifier to perform future updates of the credential.

The refresh service is only expected to be used when either the credential has expired or the issuer does not publish credential status information. Issuers are advised not to put the refreshService property in a verifiable credential that does not contain public information or whose refresh service is not protected in some way.

Example 20

: Usage of the refreshService property by an issuer

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

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 verifiable credential. The holder places their terms of use inside a verifiable presentation. This specification defines a termsOfUse property for expressing terms of use information.

The value of the termsOfUse property tells the verifier what actions it is required to perform (an obligation), not allowed to perform (a prohibition), or allowed to perform (a permission) if it is to accept the verifiable credential or verifiable presentation.

Example 21

: Usage of the termsOfUse property by an issuer

CredentialVerifiable Credential (with proof)Verifiable Credential (as JWT)

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

Example 22

: Usage of the termsOfUse property by a holder

Note

Warning: The termsOfUse property is improperly defined within the VerifiablePresentation scoped context. This is a bug with the version 1 context and will be fixed in the version 2 context. In the meantime, implementors who wish to use this feature will be required to extend the context of their verifiable presentation with an additional term that defines the termsOfUse property, which can then be used alongside the verifiable presentation type property, in order for the term to be semantically recognized in a JSON-LD processor.

In the example above, the holder (the assigner), who is also the subject, expressed a term of use prohibiting the verifier (the assignee, //wineonline.example.org) from using the information provided to correlate the holder or 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.

This feature is also expected to be used by government-issued verifiable credentials to instruct digital wallets to limit their use to similar government organizations in an attempt to protect citizens from unexpected usage of sensitive data. Similarly, some verifiable credentials issued by private industry are expected to limit usage to within departments inside the organization, or during business hours. Implementers are urged to read more about this rapidly evolving feature in the appropriate section of the Verifiable Credentials Implementation Guidelines [VC-IMP-GUIDE] document.

Evidence can be included by an issuer to provide the verifier with 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 could check physical documentation provided by the subject or 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.

This specification defines the evidence property for expressing evidence information.

Note

For information about how attachments and references to credentials and non-credential data might be supported by the specification, see the Verifiable Credentials Implementation Guidelines [VC-IMP-GUIDE] document.

{ "@context": [ "//www.w3.org/2018/credentials/v1", "//www.w3.org/2018/credentials/examples/v1" ], "id": "//example.edu/credentials/3732", "type": ["VerifiableCredential", "UniversityDegreeCredential"], "issuer": "//example.edu/issuers/14", "issuanceDate": "2010-01-01T19:23:24Z", "credentialSubject": { "id": "did:example:ebfeb1f712ebc6f1c276e12ec21", "degree": { "type": "BachelorDegree", "name": "Bachelor of Science and Arts" } }, "evidence": [{ "id": "//example.edu/evidence/f2aeec97-fc0d-42bf-8ca7-0548192d4231", "type": ["DocumentVerification"], "verifier": "//example.edu/issuers/14", "evidenceDocument": "DriversLicense", "subjectPresence": "Physical", "documentPresence": "Physical", "licenseNumber": "123AB4567" }], "proof": { } }

Note

In this evidence example, the issuer is asserting that they physically matched the subject of the credential to a physical copy of a driver's license with the stated license number. This driver's license was used in the issuance process to verify that "Example University" verified the subject before issuance of the credential and how they did so (physical verification).

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.

The key capabilities introduced by zero-knowledge proof mechanisms are the ability of a holder to:

• Combine multiple verifiable credentials from multiple issuers into a single verifiable presentation without revealing verifiable credential or subject identifiers to the verifier. This makes it more difficult for the verifier to collude with any of the issuers regarding the issued verifiable credentials.
• Selectively disclose the claims in a verifiable credential to a verifier without requiring the issuance of multiple atomic verifiable credentials. This allows a holder to provide a verifier with precisely the information they need and nothing more.
• Produce a derived verifiable credential that is formatted according to the verifier's data schema instead of the issuer's, without needing to involve the issuer after verifiable credential issuance. This provides a great deal of flexibility for holders to use their issued verifiable credentials.

This specification describes a data model that supports selective disclosure with the use of zero-knowledge proof mechanisms. The examples below highlight how the data model can be used to issue, present, and verify zero-knowledge verifiable credentials.

