Status List 2021

Abstract

This specification describes a privacy-preserving, space-efficient, and high-performance mechanism for publishing status information such as suspension or revocation of Verifiable Credentials.

This section is non-normative.

It is often useful for an issuer of verifiable credentials [VC-DATA-MODEL] to link to a location where a verifier can check to see if a credential has been suspended or revoked. There are a variety of privacy and performance considerations that are made when designing, publishing, and processing status lists.

One such privacy consideration happens when there is a one-to-one mapping between a verifiable credential and a URL where the status is published. This type of mapping enables the website that publishes the URL to correlate the holder, time, and verifier when the status is checked. This could enable the issuer to discover the type of interaction the holder is having with the verifier, such as providing an age verification credential when entering a bar. Being tracked by the issuer of a driver's license when entering an establishment violates a privacy expectation that many people have today.

Similarly, there are performance considerations that are explored when designing status lists. One such consideration is where the list is published and the burden it places from a bandwidth and processing perspective, both on the server and the client fetching the information. In order to meet privacy expectations, it is useful to bundle the status of large sets of credentials into a single list to help with herd privacy. However, doing so can place an impossible burden on both the server and client if the status information is as much as a few hundred bytes in size per credential across a population of hundreds of millions of holders.

The rest of this document proposes a highly compressible, bitstring-based status list mechanism with strong privacy-preserving characteristics, that is compatible with the architecture of the Web, is highly space-efficient, and lends itself well to content distribution networks. As an example of using this specification to achieve a number of beneficial privacy and performance goals, it is possible to create a status list that can be constructed for 100,000 verifiable credentials that is roughly 12,500 bytes in size in the worst case. In a case where a few hundred credentials have been revoked, the size of the list is less than a few hundred bytes while providing privacy in a herd of 100,000 individuals.

This section is non-normative.

This section outlines the core concept utilized by the status list mechanism described in this document. At the most basic level, status information for all verifiable credentials issued by an issuer are expressed as simple binary values. The issuer keeps a bitstring list of all verifiable credentials it has issued. Each verifiable credential is associated with a position in the list. If the binary value of the position in the list is 1 (one), the verifiable credential is revoked, if it is 0 (zero) it is not revoked.

One of the benefits of using a bitstring is that it is a highly compressible data format since, in the average case, large numbers of credentials will remain unrevoked. This will ensure long sections of bits that are the same value and thus highly compressible using run-length compression techniques such as GZIP [RFC1952]. The default bitstring size is 16KB (131,072 entries), and when only a handful of verifiable credentials are revoked, the compressed bitstring size is reduced down to a few hundred bytes.

Another benefit of using a bitstring is that it enables large numbers of verifiable credential statuses to be placed in the same list. This specification utilizes a minimum bitstring length of 131,072 (16KB). This population size ensures an adequate amount of herd privacy in the average case. If better herd privacy is required, the bitstring can be made to be larger.

diagram showing a
list of boxes at the top of the image with two of them in
red depicting revoked credentials. Text beside the boxes to the
right reads 16 kilobytes. An depiction of the boxes being
GZIP compressed into a cylinder on the bottom of the page shows
that compression has resulted in a final size of 135 bytes. — Figure 1 A visual depiction of the concepts outlined in this section.

This section is non-normative.

The following terms are used to describe concepts in this specification.

