Verifier Universal Interface

Definition of the standards and interfaces that might be consumed by any verifier to be interoperable with any wallet

Decentralized digital identity, or "self-sovereign" identity (SSI), is a new paradigm proposed for managing digital identities. Relying on Decentralized identifiers (DIDs) and Verifiable Credentials (VC), the subject and his personal credentials are independent from any centralized registry, identity provider, or certificate authority.

On the Decentralized Identity model, the role associated to requesting and verifying credentials is defined as the Verifier.

The verifier functionality is to support the data collector as it tries to acquire credentials from some other party for the purpose of negotiating a business transaction. It does so by creating presentation requests (or Presentation Definition as it is called in the draft DIF specfication for Presentation Exchange) that ask for such credentials, sending them to a holder component of another party, receiving a response to such a request (which we call a 'presentation'), verifying the presentation, i.e. checking the signature and other proofs of the veracity of both the construction of the presentation as well as its contents, thus providing the party with verified data.

This document specifies the interfaces that MAY be consumed by any verifier allowing him to be interoperable with any wallet fulfilling this interface, regardless of:

Despite its format, this document does not aim to be published on the W3C as a standard normative, rather it aims to propose a universal interfaces for a verifier. However, this format is however extremely adequate to allow work and contribution on the protocol definition.

Context

As different technology providers are offering SSI solutions, it becomes increasingly relevant to ensure interoperability between them. On a not yet stable market, most providers aim to offer the whole SSI stack: components for holders, verifiers, issuers and did-resolver.

In order to avoid vendor lock-ins, the APIs exploited by the Verifiers component can be standarized. This document aims to offer the necessary capabilities for a Verifier component of a technology provider to use the technology stack of another SSI provider.

This document presents an overview of all the different interfaces covered by the Verifier Universal Interface working group. It refers to each of their specs and tries to provide (if needed) additional information on how they relate.

Terminology

Decentralized Identifier(DID)
As defined in [[DID-CORE]].
DID document
As defined in [[DID-CORE]].
DID resolution
As defined in [[DID-RESOLUTION]].
DID resolver
As defined in [[DID-RESOLUTION]].
Verifiable Credential(VC)
As defined in [[VC-DATA-MODEL]].
Verifiable Presentation(VP)
As defined in [[VC-DATA-MODEL]].

Verifier operations

Any Verifier MUST perform the following operations:

  1. Request and receive specific information to holders in the form of Verifiable Credentials (presented in a Verifiable Presentation)
  2. Verify received VCs. That includes:
    1. Verify the proofs of every credential
    2. Verify the proof of the presentation
    3. Verify that the information received matches the information requested
Additionally, any Verifier SHALL perform the following operations:
  1. While verifying the received VCs:
    1. Verify the Issuer of the credential
      1. Its authority over the credential
      2. Its associated trust level
    2. Verify the status of every credential
    3. Verify and match the ownership of the Verifiable Credentials and the Verifiable Presentation
    4. Verify that all credentials can be linked to the same entity
    5. Verify that the credentials requested satisfy some stablished constraints
  2. Verify that the credentials have been handled from a secure wallet
  3. Authenticate the wallet owner
  4. Manage the consent of the usage of the information inside the credentials
  5. Ensure user anonimity and non external traceability
  6. Manage selective disclosure of the information contained on the credentials

Interfaces

In order to allow Verifier operation as decribed in the previous section, this document proposes a definition for the Interfaces between the Verifier and other components.

Those interfaces MUST be independent from any framework or programming language. Interfaces shall be implemented with standard communication protocols through web services. Usage of REST is recommended.

