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

The Working Group is actively seeking implementation feedback for this specification. In order to exit the Candidate Recommendation phase, the Working Group has set the requirement of at least two independent implementations for each mandatory feature in the specification. For details on the conformance testing process, see the implementation report.

Introduction

It is often useful for an [=issuer=] of [=verifiable credentials=] [[VC-DATA-MODEL-2.0]] 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 group 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 group of 100,000 individuals.

Conceptual Framework

This section outlines the core concept utilized by the status list mechanism described in this specification. At the most basic level, status information for all [=verifiable credentials=] issued by an [=issuer=] is expressed as items in a list. Each [=issuer=] manages a list of all [=verifiable credentials=] that it has issued. Each [=verifiable credential=] is associated with an item in its list. When a single bit specifies a status, such as "revoked" or "suspended", then that status is expected to be true when the bit is set (`1`) and false when unset (`0`).

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 status list size is 131,072 entries, equivalent to 16 KB of single bit values. When only a handful of [=verifiable credentials=] are revoked, GZIP compresses the bitstring 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 uses a minimum list length of 131,072. This size ensures an adequate amount of group privacy in the average case. If better group privacy is required, the bitstring can be made 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.
A visual depiction of the concepts outlined in this section.

The status information associated with a particular [=verifiable credential=] is about the [=verifiable credential=] itself and might not apply to any underlying or backing [=credential=], such as an educational degree. For example, in the case of such an educational degree, it is possible for a [=verifiable credential=] to be revoked because the mechanism used to create its digital signature has been compromised, while the backing educational degree remains valid.

Terminology

Terminology used throughout this document is defined in the Terminology section of the [[[VC-DATA-MODEL-2.0]]] specification.

A conforming document is any concrete expression of the data model that follows the relevant normative requirements in Section .

A conforming processor is any algorithm realized as software and/or hardware that generates and/or consumes a [=conforming document=] according to the relevant normative statements in Section . Conforming processors MAY choose to only support bitstring entry sizes of 1. Conforming processors MUST produce errors when non-conforming documents are consumed.

Data Model

There are numerous ways to express status information associated with digital credentials. Some of these mechanisms include Certificate Revocation Lists (CRL) [[?RFC5280]], the Online Certificate Status Protocol (OCSP) [[?RFC2560]], Bloom Filters [[?RFC8932]], and cryptographic accumulators [[?ALLOSAUR]]. This specification optimizes for a variety of requirements that are different from other mechanisms. These requirements include:

Technology CRL OCSP Bloom Accumulator Bitstring
Provides tunable group privacy ✓ ✗ ✓ ✓ ✓
Does not require signed assertion for each credential ✓ ✗ ✓ ✓ ✓
Resistant to [=issuer=] tracking when fetched by [=verifier=] ✓ ✓ ✓ ✓ ✓
Caching is space efficient with many revocations ✗ ✗ ✓ ✓ ✓
Highly compressible (>90% average compression) ✗ ✗ ✓ ✓ ✓
Updates are efficient (fast and entire population does not need to update) ✓ ✗ ✓ ✗ ✓
Uses cryptographic primitives approved by IETF ✓ ✓ ✓ ✗ ✓
No false positives ✓ ✓ ✗ ✓ ✓
Can be delivered by [=holder=] (stapling) ✗ ✓ ✗ ✓ ✓
Easily profiled for usage with [=verifiable credentials=] ✗ ✗ ✗ ✗ ✓

BitstringStatusListEntry

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 section. Any expression of the data model in this section MUST be expressed in a conforming [=verifiable credential=] as defined in [[VC-DATA-MODEL-2.0]].

The Working Group is currently seeking implementer feedback regarding the utility of bitstring entries that have sizes greater than one. Supporting such entries adds complexity to the solution, and it's not clear whether there is enough of an implementation community to support the feature. The WG is considering three options: (1) require conforming implementations to support the feature; (2) allow implementations to optionally support the feature; or (3) remove the feature. At present, the specification implements option (2).