For a holder to use a zero-knowledge verifiable presentation, they need an issuer to have issued a verifiable credential in a manner that enables the holder to derive a proof from the originally issued verifiable credential, so that the holder can present the information to a verifier in a privacy-enhancing manner. This implies that the holder can prove the validity of the issuer's signature without revealing the values that were signed, or when only revealing certain selected values. The standard practice is to do so by proving knowledge of the signature, without revealing the signature itself. There are two requirements for verifiable credentials when they are to be used in zero-knowledge proof systems.

• The verifiable credential MUST contain a Proof, using the proof property, so that the holder can derive a verifiable presentation that reveals only the information than the holder intends to reveal.
• If a credential definition is being used, the credential definition MUST be defined in the credentialSchema property, so that it can be used by all parties to perform various cryptographic operations in zero-knowledge.

The following example shows one method of using verifiable credentials in zero-knowledge. It makes use of a Camenisch-Lysyanskaya Signature [CL-SIGNATURES], which allows the presentation of the verifiable credential in a way that supports the privacy of the holder and subject through the use of selective disclosure of the verifiable credential values. Some other cryptographic systems which rely upon zero-knowledge proofs to selectively disclose attributes can be found in the [LDP-REGISTRY] as well.

Example 24

: A verifiable credential that supports CL Signatures

The example above provides the verifiable 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, so that the verifiable credential discloses only the claims and additional credential metadata that the holder intended. To do this, all of the following requirements are expected to be met:

• Each derived verifiable credential within a verifiable presentation MUST contain all information necessary to verify the verifiable credential, either by including it directly within the credential, or by referencing the necessary information.
• A verifiable presentation MUST NOT leak information that would enable the verifier to correlate the holder across multiple verifiable presentations.
• The verifiable presentation SHOULD contain a proof property to enable the verifier to check that all derived verifiable credentials in the verifiable presentation were issued to the same holder without leaking personally identifiable information that the holder did not intend to share.

Example 25

: A verifiable presentation that supports CL Signatures

Note

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 who want to extend verifiable credentials and verifiable presentations to support zero-knowledge proof systems.

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

• A subject disputes a claim made by the issuer. For example, the address property is incorrect or out of date.
• An entity disputes a potentially false claim made by the issuer about a different subject. For example, an imposter claims the social security number for an entity.

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 //example.org/credentials/245 is disputed, the subject can issue the credential shown below and present it to the verifier along with the disputed credential.

{ "@context": [ "//www.w3.org/2018/credentials/v1", "//www.w3.org/2018/credentials/examples/v1" ], "id": "//example.com/credentials/123", "type": ["VerifiableCredential", "DisputeCredential"], "credentialSubject": { "id": "//example.com/credentials/245", "currentStatus": "Disputed", "statusReason": { "value": "Address is out of date.", "lang": "en" }, }, "issuer": "//example.com/people#me", "issuanceDate": "2017-12-05T14:27:42Z", "proof": { } }

In the above verifiable credential the issuer is claiming that the address in the disputed verifiable credential is wrong.

Note

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.

Note

This area of study is rapidly evolving and developers that are interested in publishing credentials that dispute the veracity of other credentials are urged to read the section related to disputes in the Verifiable Credentials Implementation Guidelines [VC-IMP-GUIDE] document.

This section is non-normative.

Verifiable credentials are intended as a means of reliably identifying subjects. While it is recognized that Role Based Access Controls (RBACs) and Attribute Based Access Controls (ABACs) 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.

The data model, as described in Section 3. Core Data Model, can be encoded in JavaScript Object Notation (JSON) [RFC8259] by mapping property values to JSON types as follows:

• Numeric values representable as IEEE754 SHOULD be represented as a Number type.
• Boolean values SHOULD be represented as a Boolean type.
• Sequence value SHOULD be represented as an Array type.
• Unordered sets of values SHOULD be represented as an Array type.
• Sets of properties SHOULD be represented as an Object type.
• Empty values SHOULD be represented as a null value.
• Other values MUST be represented as a String type.

Note

As the transformations listed herein have potentially incompatible interpretations, additional profiling of the JSON format is required to provide a deterministic transformation to the data model.

