Revocation Timeframe 2024

Abstract

This specification describes a new revocation mechanism - RevocationTimeframe2024 - that extends RevocationBitmap2022 to address its linkability security concerns, at least when used in combination with selectively disclosable (SD-able) VCs.

Introduction

RevocationBitmap2022 allows for a simple and straightforward way for an issuer to invalidate an issued VC before its expiration. While this method prevents the analysis of usage by the issuer through hosting the revocation information on-chain it comes with a high risk of linkability by colluding verifiers as they can store the bitmap index and use it to identify users. Additionally, verifiers can monitor the revocation information and at a later point, outside of any interaction, check the revocation status. To address this privacy concern, RevocationTimeframe2024 was designed as an extension that builds on top of RevocationBitmap2022, therefore sharing most of its logic.

Concepts

RevocationTimeframe2024 should be used together with selectively disclosable credentials - either SD-JWT or ZK-SD - in order to conceal to the verifier the credential's index in the issuer's revocation bitmap to avoid linkability.

Validity Timeframe

If the revocation index is concealed from the verifier how can it assert the validity of the presented credential? To solve this issue RevocationTimeframe2024 introduces the concept of a validity timeframe, i.e. a limited time span in which the credential is guaranteed to be non-revoked. By having a validity timeframe embedded in the credential's status the verifier can prove the credential's validity by verifying the credential's signature.

The downside of this mechanism is that the credential holder using RevocationTimeframe2024 has to contact the credential's issuer to update the signature - i.e. re-issue the credential - at the end of the credentials validity timeframe. Furthermore, given how a credentials validity timeframe proves the validity of the credential itself, the updates made by the issuer to the credential's status - i.e., revoking or un-revoking it - won't be reflected on the credential until a new validity timeframe starts. For this reason, issuers should choose a validity timeframe length with respect to how frequently they expect to change the credential's status; frequent updates deem shorter validity timeframes.

note

A credential holder does not need to have its credential re-issued at the end of every validity timeframe, but only when they need to present the credential to a verifier and its validity timeframe has expired.

Data Model

Revocation Timeframe Status

For an issuer to enable verifiers to check the status of a verifiable credential, the credentialStatus property MUST be specified with the following properties:

Property	Description
id	The constraints on the id property are listed in the Verifiable Credentials Data Model specification. The id MUST be a DID URL that is the URL to a Revocation Timeframe Service in the DID Document of the issuer.
type	The type property MUST be "RevocationTimeframe2024".
revocationBitmapIndex	The revocationBitmapIndex property MUST be an unsigned, 32-bit integer expressed as a string. This is the index of the credential in the issuers revocation bitmap. Each index SHOULD be unique among all credentials linking to the same Revocation Timeframe Service. To ensure user unlinkability, this value MUST NOT be disclosed to the verifier (this is done by default).
startValidityTimeframe	The startValidityTimeframe property MUST be a RFC 3339-compliant timestamp.
endValidityTimeframe	The endValidityTimeframe property MUST be a RFC 3339-compliant timestamp.

Example

An example of a verifiable credential with a credentialStatus of type RevocationTimeframe2024.

{
  "@context": "https://www.w3.org/2018/credentials/v1",
  "id": "https://example.edu/credentials/3732",
  "type": ["VerifiableCredential", "UniversityDegreeCredential"],
  "credentialSubject": {
    "id": "did:iota:B8DucnzULJ9E8cmaReYoePU2b7UKE9WKxyEVov8tQA7H",
    "GPA": "4.0",
    "degree": "Bachelor of Science and Arts",
    "name": "Alice"
  },
  "issuer": "did:iota:EvaQhPXXsJsGgxSXGhZGMCvTt63KuAFtaGThx6a5nSpw",
  "issuanceDate": "2022-06-13T08:04:36Z",
  "credentialStatus": {
    "id": "did:iota:EvaQhPXXsJsGgxSXGhZGMCvTt63KuAFtaGThx6a5nSpw#my-revocation-service",
    "type": "RevocationTimeframe2024",
    "revocationBitmapIndex": "5"
    "startValidityTimeframe": "2024-05-03T08:00:00Z",
    "endValidityTimeframe": "2024-05-03T08:05:00Z",
  },
}

Revocation Timeframe Service

To allow verifiers to check the status of a Revocation Timeframe Status, the DID document of the credential issuer MUST contain a service with the following properties:

Property	Description
id	The constraints on the id property are listed in the DID Core service specification. The id property MUST be a DID URL uniquely identifying the revocation bitmap.
type	The type property MUST be "RevocationTimeframe2024".
serviceEndpoint	The serviceEndpoint MUST be generated according to the service endpoint generation algorithm described in RevocationBitmap2022.

Example

An example of an issuers DID document where credential "5" in the #revocation service is revoked:

{
  "id": "did:iota:EvaQhPXXsJsGgxSXGhZGMCvTt63KuAFtaGThx6a5nSpw",
  "verificationMethod": [
    {
      "id": "did:iota:EvaQhPXXsJsGgxSXGhZGMCvTt63KuAFtaGThx6a5nSpw#key-1",
      "controller": "did:iota:EvaQhPXXsJsGgxSXGhZGMCvTt63KuAFtaGThx6a5nSpw",
      "type": "Ed25519VerificationKey2018",
      "publicKeyMultibase": "z3hgM9fNkhwgT5mECbj1HdKoFNZgpffwQYEV8WBVHphXq"
    }
  ],
  "capabilityInvocation": [
    {
      "id": "did:iota:EvaQhPXXsJsGgxSXGhZGMCvTt63KuAFtaGThx6a5nSpw#sign-0",
      "controller": "did:iota:EvaQhPXXsJsGgxSXGhZGMCvTt63KuAFtaGThx6a5nSpw",
      "type": "Ed25519VerificationKey2018",
      "publicKeyMultibase": "z83F6zbD3KqaxvQhqo25LvSXzoDdpZmp3EpPVonSVACwZ"
    }
  ],
  "service": [
    {
      "id": "did:iota:EvaQhPXXsJsGgxSXGhZGMCvTt63KuAFtaGThx6a5nSpw#revocation",
      "type": "RevocationTimeframe2024",
      "serviceEndpoint": "data:application/octet-stream;base64,ZUp5ek1tQmdZR1NBQUFFZ1ptVUFBQWZPQUlF"
    }
  ]
}

Algorithms

For generation and expansion of the service endpoint see RevocationBitmap2022.

Validation Algorithm

The following steps MUST be followed when checking whether a verifiable credential is revoked:

1. Let credential be a verifiable credential containing a credentialStatus whose type is RevocationTimeframe2024.
2. Let now be the string serialization of the RFC3339 timestamp representing the current UTC time.
3. Let start validity timeframe and end validity timeframe be the RFC3339 timestamps of the startValidityTimeframe and endValidityTimeframe (respectively) properties contained in the credentialStatus of the credential.
4. Return true if startValidityTimeframe <= now < endValidityTimeframe, false otherwise.

Test Vectors

See RevocationBitmap2022.