Property Description
id An optional identifier for the status list entry. The constraints on the `id` property are listed in the Verifiable Credentials Data Model specification [[VC-DATA-MODEL-2.0]]. If present, 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. The value is not used during the verification or validation process, and does not need to be related to the `statusListCredential` value. If necessary, the value can be used to uniquely identify the `BitstringStatusListEntry` object, such as when it is stored in a database.
type The `type` property MUST be `BitstringStatusListEntry`.
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.
`message` Used to convey an arbitrary message related to the status of the [=verifiable credential=].
statusListIndex The `statusListIndex` property MUST be an arbitrary size integer greater than or equal to 0, expressed as a string in base 10. The value identifies the position of the status of the [=verifiable credential=]. Implementations SHOULD assign indexes randomly, such that inferences — such as the recency of the assignment or the size of the group — cannot be easily drawn from that position.
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 `BitstringStatusListCredential` value.
statusSize The `statusSize` indicates the size of the status entry in bits. `statusSize` MAY be provided. If `statusSize` is not present as a property of the `credentialStatus`, then `statusSize` MUST be processed as `1`. If present, `statusSize` MUST be an integer greater than zero. If `statusSize` is provided and is greater than `1`, then the property `credentialStatus.statusMessage` MUST be present, and the number of status messages MUST equal the number of possible values.
statusMessage If present, the `statusMessage` property MUST be an array, the length of which MUST equal the number of possible status messages indicated by `statusSize` (e.g., `statusMessage` array MUST have 2 elements if `statusSize` has 1 bit, 4 elements if `statusSize` has 2 bits, 8 elements if `statusSize` has 3 bits, etc.). `statusMessage` MAY be present if `statusSize` is `1`, and MUST be present if `statusSize` is greater than `1`. If the `statusMessage` array is not present, the message values associated with the `status` bit values of `1` and `0` are "set" and "unset", respectively. If the `statusMessage` array is present, each element MUST contain the two properties described below, and MAY contain additional properties.
  • `status`, a string representing the hexadecimal value of the status prefixed with `0x`
  • `message`, a string used by software developers to assist with debugging which SHOULD NOT be displayed to end users.
Implementers MAY add additional values to objects in the `statusMessage` array. Implementers MAY use the string value of `undefined` in the value to indicate that a corresponding status is not defined for the associated status value, but that it may be defined in the future. Rules for how to handle various status messages are outside the scope of normative requirements in this document, but it is assumed that implementers will document rules for processing various status codes.
statusReference An implementer MAY include the `statusReference` property. If present, its value MUST be a URL or an array of URLs [[URL]] which dereference to material related to the status. Implementers using a `statusPurpose` of `message` are strongly encouraged to provide a `statusReference`.

`statusReference` is especially important when interpretation of the status for a credential may involve some understanding of the business case involved.

Status list entries can be used to express the purpose of a status associated with a [=verifiable credential=] by using the `statusPurpose` property.

The use of `revocation` or `suspension` as the status purpose includes the semantics of the status, with `revocation` indicating that a status bit expresses whether a [=verifiable credential=] has been revoked and `suspension` indicating that a status bit expresses whether a [=verifiable credential=] has been suspended. The example below demonstrates the use of these status purposes: 

{
  "@context": [
    "https://www.w3.org/ns/credentials/v2",
    "https://www.w3.org/ns/credentials/examples/v2"
  ],
  "id": "https://example.com/credentials/23894672394",
  "type": ["VerifiableCredential", "EmployeeIdCredential"],
  "issuer": "did:example:12345",
  "validFrom": "2024-04-05T14:27:42Z",
  "credentialStatus": [{
    "id": "https://example.com/credentials/status/3#94567",
    "type": "BitstringStatusListEntry",
    "statusPurpose": "revocation",
    "statusListIndex": "94567",
    "statusListCredential": "https://example.com/credentials/status/3"
  }, {
    "id": "https://example.com/credentials/status/4#23452",
    "type": "BitstringStatusListEntry",
    "statusPurpose": "suspension",
    "statusListIndex": "23452",
    "statusListCredential": "https://example.com/credentials/status/4"
  }],
  "credentialSubject": {
    "id": "did:example:6789",
    "type": "Person",
    "employeeId": "A-123456"
  }
}

The use of `message` as the status purpose enables an issuer to define an arbitrary number of custom, descriptive messages about the status of the [=verifiable credential=]. The [=issuer=] commits to the set of messages that may be associated with a particular entry (i.e., with a particular [=verifiable credential=]) through the `statusSize`, `statusMessage`, and optional `statusReference` properties, at the time of [=verifiable credential=] issuance. This is to ensure that the [=holder=] knows what sort of information might be associated with a particular [=verifiable credential=] they keep in their possession, that could then be discoverable by a [=verifier=] that later receives that credential. 