claim: An assertion made about a subject.
credential: A set of one or more claims made by an issuer. A verifiable credential is a tamper-evident credential that has authorship that can be cryptographically verified. Verifiable credentials can be used to build verifiable presentations, which can also be cryptographically verified. The claims in a credential can be about different subjects.
data minimization: The act of limiting the amount of shared data strictly to the minimum necessary to successfully accomplish a task or goal.
decentralized identifier: A portable URL-based identifier, also known as a DID, associated with an entity. These identifiers are most often used in a verifiable credential and are associated with subjects such that a verifiable credential itself can be easily ported from one repository to another without the need to reissue the credential. An example of a DID is did:example:123456abcdef.
entity: A thing with distinct and independent existence, such as a person, organization, or device that performs one or more roles in the ecosystem.
holder: A role an entity might perform by possessing one or more verifiable credentials and generating presentations from them. A holder is usually, but not always, a subject of the verifiable credentials they are holding. Holders store their credentials in credential repositories.
identity provider: An identity provider, sometimes abbreviated as IdP, is a system for creating, maintaining, and managing identity information for holders, while providing authentication services to relying party applications within a federation or distributed network. In this case the holder is always the subject. Even if the verifiable credentials are bearer credentials, it is assumed the verifiable credentials remain with the subject, and if they are not, they were stolen by an attacker. This specification does not use this term unless comparing or mapping the concepts in this document to other specifications. This specification decouples the identity provider concept into two distinct concepts: the issuer and the holder.
issuer: A role an entity can perform by asserting claims about one or more subjects, creating a verifiable credential from these claims, and transmitting the verifiable credential to a holder.
presentation: Data derived from one or more verifiable credentials, issued by one or more issuers, that is shared with a specific verifier. A verifiable presentation is a tamper-evident presentation encoded in such a way that authorship of the data can be trusted after a process of cryptographic verification. Certain types of verifiable presentations might contain data that is synthesized from, but do not contain, the original verifiable credentials (for example, zero-knowledge proofs).
repository: A program, such as a storage vault or personal verifiable credential wallet, that stores and protects access to holders' verifiable credentials.
subject: A thing about which claims are made.
verifiable data registry: A role a system might perform by mediating the creation and verification of identifiers, keys, and other relevant data, such as verifiable credential schemas, revocation registries, issuer public keys, and so on, which might be required to use verifiable credentials. Some configurations might require correlatable identifiers for subjects. Some registries, such as ones for UUIDs and public keys, might just act as namespaces for identifiers.
verification: The evaluation of whether a verifiable credential or verifiable presentation is an authentic and timely statement of the issuer or presenter, respectively. This includes checking that: the credential (or presentation) conforms to the specification; the proof method is satisfied; and, if present, the status check succeeds. Verification of a credential does not imply evaluation of the truth of claims encoded in the credential..
verifier: A role an entity performs by receiving one or more verifiable credentials, optionally inside a verifiable presentation for processing. Other specifications might refer to this concept as a relying party.

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

The following sections outlines the data model for this document.

When an issuer desires to enable status information for a verifiable credential, they MAY add a credentialStatus property that uses the data model described in this specification.

Property Description

id The constraints on the id property are listed in the Verifiable Credentials Data Model specification [VC-DATA-MODEL]. The value is expected to be a URL that identifies the status information associated with the verifiable credential. It MUST NOT be the URL for the status list.

type The type property MUST be StatusList2021Entry.

statusPurpose

The purpose of the status entry MUST be a string. While the value of the string is arbitrary, the following values MUST be used for their intended purpose:

Value	Description
`revocation`	Used to cancel the validity of a verifiable credential. This status is not reversible.
`suspension`	Used to temporarily prevent the acceptance of a verifiable credential. This status is reversible.

statusListIndex The statusListIndex property MUST be an arbitrary size integer greater than or equal to 0, expressed as a string. The value identifies the bit position of the status of the verifiable credential.

statusListCredential The statusListCredential property MUST be a URL to a verifiable credential. When the URL is dereferenced, the resulting verifiable credential MUST have type property that includes the StatusList2021Credential value.

Example 1: Example StatusList2021Credential

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://w3id.org/vc/status-list/2021/v1"
  ],
  "id": "https://example.com/credentials/23894672394",
  "type": ["VerifiableCredential"],
  "issuer": "did:example:12345",
  "issued": "2021-04-05T14:27:42Z",
  "credentialStatus": {
    "id": "https://example.com/credentials/status/3#94567"
    "type": "StatusList2021Entry",
    "statusPurpose": "revocation",
    "statusListIndex": "94567",
    "statusListCredential": "https://example.com/credentials/status/3"
  },
  "credentialSubject": {
    "id": "did:example:6789",
    "type": "Person"
  },
  "proof": { ... }
}

When a status list is published, the result is a verifiable credential that encapsulates the status list. The following section describes the format of the verifiable credential that encapsulates the status list:

Property Description

id The verifiable credential that contains the status list MAY express an id property that matches the value specified in statusListCredential for the corresponding StatusList2021Entry (see 2.1 StatusList2021Entry).

type The verifiable credential that contains the status list MUST express a type property that includes the StatusList2021Credential value.

credentialSubject.type The type of the credential subject, which is the status list, MUST be StatusList2021.

credentialSubject.statusPurpose

The purpose of the status entry MUST be a string. While the value of the string is arbitrary, the following values MUST be used for their intended purpose:

Value	Description
`revocation`	Used to cancel the validity of a verifiable credential. This status is not reversible.
`suspension`	Used to temporarily prevent the acceptance of a verifiable credential. This status is reversible.

credentialSubject.encodedList The encodedList property of the credential subject MUST be the GZIP-compressed [RFC1952], base-64 encoded [RFC4648] bitstring values for the associated range of verifiable credential status values. The uncompressed bitstring MUST be at least 16KB in size.