[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 6.1 JSON), 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 5.3 Extensibility.

Multiple contexts MAY be used or combined to express any arbitrary information about verifiable credentials in idiomatic JSON. The JSON-LD context, available at //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 //www.w3.org/2018/credentials.

The data model described in this specification is designed to be proof format agnostic. This specification does not normatively require any particular digital proof or signature format. While the data model is the canonical representation of a credential or presentation, the proofing mechanisms for these are often tied to the syntax used in the transmission of the document between parties. As such, each proofing mechanism has to specify whether the verification of the proof is calculated against the state of the document as transmitted, against the possibly transformed data model, or against another form. At the time of publication, at least two proof formats are being actively utilized by implementers and the Working Group felt that documenting what these proof formats are and how they are being used would be beneficial to implementers. The sections detailing the current proof formats being actively utilized to issue verifiable credentials are:

• Section 6.3.1 JSON Web Token, and
• Section 6.3.2 Data Integrity Proofs.

This section is non-normative.

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.

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.

Note

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.

This section is non-normative.

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 verifiable credentials when possible. For example, issuing ageOver verifiable credentials instead of date of birth verifiable credentials when a verifier wants to determine if an entity is over the age of 18.

Because a verifiable credential often contains personally identifiable information (PII), implementers are strongly advised to use mechanisms while storing and transporting verifiable credentials that protect the data from those who should not access it. Mechanisms that could be considered include Transport Layer Security (TLS) or other means of encrypting the data while in transit, as well as encryption or data access control mechanisms to protect the data in a verifiable credential while at rest.

This section is non-normative.

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 verifiable credential schemes that allow hiding the identifier during verifiable 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 verifiable credential.

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

• Bound to a single origin
• Single-use
• Not used at all, but instead replaced by short-lived, single-use bearer tokens.

This section is non-normative.

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 verificationMethod, created, proofPurpose, and jws 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.

Note

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

This section is non-normative.

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, verifiable credential-specific JSON-LD contexts, and many other sorts of long-lived identifiers.

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

This section is non-normative.

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.

This section is non-normative.

To enable recipients of verifiable credentials to use them in a variety of circumstances without revealing more PII 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 PII 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 PII. 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 verifiable credential claiming that subjects have met that requirement instead of offering verifiable credentials containing claims about specific birthdates. This enables individual customers to make purchases without revealing specific PII.

This section is non-normative.

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 verifiable 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 verifiable credential containing every attribute that appears on a driver's license, as well as a set of verifiable credentials where every verifiable credential contains only a single attribute, such as a person's birthday. It could also issue more abstract verifiable credentials (for example, a verifiable 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 verifiable 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:

• Reduces the liability on the verifier for handling highly sensitive information that it does not need to.
• Enhances the privacy of the individual by only asking for information required for a specific transaction.

Note

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.

This section is non-normative.

A bearer credential is a privacy-enhancing piece of information, such as a concert ticket, which entitles the holder of the bearer credential to a specific resource without divulging sensitive information about the holder. Bearer credentials are often used in low-risk use cases where the sharing of the bearer credential is not a concern or would not result in large economic or reputational losses.

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:

CredentialVerifiable Credential (with proof)Verifiable Credential (as JWT)

While bearer credentials can be privacy-enhancing, they must be carefully crafted so as not accidentally divulge more information than the holder of the bearer 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 bearer credential or session.

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

• Are single-use, where possible.
• Do not contain personally identifying information.
• Are not unduly correlatable.

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.

This section is non-normative.

When processing verifiable credentials, verifiers are expected to perform many of the checks listed in Appendix A. Validation as well as a variety of specific business process checks. Validity checks might include checking:

• The professional licensure status of the holder.
• A date of license renewal or revocation.
• The sub-qualifications of an individual.
• If a relationship exists between the holder and the entity with whom the holder is attempting to interact.
• The geolocation information associated with the holder.

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 such as 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.

This section is non-normative.

When a holder receives a verifiable credential from an issuer, the verifiable 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.

Some effective mitigations for data mining and profiling include using:

• Service providers that do not sell your information to third parties.
• Software that encrypts verifiable credentials such that a service provider cannot view the contents of the credential.
• Software that stores verifiable credentials locally on a device that you control and that does not upload or analyze your information beyond your expectations.