{
  "@context": [
    "https://www.w3.org/ns/credentials/v2",
    "https://www.w3.org/ns/credentials/examples/v2"
  ],
  "id": "https://example.com/credentials/2947478373",
  "type": ["VerifiableCredential", "BillOfLadingExampleCredential"],
  "issuer": "did:example:12345",
  "validFrom": "2024-04-05T03:52:31Z",
  "credentialStatus": {
    "id": "https://example.com/credentials/status/8#492847",
    "type": "BitstringStatusListEntry",
    "statusPurpose": "message",
    "statusListIndex": "492847",
    "statusSize": 2,
    "statusListCredential": "https://example.com/credentials/status/8",
    "statusMessage": [
        {"status":"0x0", "message":"pending_review"},
        {"status":"0x1", "message":"accepted"},
        {"status":"0x2", "message":"rejected"},
        ...
    ],
    "statusReference": "https://example.org/status-dictionary/"
  },
  "credentialSubject": {
    "id": "did:example:6789",
    "type": "BillOfLading",
    ...
  }
}

BitstringStatusListCredential

When a status list [=verifiable credential=] is published, it MUST be a conforming document, as defined in [[VC-DATA-MODEL-2.0]], that expresses the data model in this section. The following section describes the format of the [=verifiable credential=] that encapsulates the status list.

The status list is expressed inside a [=verifiable credential=] in order to enable a [=holder=] to provide it directly to a [=verifier=]. This mechanism, sometimes called "certificate stapling", increases privacy for the [=holder=] by ensuring that the [=verifier=] does not need to contact the [=issuer=] to retrieve the status list. Still, a [=verifier=] might choose to ignore the [=holder=]-provided status list, even when its authenticity is verifiable, if it desires a more recent version of a status list, for instance.

[=Issuers=] and [=verifiers=] are advised that the [=issuer=] of a [=verifiable credential=] and the [=issuer=] of an associated `BitstringStatusListCredential` might not be the same. There are technical, legal, institutional, political, and other reasons that might make it appropriate to separate the authority over the original credential from the authority to revoke, or otherwise change the status of, such a credential. Therefore, the `issuer` value of a [=verifiable credential=] containing a `BitstringStatusListEntry` MAY be different from the `issuer` value of a `BitstringStatusListCredential`.

The Working Group is considering the removal of the `ttl` ("time to live") feature because its semantics conflict with the semantics of the `validUntil` feature of [=verifiable credentials=]. When a [=verifier=] performs validation and evaluates a `BitstringStatusListCredential` that contains both a `ttl` property and a `validUntil` property, each with a different value (i.e., each indicating a different point in time when the credential is to "expire"), it is not clear which (if either) property a validator can be expected to ignore. In other words, if a `ttl` value specifies an expiration datetime of midnight today, but the `validUntil` property specifies an expiration datetime of midnight tomorrow, then what is a [=verifier=] expected to do? Fundamentally, `ttl` and `validUntil` have conflicting semantics. One way to resolve this conflict is to remove `ttl` and specify that caching behavior can be expressed using protocol mechanisms (such as the `expires` header in HTTP), and that any caching performed MUST align with the `validUntil` value for the [=verifiable credential=]. The Working Group is seeking feedback from the implementer community regarding this feature.

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 `BitstringStatusListEntry` (see ).
type The [=verifiable credential=] that contains the status list MUST express a `type` property that includes the `BitstringStatusListCredential` value.
validFrom The earliest point in time at which the status list is valid. This property is defined in the Verifiable Credentials Data Model specification in Section 4.6: Validity Period.
validUntil The latest point in time at which the status list is valid. This property is defined in the Verifiable Credentials Data Model specification in Section 4.6: Validity Period.
credentialSubject.type The `type` of the credential [=subject=], which is the status list, MUST be `BitstringStatusList`.
credentialSubject.statusPurpose The value of the purpose property of the status entry, `statusPurpose`, MUST be one or more strings. While the value of each 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.
`message` Used to indicate a status message associated with a verifiable credential. The status message descriptions MUST be defined in `credentialSubject.statusMessages`. `credentialSubject.statusSize` MUST be specified when this `statusPurpose` value is used.
credentialSubject.encodedList The `encodedList` property of the credential [=subject=] MUST be a Multibase-encoded base64url (with no padding) [[RFC4648]] representation of the GZIP-compressed [[RFC1952]] bitstring values for the associated range of [=verifiable credential=] status values. The uncompressed bitstring MUST be at least 16KB in size. The bitstring MUST be encoded such that the first index, with a value of zero (`0`), is located at the left-most bit in the bitstring and the last index, with a value of one less than the length of the bitstring (`bitstring_length - 1`), is located at the right-most bit in the bitstring. Further information on bitstring encoding can be found in Section .
credentialSubject.ttl The `ttl` indicates the "time to live" in milliseconds. This property MAY be present. If not present, implementers MUST use a value of `300000` for this property. A verifier MUST NOT use a cached `BitstringStatusListCredential` that was cached for more than the `ttl` duration prior to the start of verification operation on a [=verifiable credential=]. Implementations that publish the status list SHOULD align any protocol-specific caching information, such as the HTTP `Cache-Control` header, with the value in this field.