Example 2: Example StatusList2021Credential

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://w3id.org/vc/status-list/2021/v1"
  ],
  "id": "https://example.com/credentials/status/3",
  "type": ["VerifiableCredential", "StatusList2021Credential"],
  "issuer": "did:example:12345",
  "issued": "2021-04-05T14:27:40Z",
  "credentialSubject": {
    "id": "https://example.com/status/3#list",
    "type": "StatusList2021",
    "statusPurpose": "revocation",
    "encodedList": "H4sIAAAAAAAAA-3BMQEAAADCoPVPbQwfoAAAAAAAAAAAAAAAAAAAAIC3AYbSVKsAQAAA"
  },
  "proof": { ... }
}

The following section outlines the algorithms that are used to generate and validate status lists as described by this document.

The following process, or one generating the exact output, MUST be followed when producing a StatusList2021Credential:

Let issued credentials be a list of all issued verifiable credentials.
Let RLC be an unsigned StatusList2021Credential without the encodedList property set.
Generate a compressed bitstring by passing issued credentials to the Bitstring Generation Algorithm.
Set the encodedList to compressed bitstring.
Generate a proof for the RLC and publish it to the endpoint listed in the verifiable credential.

The following process, or one generating the exact output, MUST be followed when validating a verifiable credential that is contained in a StatusList2021Credential:

Let credentialToValidate be a verifiable credentials containing a credentialStatus entry that is a StatusList2021Entry.
Let status purpose be the value of statusPurpose in the credentialStatus entry in the credentialToValidate.
Verify all proofs associated with the credentialToValidate. If a proof fails, return a validation error.
Verify that the status purpose matches the statusPurpose value in the statusListCredential.
Let compressed bitstring be the value of the encodedList property of the StatusList2021Credential.
Let credentialIndex be the value of the statusListIndex property of the StatusList2021Entry.
Generate a revocation bitstring by passing compressed bitstring to the Bitstring Expansion Algorithm.
Let status be the value of the bit at position credentialIndex in the revocation bitstring.
Return true if status is 1, false otherwise.

The following process, or one generating the exact output, MUST be followed when generating a status list bitstring. The algorithm takes a issuedCredentials list as input and returns a compressed bitstring as output.

Let bitstring be a list of bits with a minimum size of 16KB, where each bit is initialized to 0 (zero).
For each bit in bitstring, if there is a corresponding statusListIndex value in a revoked credential in issuedCredentials, set the bit to 1 (one), otherwise set the bit to 0 (zero).
Generate a compressed bitstring by using the GZIP compression algorithm [RFC1952] on the bitstring and then base64-encoding [RFC4648] the result.
Return the compressed bitstring.

The following process, or one generating the exact output, MUST be followed when expanding a compressed status list bitstring. The algorithm takes a compressed bitstring as input and returns a uncompressed bitstring as output.

Let compressed bitstring be a compressed status list bitstring.
Generate an uncompressed bitstring by using the base64-decoding [RFC4648] algorithm on the compressed bitstring and then expanding the output using the GZIP decompression algorithm [RFC1952].
Return the uncompressed bitstring.

This section is non-normative.

This section details the general privacy considerations and specific privacy implications of deploying this specification into production environments.

This section is non-normative.

This document specifies a minimum revocation bitstring length of 131,072, or 16KB uncompressed. This is enough to give holders an adequate amount of herd privacy if the number of verifiable credentials issued is large enough. However, if the number of issued verifiable credentials is a small population, the ability to correlate an individual increases because the number of allocated slots in the bitstring is small. Correlating this information with, for example, where the geographic request came from can also help to correlate individuals that have received a credential from the same geographic region.

This section is non-normative.

It is possible for verifiers to increase the privacy of the holder whose verifiable credential is being checked by caching status lists that have been fetched from remote servers. By caching the content locally, less correlatable information can be inferred from verifier-based access patterns on the status list.

This section is non-normative.

The use of content distribution networks by issuers can increase the privacy of holders by reducing or eliminating requests for the status lists from the issuer. Often, a request for a revocation list will be served by an edge device and thus be faster and reduce the load on the server as well as cloaking verifiers and holders from issuers.

Status List 2021

Privacy-preserving status information for Verifiable Credentials

Abstract

Status of This Document

1. Introduction

1.1 Conceptual Framework

1.2 Terminology

1.3 Conformance

2. Data Model

2.1 StatusList2021Entry

2.2 StatusList2021Credential

3. Algorithms

3.1 Generate Algorithm

3.2 Validate Algorithm

3.3 Bitstring Generation Algorithm

3.4 Bitstring Expansion Algorithm

4. Privacy Considerations

4.1 Revocation Bitstring Length

4.2 Verifier Caching

4.3 Content Distribution Networks

5. Security Considerations

6. Accessibility Considerations

7. Internationalization Considerations

A. References

A.1 Normative references

A.2 Informative references