This section is non-normative.

Holding two pieces of information about the same subject almost always reveals more about the subject than just the sum of the two pieces, even when the information is delivered through different channels. The aggregation of verifiable 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 as well as age-related information for that 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 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 tends 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.

This section is non-normative.

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:

• The same verifiable credential is presented to the same verifier more than once. The verifier could infer that the holder is the same individual.
• The same verifiable credential is presented to different verifiers, and either those verifiers collude or a third party has access to transaction records from both verifiers. An observant party could infer that the individual presenting the verifiable credential is the same person at both services. That is, the accounts are controlled by the same person.
• A subject identifier of a credential refers to the same subject across multiple presentations or verifiers. Even when different credentials are presented, if the subject identifier is the same, verifiers (and those with access to verifier logs) could infer that the holder of the credential is the same person.
• The underlying information in a credential can be used to identify an individual across services. In this case, using information from other sources (including information provided directly by the holder), verifiers can use information inside the credential to correlate the individual with an existing profile. For example, if a holder presents credentials that include postal code, age, and gender, a verifier can potentially correlate the subject of that credential with an established profile. For more information, see [DEMOGRAPHICS].
• Passing the identifier of a credential to a centralized revocation server. The centralized server can correlate the credential usage across interactions. For example, if a credential is used for proof of age in this manner, the centralized service could know everywhere that credential was presented (all liquor stores, bars, adult stores, lottery purchases, and so on).

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

• Using a globally-unique identifier as the subject for any given credential and never re-use that credential.
• If the credential supports revocation, using a globally-distributed service for revocation.
• Designing revocation APIs that do not depend on submitting the ID of the credential. For example, use a revocation list instead of a query.
• Avoiding the association of personally identifiable information with any specific long-lived subject identifier.

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 credentials.

This section is non-normative.

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.

This section is non-normative.

As detailed in Section 7.13 Usage Patterns, 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 verifiable credentials short lived and renewal automatic.

For example, an ageOver verifiable credential is useful for gaining access to a bar. If an issuer issues such a verifiable 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.

This section is non-normative.

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 verifiable credentials whenever possible.

Using single-use verifiable credentials provides several benefits. The first benefit is to verifiers who can be sure that the data in a verifiable credential is fresh. The second benefit is to holders, who know that if there are no long-lived identifiers in the verifiable credential, the verifiable 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.

This section is non-normative.

In an ideal private browsing scenario, no PII will be revealed. Because many credentials include PII, organizations providing software to holders should warn them about the possibility of revealing this information if they wish to use credentials and presentations while in private browsing mode. As each browser vendor handles private browsing differently, and some browsers might not have this feature at all, it is important for implementers to be aware of these differences and implement solutions accordingly.

This section is non-normative.

It cannot be overstated that verifiable credentials rely on a high degree of trust in issuers. The degree to which a holder might take advantage of possible privacy protections often depends strongly on the support an issuer provides for such features. In many cases, privacy protections which make use of zero-knowledge proofs, data minimization techniques, bearer credentials, abstract claims, and protections against signature-based correlation, require the issuer to actively support such capabilities and incorporate them into the verifiable credentials they issue.

It should also be noted that, in addition to a reliance on issuer participation to provide verifiable credential capabilities that help preserve holder and subject privacy, holders rely on issuers to not deliberately subvert privacy protections. For example, an issuer might sign verifiable credentials using a signature scheme that protects against signature-based correlation. This would protect the holder from being correlated by the signature value as it is shared among verifiers. However, if the issuer creates a unique key for each issued credential, it might be possible for the issuer to track presentations of the credential, regardless of a verifier's inability to do so.

This section is non-normative.

Some aspects of the data model described in this specification can be protected through the use of cryptography. It is important for implementers to understand the cryptography suites and libraries used to create and process credentials and presentations. Implementing and auditing cryptography systems generally requires substantial experience. Effective red teaming can also help 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 need to take this into account and ensure mechanisms exist to easily and proactively upgrade expired or broken cryptography suites and libraries, and to invalidate and replace existing credentials. Regular monitoring is important to ensure the long term viability of systems processing credentials.

This section is non-normative.