The example below demonstrates how the `BitstringStatusListEntry` is used with a `BitstringStatusListCredential` to provide the [=verifier=] with the information necessary to determine the status of a particular [=verifiable credential=]. 

{
  "@context": [
    "https://www.w3.org/ns/credentials/v2"
  ],
  "id": "https://example.com/credentials/status/3",
  "type": ["VerifiableCredential", "BitstringStatusListCredential"],
  "issuer": "did:example:12345",
  "validFrom": "2021-04-05T14:27:40Z",
  "credentialSubject": {
    "id": "https://example.com/status/3#list",
    "type": "BitstringStatusList",
    "statusPurpose": "revocation",
    "encodedList": "uH4sIAAAAAAAAA-3BMQEAAADCoPVPbQwfoAAAAAAAAAAAAAAAAAAAAIC3AYbSVKsAQAAA"
  }
}

Algorithms

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

If an implementation of any of the algorithms in this section processes a property defined in Section whose value is malformed due to not complying with associated "MUST" statements, a MALFORMED_VALUE_ERROR MUST be raised.

The Working Group is seeking feedback related to implementer desire to align with the IETF OAuth Working Group Token Status List specification (formerly titled OAuth Status List, and JWT and CWT Status List). If there is interest, and to the extent possible, this specification might align more closely with the Token Status List bitstring format, as it could be beneficial to have one code base able to process bitstring values from both lists. If there is implementer support for such changes, they might be made during the Candidate Recommendation phase.

Generate Algorithm

The following process, or one generating the exact output, MUST be followed when producing a BitstringStatusListCredential. The algorithm takes a list of |issued credentials| as input and either throws an error or returns a status list credential as output.

  1. Let |issued credentials| be a list of all issued [=verifiable credentials=].
  2. Let |statusListCredential| be an unsigned BitstringStatusListCredential without the `encodedList` property set.
  3. Generate a |compressed bitstring| by passing |issued credentials| to the Bitstring Generation Algorithm.
  4. Set the `encodedList` to |compressed bitstring|.
  5. Generate a proof for the |statusListCredential| and publish it to the endpoint listed in the [=verifiable credential=].

[=Issuers=] SHOULD publish status list credentials in a way that can be cached and that does not track who retrieves the status list credential, such as through [[[RFC9458]]], a content distribution network that is not operated by the [=issuer=], or business processes for which the access logs are not accessible by data analysts or systems administrators.

Validate Algorithm

The following process, or one generating the exact output, MUST be followed when validating a [=verifiable credential=] that is contained in a BitstringStatusListCredential. The algorithm takes a status list [=verifiable credential=] as input and either throws an error or returns a status list credential as output.

  1. Let |credentialToValidate| be a [=verifiable credential=] containing a `credentialStatus` entry that is a BitstringStatusListEntry.
  2. Let |minimumNumberOfEntries| be 131,072 unless a different lower bound is established by a specific ecosystem specification.
  3. Let |status purpose| be the value of `statusPurpose` in the `credentialStatus` entry in the |credentialToValidate|.
  4. Dereference the `statusListCredential` URL, and ensure that all proofs verify successfully. If the dereference fails, raise a STATUS_RETRIEVAL_ERROR. If any of the proof verifications fail, raise a STATUS_VERIFICATION_ERROR.
  5. Verify that the |status purpose| is equal to a `statusPurpose` value in the |statusListCredential|. Note: The |statusListCredential| might contain multiple status purposes in a single list. If the values are not equal, raise a STATUS_VERIFICATION_ERROR.
  6. Let |compressed bitstring| be the value of the `encodedList` property of the BitstringStatusListCredential.
  7. Let |credentialIndex| be the value of the `statusListIndex` property of the BitstringStatusListEntry.
  8. Generate a |revocation bitstring| by passing |compressed bitstring| to the Bitstring Expansion Algorithm.
  9. If the length of the |revocation bitstring| divided by `statusSize` is less than |minimumNumberOfEntries|, raise a STATUS_LIST_LENGTH_ERROR.
  10. Let |status| be the value in the |bitstring| at the position indicated by the |credentialIndex| multiplied by the |size|. If the |credentialIndex| multiplied by the |size| is a value outside of the range of the |bitstring|, a RANGE_ERROR MUST be raised.
  11. Let |result| be an empty [=map=].
  12. Set the `status` key in |result| to |status|, and set the `purpose` key in |result| to the value of `statusPurpose`.
  13. If |status| is `0`, set the `valid` key in |result| to `true`; otherwise, set it to `false`.
  14. If the `statusPurpose` is `message`, set the `message` key in |result| to the corresponding `message` of the `value` as indicated in the `statusMessages` array.
  15. Return |result|.

