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Version: 1.4

Revoke a Verifiable Credential

The example below demonstrates two methods that an issuer can use to revoke a verifiable credential using the IOTA Identity Framework:

  1. By using the credentialStatus field in a credential and linking to a revocation method.
  2. By removing the verification method that signed the credential. This invalidates all credentials that were signed with that verification method.

Revocation methods​

The IOTA Identity Framework supports two different revocation methods: RevocationBitmap2022 and StatusList2021.

Revocation Bitmap​

RevocationBitmap2022 is the default credential revocation method used in the IOTA Identity Framework. It allows issuers to control whether a credential is valid or revoked. To do so, a revocation list (represented as a bitmap) is stored in the issuer's DID document. When a credential is issued, a unique index from the issuer's revocation list is chosen, linking the credential's status to the value of the list entry. To change the status of a credential, the issuer simply updates the corresponding entry in its revocation list.

With RevocationBitmap2022 the identity.rs library completely handles the processes required to handle credentials revocation; from creation and storage of the revocation list to its lookup. This makes RevocationBitmap2022 the preferred way for users to handle credential revocation, but it requires sufficient funds to rent out the required on-tangle space.

note

DLT's size constraints limit the size of the revocation list. With the assumption of only one such revocation list per the issuer's DID document, one may expect to be able to handle roughly 50k entries.

examples/0_basic/7_revoke_vc.rs
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If the binary value of the index in the bitmap is 1 (one), the verifiable credential is revoked, if it is 0 (zero) it is not revoked.

For example, with this approach the issuer adds an index to a credential in the credentialStatus field, such as "5". This part of the credential might then look like this:

"credentialStatus": {
  "id": "did:iota:EvaQhPXXsJsGgxSXGhZGMCvTt63KuAFtaGThx6a5nSpw#revocation",
  "type": "RevocationBitmap2022",
  "revocationBitmapIndex": "5"
},

The verifier uses the id field (did:iota:EvaQhPXXsJsGgxSXGhZGMCvTt63KuAFtaGThx6a5nSpw#revocation) to look up the service in the issuer's DID document:

{
  "id": "did:iota:EvaQhPXXsJsGgxSXGhZGMCvTt63KuAFtaGThx6a5nSpw#revocation",
  "type": "RevocationBitmap2022",
  "serviceEndpoint": "data:application/octet-stream;base64,ZUp5ek1tQmdZR1NBQUFFZ1ptVUFBQWZPQUlF"
}

During verification, the verifier decodes the revocation bitmap embedded in the data URL. This bitmap written as a bitstring looks like this: 000001. Here, the 5th bit is set, which means the credential with that index is revoked, while all other credentials aren't revoked.

StatusList2021​

StatusList2021 offers similar functionalities to RevocationBitmap2022 but in a more flexible and scalable way. The main difference is that StatusList2021 is completely agnostic in regards to how the issuer's status list is stored and fetched, as long as its location can be encoded through a URL. For instance, the status list can be made available over HTTP (e.g. https://example.com/credentials/status) or through the Interplanetary File System (e.g. ipfs://QmXDWGdVBhbDoXXzKNMhJk5ejnZgxpMBVzW4EhQaHPD3Mi).

This flexibility, although it requires the issuer to manually fetch and update its status list, allows for an arbitrary number of entries in the status list, in contrast with RevocationBitmap2022, where the length of the list is limited by the DLT's constraints.

Furthermore, StatusList2021 introduces a new credential state: suspended. Suspended credentials are credentials that a validator will not accept as valid, but that might become valid again in the future. Instead, revoked credentials cannot ever be valid again, as the revoked state is irreversible.

examples/1_advanced/8_status_list_2021.rs
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First, an issuer creates a credential that encodes a certain status list, specifying its purpose (either revocation or suspension) and the location at which it will be available (https://example.com/credentials/status in this case). After creation, the issuer must make the credential available at the chosen URL so that verifiers can fetch it.

Upon issuing a credential, to revoke it or suspend it later, the issuer sets the credentialStatus field, linking to an entry in its status list. The snippet below shows what credentialStatus would look like when linking to the previously created status list credential. 

{
    "id": "https://example.com/credentials/status#94567",
    "type": "StatusList2021Entry",
    "statusPurpose": "revocation",
    "statusListIndex": "94567",
    "statusListCredential": "https://example.com/credentials/status"
}
examples/1_advanced/8_status_list_2021.rs
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To set the status of a credential, the issuer retrieves the status list credential and sets the value of the chosen entry index.

Removing the Verification Method​

A less efficient alternative is to remove the verification method that signed the credential from the DID Document of the issuer. This means the VC can no longer be validated.

However, this will also invalidate every VC signed with that verification method, meaning that the issuer will have to sign every VC with a different key to retain precise control over which credential is revoked.

examples/0_basic/7_revoke_vc.rs
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Full Example Code​

The following code exemplifies how you can revoke a Verifiable Credential (VC).

examples/0_basic/7_revoke_vc.rs
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