Verifiable credentials often contain URLs to data that resides outside of the verifiable credential itself. Linked content that exists outside a verifiable credential, such as 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 but probably should be:

{ "@context": [ "//www.w3.org/2018/credentials/v1", "//www.w3.org/2018/credentials/examples/v1" ], "id": "//example.edu/credentials/58473", "type": ["VerifiableCredential", "AlumniCredential"], "credentialSubject": { "id": "did:example:ebfeb1f712ebc6f1c276e12ec21", "image": "//example.edu/images/58473", "alumniOf": { "id": "did:example:c276e12ec21ebfeb1f712ebc6f1", "name": [{ "value": "Example University", "lang": "en" }, { "value": "Exemple d'Université", "lang": "fr" }] } }, "proof": { } }

While this specification does not recommend any specific content integrity protection, document authors who want to ensure 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 [IPFS]. The example below transforms the previous example 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.

Example 35

: Content-integrity protection for links to external data

Note

It is debatable whether the JSON-LD Contexts above need protection because production implementations are expected to 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 might be better suited for certain applications. Implementers are urged to understand how links to external machine-readable content that are not content-integrity protected could result in successful attacks against their applications.

This section is non-normative.

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 be aware that these types of credentials are not verifiable because the authorship either is not known or cannot be trusted.

This section is non-normative.

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. Approaches such as token binding [RFC8471], which ties the request for a verifiable presentation to the response, can secure the protocol. Any unsecured protocol is susceptible to man-in-the-middle attacks.

This section is non-normative.

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 verifiable credentials to a person, each containing two properties, which must be taken together to to designate the "role" of that person in a given "department", such as "Staff Member" in the "Department of Computing", or "Post Graduate Student" in the "Department of Economics". If these verifiable credentials are atomized to put only one of these properties into each credential , then the university would issue four credentials to the person, each containing one of the following designations: "Staff Member", "Post Graduate Student", "Department of Computing", and "Department of Economics". The holder might then transfer the "Staff Member" and "Department of Economics" verifiable credentials to a verifier, which together would comprise a false claim.

This section is non-normative.

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 verifiable credential is valid might create exploitable security vulnerabilities. Expiration periods shorter than the timeframe where the information expressed by the verifiable credential is valid creates a burden on holders and verifiers. It is therefore important to set validity periods for verifiable credentials that are appropriate to the use case and the expected lifetime for the information contained in the verifiable credential.

This section is non-normative.

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:

• Enabling password, pin, pattern, or biometric screen unlock protection on the device.
• Enabling password, biometric, or multi-factor authentication for the credential repository.
• Enabling password, biometric, or multi-factor authentication when accessing cryptographic keys.
• Using a separate hardware-based signature device.
• All or any combination of the above.

This section is non-normative.

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 relying on a viewer's interpretation of 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.

This section is non-normative.

Data publishers are strongly encouraged to read the section on Cross-Syntax Expression in the Strings on the Web: Language and Direction Metadata document [STRING-META] to ensure that the expression of language and base direction information is possible across multiple expression syntaxes, such as [JSON-LD], [JSON], and CBOR [RFC7049].

The general design pattern is to use the following markup template when expressing a text string that is tagged with a language and, optionally, a specific base direction.

Example 36

: Design pattern for natural language strings

Using the design pattern above, the following example expresses the title of a book in the English language without specifying a text direction.

Example 37

: Expressing natural language text as English

The next example uses a similar title expressed in the Arabic language with a base direction of right-to-left.

Example 38

: Arabic text with a base direction of right-to-left

Note

The text above would most likely be rendered incorrectly as left-to-right without the explicit expression of language and direction because many systems use the first character of a text string to determine text direction.

Implementers utilizing JSON-LD are strongly urged to extend the JSON-LD Context defining the internationalized property and use the Scoped Context feature of JSON-LD to alias the @value, @language, and @direction keywords to value, lang, and dir, respectively. An example of a JSON-LD Context snippet doing this is shown below.

Example 39

: Specifying scoped aliasing for language information

This section is non-normative.

When multiple languages, base directions, and annotations are used in a single natural language string, more complex mechanisms are typically required. It is possible to use markup languages, such as HTML, to encode text with multiple languages and base directions. It is also possible to use the rdf:HTML datatype to encode such values accurately in JSON-LD.