When a `statusListCredential` URL is dereferenced, server implementations MAY provide a mechanism to dereference the status list as of a particular point in time. When an [=issuer=] provides such a mechanism, it enables a [=verifier=] to determine changes in status to a precision chosen by the issuer, such as hourly, daily, or weekly. If such a feature is supported, and if query parameters are supported by the URL scheme, then the name of the query parameter MUST be `timestamp` and the value MUST be a valid URL-encoded [[XMLSCHEMA11-2]] dateTimeStamp string value. The result of dereferencing such a timestamp-parameterized URL MUST be either a status list credential containing the status list as it existed at the given point in time, or a STATUS_RETRIEVAL_ERROR. If the result is an error, implementations MAY attempt the retrieval again with a different timestamp value, or without a timestamp value, as long as the [=verifier=]'s validation rules permit such an action.

[=Verifiers=] SHOULD cache the retrieved status list and SHOULD use proxies or other mechanisms, such as [[[RFC9458]]], that hide retrieval behavior from the [=issuer=].

It is expected that a [=verifier=] will ensure that it trusts the issuer of a [=verifiable credential=], as well as the issuer of the associated BitstringStatusListCredential, before using the information contained in either credential for further decision making purposes. Implementers are advised that the issuers of these credential might differ, such as when the original issuer of the [=verifiable credential=] does not maintain a record of its validity.

Bitstring Generation Algorithm

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

  1. Let |bitstring| be a list of bits with a minimum size of 16KB, where each bit is initialized to 0 (zero).
  2. For each value in `bitstring`, if there is a corresponding `statusListIndex` value in a credential in `issuedCredentials`, set the value to the appropriate status. The position of the value is computed as `statusListIndex` times the `statusSize`.
  3. Generate a |compressed bitstring| by using the GZIP compression algorithm [[RFC1952]] on the |bitstring| and then base64url-encoding (with no padding) [[RFC4648]] the result.
  4. Return the |compressed bitstring|.

Bitstring Expansion Algorithm

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 either throws an error or returns a |uncompressed bitstring| as output.

  1. Let |compressed bitstring| be a compressed status list bitstring.
  2. Generate an |uncompressed bitstring| by using the base64url-decoding (with no padding) [[RFC4648]] algorithm on the |compressed bitstring| and then expanding the output using the GZIP decompression algorithm [[RFC1952]].
  3. Return the |uncompressed bitstring|.

Processing Errors

The algorithms described in this specification throw specific types of errors. Implementers might find it useful to convey these errors to other libraries or software systems. This section provides specific URLs, descriptions, and error codes for the errors, such that an ecosystem implementing technologies described by this specification might interoperate more effectively when errors occur.

When exposing these errors through an HTTP interface, implementers SHOULD use [[[RFC9457]]] [[RFC9457]] to encode the error data structure. If [[RFC9457]] is used:

STATUS_RETRIEVAL_ERROR (-128)
Retrieval of the status list failed. See Section .
STATUS_VERIFICATION_ERROR (-129)
Validation of the status entry failed. See Section .
STATUS_LIST_LENGTH_ERROR (-130)
The status list length does not satisfy the minimum length required for herd privacy. See Section .

Securing Algorithms

There are multiple ways that the information in Section can be secured. These mechanisms are elaborated upon in the Securing Mechanisms section of the [[[VC-DATA-MODEL-2.0]]].

When securing a [=verifiable credential=] that contains a reference to a BitstringStatusListCredential, implementers SHOULD use the same securing mechanism with the same cryptographic parameters and the same media type for both [=verifiable credentials=].

Media Types

When dereferencing `statusListCredential`, the content of the returned `statusListCredential` might be any media type registered for the purpose of expressing a verifiable credential with one or more proofs.

For example, a verifiable credential secured with Data Integrity Proofs might have media type `application/vc+ld+json`, while a verifiable credential secured with SD-JWT might have media type `application/sd-jwt`.