Depending on the party to interact and the functionality described to cover, a Verifier SHALL make use of the following defined interfaces for his operation: 
                 +------------------------------------++------------------------------------++-----------------++-----------------+
                 |            Holder                  ||             Governance             ||  DID Resolver   ||     Issuer      |
                 +------------------------------------++------------------------------------++-----------------++-----------------+
                +------------------------------------------------------------------------------------------------------------------+ 
                |+-----------------++-----------------++-----------------++-----------------++-----------------++-----------------+|
                ||                 ||                 ||                 ||                 ||                 ||                 ||
                || Presentation    || Data            || Schema          || Issuer          || DID Resolution  || Credential      ||
                || Exchange        || Agreement       || Resolution      || Resolution      ||                 || Status          ||
                ||                 ||                 ||                 ||                 ||                 || Query           ||
                ||                 ||                 ||                 ||                 ||                 ||                 ||
                ||                 ||                 ||                 ||                 ||                 ||                 ||
                ||       API       ||       API       ||       API       ||       API       ||       API       ||      API        ||
                ++-----------------++------------------------------------++-----------------++-----------------++-----------------++
                |   1, 2.3, 3,      |     2.6, 2.7,    |       2.6, 7     |      2.2         |      2.1, 2.2,   |      2.3         |
                |    4, 6, 7        |       5, 6       |                  |                  |       2.4, 4     |                  |
                |                                                                                                                  |
                |                                                      Verifier                                                    |
                |                                                                                                                  |
                +------------------------------------------------------------------------------------------------------------------+
Alignment of Interfaces with functionalities to cover

DID Resolver interfaces

DID Resolution

Standard protocol and considerations for DID Resolution are being defined by a specific working group. See [DID-RESOLUTION] .

Issuer interfaces

Credential Status

Standard protocol and considerations for querying the status of the credentials and managing their revocation are being defined by a specific working group. See [VC-STATUS-RL-2020] .

Governance interfaces

The Identity platform governance should determine what kind of credentials could be issued by the different parties, the specific formats and contexts of those credentials, as well as which entities fulfill the requirements to issue each specific credential. In order to do that, it might use different registries, that should be queried from any interested verifier cooperating with the platform, to allow them to :

  1. Discover the credentials available on the Identity platform
  2. Manage the format of every credential
  3. Establish the relationship between different credentials, taken as attributes to build more complex credentials
  4. Define and restrict the entities enabled to assert and validate the information from the subjects
  5. Assign different levels of trust to the identity validation processes

Registries of Issuers and Credential Schemas MAY be separated and managed by different entities. Multiple registries of Issuers and/or Schemas MAY be queried by a Verifier.

Verifiers MAY make use of internal private registries to complement the public registries. Defining a protocol to merge different registries is outside the scope of this document.

Verifiers MAY query and assign different levels of trust to different governance platforms, but all the platforms should present the same query interface.

Registries SHALL be publicly available and auditable. Persisting registries on DLTs is OPTIONAL but RECOMMENDED as good practice in order to maintain neutrality.

Registries MUST offer a web interface. REST protocol SHOULD be employed with HTTP Method GET and JSON formatting as default.

Discovery

To be defined.

This section should be filled with discovery of registry endpoints for automatic configuration, as well as protocols to discover other registries.

Issuer Registry

The VUI protocol and considerations for querying the issuer registry are being defined by a specific working package inside the VUI. See [WP-VUI-IssuerResolution] .

Schema Registry

This interface is not yet defined. There are no active working groups with this effort.

Holder interfaces

Presentation Exchange

The VUI protocol and considerations for the Presentation Exchange is being defined by a specific working package. See [WP-VUI-PresentationExchange] .

The data model builds upon DIF Presentation Exchange with some optional additions to support additional features and connect to Data Agreements.

For the communication protocol, multiple flavors could be eligible, such as:

At this point of in the process of definition and implementation, VUI Presentation Exchange is focused on having support on the last one DID SIOP.

Data Agreements

Data Agreements includes Data privacy management oriented for the enforcement of DPIA controls in privacy frameworks such as GDPR.

The VUI protocol to manage and generate data agreements can be found on its own working package. See [WP-VUI-DataAgreements] .