Despite the ability to encode information as HTML, implementers are strongly discouraged from doing this because it:

• Requires some version of an HTML processor, which increases the burden of processing language and base direction information.
• Increases the security attack surface when utilizing this data model because blindly processing HTML could result in executing a script tag that an attacker injected at some point during the data production process.

If implementers feel they must use HTML, or other markup languages capable of containing executable scripts, to address a specific use case, they are advised to analyze how an attacker would use the markup to mount injection attacks against a consumer of the markup and then deploy mitigations against the identified attacks.

This section is non-normative.

In the verifiable credentials presented by a holder, the value associated with the id property for each credentialSubject is expected to identify a subject to the verifier. If the holder is also the subject, then the verifier could authenticate the holder if they have public key metadata related to the holder. The verifier could then authenticate the holder using a signature generated by the holder contained in the verifiable presentation. The id property is optional. Verifiers could use other properties in a verifiable credential to uniquely identify a subject.

Note

For information on how authentication and WebAuthn might work with verifiable credentials, see the Verifiable Credentials Implementation Guidelines [VC-IMP-GUIDE] document.

This section is non-normative.

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

Relevant metadata about the issuer property is expected to 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 credentials.

This section is non-normative.

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

This section is non-normative.

The cryptographic mechanism used to prove that the information in a verifiable credential or verifiable presentation was not 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 are expected to ensure:

• The proof is available in the form of a known proof suite.
• All required proof suite properties are present.
• The proof suite verification algorithm, when applied to the data, results in an acceptable proof.

Some proofs are digital signatures. In general, when verifying digital signatures, implementations are expected to ensure:

• Acceptably recent metadata regarding the public key associated with the signature is available. For example, the metadata might include properties related to expiration, key owner, or key purpose.
• The key is not suspended, revoked, or expired.
• The cryptographic signature is expected to verify.
• If the cryptographic suite expects a proofPurpose property, it is expected to exist and be a valid value, such as assertionMethod.

Note

The digital signature provides a number of protections, other than tamper resistance, which 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 verificationMethod property specifies, for example, the public key that can be used to verify the digital signature. Dereferencing a public key URL reveals information about the controller of the key, which can be checked against the issuer of the credential. The proofPurpose property clearly expresses the purpose for the proof and ensures this information is protected by the signature. A proof is typically attached to a verifiable presentation for authentication purposes and to a verifiable credential as a method of assertion.

This section is non-normative.

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

This section is non-normative.

If the credentialStatus property is available, the status of a verifiable credential is expected to be evaluated by the verifier according to the credentialStatus type definition for the verifiable credential and the verifier's own status evaluation criteria. For example, a verifier can ensure the status of the verifiable credential is not "withdrawn for cause by the issuer".

This section is non-normative.

Fitness for purpose is about whether the custom properties in the verifiable 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 a specific birthdate property, or on more 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 the 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.

This section is non-normative.

The base context, located at //www.w3.org/2018/credentials/v1 with a SHA-256 digest of ab4ddd9a531758807a79a5b450510d61ae8d147eab966cc9a200c07095b0cdcc, can be used to implement a local cached copy. For convenience, the base context is also provided in this section.

This section is non-normative.

The verifiable credential and verifiable presentation data models leverage a variety of underlying technologies including [JSON-LD] and [JSON-SCHEMA-2018]. This section will provide a comparison of the @context, type, and credentialSchema properties, and cover some of the more specific use cases where it is possible to use these features of the data model.

The type property is used to uniquely identify the type of the verifiable credential in which it appears, i.e., to indicate which set of claims the verifiable credential contains. This property, and the value VerifiableCredential within the set of its values, are mandatory. Whilst it is good practice to include one additional value depicting the unique subtype of this verifiable credential, it is permitted to either omit or include additional type values in the array. Many verifiers will request a verifiable credential of a specific subtype, then omitting the subtype value could make it more difficult for verifiers to inform the holder which verifiable credential they require. When a verifiable credential has multiple subtypes, listing all of them in the type property is sensible. While the semantics are the same in both a [JSON] and [JSON-LD] representation, the usage of the type property in a [JSON-LD] representation of a verifiable credential is able to enforce the semantics of the verifiable credential better than a [JSON] representation of the same credential because the machine is able to check the semantics. With [JSON-LD], the technology is not only describing the categorization of the set of claims, the technology is also conveying the structure and semantics of the sub-graph of the properties in the graph. In [JSON-LD], this represents the type of the node in the graph which is why some [JSON-LD] representations of a verifiable credential will use the type property on many objects in the verifiable credential.