Some implementations might choose to support less specific media types such as `application/ld+json` or `application/json`.

When dereferencing over HTTP, the use of the accept and content-type headers, might allow some implementations to negotiate for the proof format used to secure the `statusListCredential`.

Some implementations might use the 415 Unsupported Media Type status code to signal that they do not support the requested media type.

Privacy Considerations

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

Readers are urged to familiarize themselves with the general privacy advice provided in the Privacy Considerations section of the Verifiable Credentials specification before reading this section.

Revocation Bitstring Length

This document specifies a minimum revocation bitstring length of 131,072, or 16KB uncompressed. This is enough to give [=holders=] an adequate amount of group 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.

Unnecessary Correlation

There are a number of global identifiers used in a status list entry, defined in Section [[[#bitstringstatuslistentry]]], that can be used across [=verifiers=] to correlate [=subjects=]. Some of the properties that can express these values are `id`, `statusListIndex`, and `statusListCredential`.

In some cases, such as when [=presenting=] a [=verifiable credential=] that contains a global identifier (such as a driver's license identification number), adding one or more global identifier(s) for status list information does not increase correlation harm, since a single globally unique identifier is all that is required for correlation.

When global identifiers are used in [=presentations=] that use [=selective disclosure=] or [=unlinkable disclosure=], they can violate privacy expectations. [=Issuers=] are urged to enable status information to be selectively disclosable/concealable when a particular [=verifiable credential=] is expected to be disclosed in a way that does not need correlation, such as when proving that an individual is above a certain age. [=Verifiers=] can require that status information be revealed in situations that require them to know the current status of a credential, and the holder might then consent or refuse to reveal that information for a given transaction. In all cases, both [=issuers=] and [=verifiers=] are urged to avoid the use of global identifiers in order to prevent correlation, unless it is required for or by a particular exchange.

For information on other types of potential correlation, readers are urged to study the Privacy Considerations section of the [[[VC-DATA-MODEL-2.0]]] specification, particularly the subsections on Identifier-Based Correlation, Signature-Based Correlation, Long-Lived-Identifier-Based Correlation, and Metadata-Based Correlation.

Verifier Caching

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.

Content Distribution Networks

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=].

Decoy Values

[=Issuer=] use of decoy values in status lists has been explored as a mechanism to increase the privacy of [=subjects=]. While algorithms for employing decoy values are out of scope for this specification, implementers are advised that the use of decoy values does not improve privacy and can harm privacy in most cases.

When indexes of status list entries are allocated in a random fashion, which is the suggested mode of operation for this specification, adding decoys harms privacy because it reduces the true group size by the number of decoys added to the group. A random allocation of indexes inherently hides the true size of the group without reducing it, ensuring that decoys are not necessary.

There might be use cases where decoy values provide benefits. Implementers are cautioned that no such use cases were clearly identifiable by the group that created this specification. Therefore, the use of decoys is discouraged for most if not all use cases, as random allocation of status list entry indexes provides adequate protection.

Malicious Issuers and Verifiers

In general, the group privacy protections offered by this specification can be circumvented by malicious [=issuers=] and [=verifiers=]. Its privacy benefits can only be realized when issuers and verifiers intend to avoid tracking or sharing the presentation of particular credentials.

A malicious [=verifier=] might intentionally attack group privacy by sharing information from presented credentials with a malicious [=issuer=]. This sort of collusion is difficult to detect as it is typically performed via a secure communication channel between the [=issuer=] and the [=verifier=].

A malicious [=issuer=] might intentionally attack group privacy by creating a unique status list for each issued credential, in order to establish a one-to-one mapping to track when a [=verifier=] processes each mapped credential. Similarly, they could establish a one-to-one mapping by using a different cryptographic key for each issued credential that is tracked by a given status list.

This sort of collusion can be detected by [=holder=] software that serves multiple [=holders=] (e.g., a [=holder=] app that runs on a server) if it has, for example, an opt-in process that finds that some global identifier(s) used within a [=verifiable credential=] are not adequately shared by other credentials. [=Holders=] could then be warned when presenting a [=verifiable credential=] that contains some global identifier(s) that are unique to that credential. Such an opt-in service could represent some additional privacy concerns; whether this potential exposure via the [=holder=] software is justified by the awareness of possible global identifier correlation can only be evaluated by the users of such a system.

Monitoring Status Lists

Once a [=verifier=] knows of a status list and entry index that is associated with a specific [=holder=] or [=subject=], it becomes possible for that [=verifier=] to see updates to that status entry as long as the status list continues to be updated. This is useful to a [=verifier=] that needs to understand when a particular [=verifiable credential=] has changed status without asking the [=issuer=] directly for status information on the specific [=verifiable credential=] or when interacting with the [=holder=] to get the latest status information is not possible. The feature can also cause a privacy violation for the [=holder=] and/or [=subject=] if the [=verifier=] is able to perform near-real-time checks on the status of the [=verifiable credential=].

[=Issuers=] can provide a level of reprieve from this privacy concern [=holders=] by revoking and reissuing effectively the same [=verifiable credential=] on a timeline that is relatively short in nature. For example, an [=issuer=] could automatically reissue a [=verifiable credential=] every three months and assign a new status entry index when the reissuance occurs to break any sort of long-term monitoring of a [=verifiable credential=] as it changes status.

Correlation of Status Messages

This specification provides a means by which multiple status messages can be provided for a particular entry in a status list. While this mechanism can provide more detailed information for a particular entry in the status list, that information can provide further correlation data.

For example, if each status message is associated with a step in a particular process, or more detailed information as to why a credential was revoked or suspended, then an attacker that observes the changes in the list might be able to correlate information about the population of entities in the list that could lead to privacy violations. Understanding how a population progresses through a business process, or what percentage of the population is likely to be associated with a certain status, provides additional information to an attacker. Given such information, a phishing operation could predict what the next step of a business process is and then preemptively contact an entity whose current status is known. Then, based on that information, they could attempt to phish more lucrative information from the target using data gleaned from the status list over time.

For these reasons, issuers are urged to evaluate the potential ramifications of publishing detailed status information about a particular entity, or a population, in a public manner.

Alteration of Status Messages

When a status list uses the status messages feature, it becomes possible for the [=issuer=] to increase the types of messages that are associated with the [=verifiable credentials=] it issues over time.

This feature creates a potential privacy violation where the [=subject=] or [=holder=] of the [=verifiable credential=] might be associated with additional status information that was not present when the original [=verifiable credential=] was issued. For example, initial status messages might convey "delayed" and "canceled", but additional status messages might be added by the [=issuer=] to convey "delayed due to non-payment" and "canceled due to illegal activity". This change would not be apparent to the [=subject=] or [=holder=] unless there was monitoring software operating on their behalf that would warn them that the [=issuer=] intends to expose additional information about their activity.

Holder software can provide features to [=holders=] that warn them about the level of [=holder=] and/or [=subject=] information exposure when using [=verifiable credentials=] that are associated with status messages, and warn them when the level of information exposure changes.

Security Considerations

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

Readers are urged to familiarize themselves with the general security advice provided in the Security Considerations section of the Verifiable Credentials specification before reading this section.

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

Bitstring Encoding

It is critical that implementers pay particular attention to the way that they encode and decode bitstrings. Failure to do so can result in checking the wrong bitstring index for a given credential, leading to a misinterpretation of its present state (e.g., mistaking a revoked status for an unrevoked status). As stated in Section , bitstrings are encoded such that the first (zeroth) index refers to the left-most bit of the bitstring array. The diagram below demonstrates the proper layout for an uncompressed bitstring.

a diagram showing two wide rectangles shown side-by-side. Each rectangle is partitioned into eight boxes to represent the 8 bits in a byte. The left rectangle is labeled 'First byte' and the right rectangle is labeled 'Last byte'. The left rectangle's left-most bit box is pointed to by an arrow labeled 'index: 0'. The right rectangle's right-most bit box is pointed to by an arrow labeled 'index: length - 1'.
A visual depiction of the bitstring layout.

For example, if a bitstring is 131,072 bits in size (16KB), the first index will be 0, and the last index will be 131,071.

Accessibility Considerations

Readers are urged to familiarize themselves with the general accessibility advice provided in the Accessibility Considerations section of the Verifiable Credentials specification. No further advice is provided in this specification beyond the general advice for all [=verifiable credentials=].

Internationalization Considerations

Readers are urged to familiarize themselves with the general internationalization advice provided in the Internationalization Considerations section of the Verifiable Credentials specification. No further advice is provided in this specification beyond the general advice for all [=verifiable credentials=].