The primary purpose of the @context property, from a [JSON-LD] perspective, is to convey the meaning of the data and term definitions of the data in a verifiable credential, in a machine readable way. When encoding a pure [JSON] representation, the @context property remains mandatory and provides some basic support for global semantics. The @context property is used to map the globally unique URIs for properties in verifiable credentials and verifiable presentations into short-form alias names, making both the [JSON] and [JSON-LD] representations more human-friendly to read. From a [JSON-LD] perspective, this mapping also allows the data in a credential to be modeled in a network of machine-readable data, by enhancing how the data in the verifiable credential or verifiable presentation relates to a larger machine-readable data graph. This is useful for telling machines how to relate the meaning of data to other data in an ecosystem where parties are unable to coordinate. This property, with the first value in the set being //www.w3.org/2018/credentials/v1, is mandatory.

Since the @context property is used to map data to a graph data model, and the type property in [JSON-LD] is used to describe nodes within the graph, the type property becomes even more important when using the two properties in combination. For example, if the type property is not included within the resolved @context resource using [JSON-LD], it could lead to claims being dropped and/or their integrity no longer being protected during production and consumption of the verifiable credential. Alternatively, it could lead to errors being raised during production or consumption of a verifiable credential. This will depend on the design choices of the implementation and both paths are used in implementations today, so it's important to pay attention to these properties when using a [JSON-LD] representation of a verifiable credential or verifiable presentation.

The primary purpose of the credentialSchema property is to define the structure of the verifiable credential, and the datatypes for the values of each property that appears. A credentialSchema is useful for defining the contents and structure of a set of claims in a verifiable credential, whereas [JSON-LD] and a @context in a verifiable credential are best used only for conveying the semantics and term definitions of the data, and can be used to define the structure of the verifiable credential as well.

While it is possible to use some [JSON-LD] features to allude to the contents of the verifiable credential, it's not generally suggested to use @context to constrain the data types of the data model. For example, "@type": "@json" is useful for leaving the semantics open-ended and not strictly defined. This can be dangerous if the implementer is looking to constrain the data type of the claims in the credential, and is expected not to be used.

When the credentialSchema and @context properties are used in combination, both producers and consumers can be more confident about the expected contents and data types of the verifiable credential and verifiable presentation.

This section is non-normative.

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 can insert the nonTransferable property into the verifiable credential, as described below.

This section is non-normative.

In this case, the credentialSubject property might contain multiple properties, each providing an aspect of a description of the subject, which combine together to 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.

Example 41

: A credential uniquely identifying a subject

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

This section is non-normative.

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 letting the subject issue a new verifiable credential and give 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 C.5 Subject Passes a Verifiable Credential to Someone Else.

This section is non-normative.

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

• Issuer can include the relationship between the holder and the subject in the credentialSubject property.
• Issuer can express the relationship between the holder and the subject by issuing a new verifiable credential, which the holder utilizes.
• Subject can express their relationship with the holder by issuing a new verifiable credential, which the holder utilizes.

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.

Note

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.

Example 42

: The relationship property in a child's credential

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.

Example 43

: A relationship credential issued to a parent

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.

Example 44

: A relationship credential issued by a 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.

This section is non-normative.

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

• Issuer is the subject.
• Subject is the holder to whom the verifiable credential is being passed.
• Claim contains the properties being passed on.

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.

Example 45

: A holder presenting a verifiable credential that was passed to it by the subject

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 subject of the original verifiable credential is the issuer, and the credential is a copy of the original prescription.

This section is non-normative.

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 might insert the relationship of the holder to itself into the subject's credential.

Note

Verifiable credentials are not an authorization framework and therefore delegation is outside the scope of 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 might be appropriate for some use cases.

Example 46

: A credential issued to a holder who is not the (only) subject of the credential, who has no relationship with the subject of the credential, but who has a relationship with the issuer

This section is non-normative.

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