Appendix

Examples

Revocable Verifiable Credential

{
  "@context": [
    "https://www.w3.org/ns/credentials/v2",
    "https://www.w3.org/ns/credentials/examples/v2"
  ],
  "id": "https://example.com/credentials/23894672394",
  "type": ["VerifiableCredential"],
  "issuer": "did:example:12345",
  "validFrom": "2021-04-05T14:27:42Z",
  "credentialStatus": {
    "id": "https://example.com/credentials/status/3#94567",
    "type": "BitstringStatusListEntry",
    "statusPurpose": "revocation",
    "statusListIndex": "94567",
    "statusListCredential": "https://example.com/credentials/status/3"
  },
  "credentialSubject": {
    "id": "did:example:6789",
    "type": "Person"
  }
}

Status List Verifiable Credential

{
  "@context": [
    "https://www.w3.org/ns/credentials/v2",
    "https://www.w3.org/ns/credentials/examples/v2"
  ],
  "id": "https://example.com/credentials/status/3",
  "type": ["VerifiableCredential", "BitstringStatusListCredential"],
  "issuer": "did:example:12345",
  "validFrom": "2021-04-05T14:27:40Z",
  "credentialSubject": {
    "id": "https://example.com/status/3#list",
    "type": "BitstringStatusList",
    "statusPurpose": "revocation",
    "encodedList": "uH4sIAAAAAAAAA-3BMQEAAADCoPVPbQwfoAAAAAAAAAAAAAAAAAAAAIC3AYbSVKsAQAAA"
  }
}

Multiple Status Lists in One Verifiable Credential

This specification enables an [=issuer=] to associate multiple status lists with a single [=verifiable credential=]. 

{
  "@context": [
    "https://www.w3.org/ns/credentials/v2",
    "https://www.w3.org/ns/credentials/examples/v2"
  ],
  "id": "https://example.com/credentials/23894672394",
  "type": ["VerifiableCredential"],
  "issuer": "did:example:12345",
  "issuanceDate": "2021-04-05T14:27:42Z",
  // note the use of an array to represent the set of
  // status entries
  "credentialStatus": [{
    "id": "https://example.com/credentials/status/3#94567",
    "type": "BitstringStatusListEntry",
    "statusPurpose": "revocation",
    "statusListIndex": "94567",
    "statusListCredential": "https://example.com/credentials/status/3"
  }, {
    "id": "https://example.com/credentials/status/4#12345",
    "type": "BitstringStatusListEntry",
    "statusPurpose": "suspension",
    "statusListIndex": "12345",
    "statusListCredential": "https://example.com/credentials/status/4"
  }],
  "credentialSubject": {
    "id": "did:example:6789",
    "type": "Person"
  }
}

Multiple Status Entries in a Single List

It is possible for a single status list to contain multiple types of status purposes. Doing so can make the retrieval of a list slightly more efficient than fetching multiple status lists.

The "space efficiency" argument for this feature is weak. One list with two types of status entries must, presumably, be twice as long as a list with one type of status entries, to ensure proper privacy protections. One privacy benefit of doing so is that bit flips cannot be known to be associated with a particular status unless one is also in control of the VC that the status is about. Therefore, mixing "revocation" and "suspension" in a single list that is twice as large has positive privacy implications.

The "retrieval efficiency" argument is also weak. Performing two HTTP retrievals instead of one is probably not significant. Performing upwards of five or six, on a list that is five or six times larger, might result in fairly meager savings over modern versions of HTTP that bundle requests over a single channel (such as HTTP/2 or HTTP/3). The requests themselves would save a handful of bytes with no significant improvement in retrieval speed.

The Working Group is looking for feedback from implementers and is considering striking this feature during the Candidate Recommendation period, since it would simplify the specification for implementations to not have to support sets of `statusPurpose` values in the status list credentials (again, a meager savings in space efficiency at a small cost to retrieval efficiency). 

{
  "@context": [
    "https://www.w3.org/ns/credentials/v2",
    "https://www.w3.org/ns/credentials/examples/v2"
  ],
  "id": "https://example.com/credentials/23894672394",
  "type": ["VerifiableCredential"],
  "issuer": "did:example:12345",
  "issuanceDate": "2021-04-05T14:27:42Z",
  // note the use of a single list to store multiple
  // status entries
  "credentialStatus": [{
    "id": "https://example.com/credentials/status/5#94567",
    "type": "BitstringStatusListEntry",
    "statusPurpose": "revocation",
    "statusListIndex": "94567",
    "statusListCredential": "https://example.com/credentials/status/5"
  }, {
    "id": "https://example.com/credentials/status/5#12345",
    "type": "BitstringStatusListEntry",
    "statusPurpose": "suspension",
    "statusListIndex": "12345",
    "statusListCredential": "https://example.com/credentials/status/5"
  }],
  "credentialSubject": {
    "id": "did:example:6789",
    "type": "Person"
  }
}