Oblivious Credential State

Introduction Digitial Credentials often have a validity period, which indicates the time at which the claims become active for the subject according to the issuer, and the time at which the issuer specifies the claims are no longer to be considered asserted by the issuer. A typical example is a digital drivers license, which has an activation date, and an expiration date. When the activation date is in the past, and the expiration date is in the future, we consider the license to be valid at the current time. A verifier might wonder if such licenses are suspended or revoked, even if the validity period is acceptable. A common solution is to the issuer of the credential to provide a resource that reflects information about the state of the credential over time. Because issuer's track the presentation of digital credentials if a verifier where to ask the issuer about the state of a specific digital credential, it common to see credential states be merged into blocks, or herds, where an issuer can deliver the block to the verifier upon request, without learning which specific digital credential the verifier is interested in. Unfortunatly, the metadata associated with resolving credential state can leak time and location information about the presentation of credentials over time. This document addresses this risk by introducing a mediator which is trusted by the verifier.

Credential State To simplify interpretation of resolution of credential state resources, this document uses the following aliases for the terms defined in . The Verifier's Software is the Oblivious HTTP Client. The Mediator's Credential State Resource is the Oblivious Relay Resource. The Issuer's Gateway Resource is the Gateway Resource. The Issuer's Credential State Resource is the Target Resource. The critical privacy property is obtained by the verifier's client relying on the Mediator's Credential State Resource instead of Issuer's Credential State Resource. In order to achieve this property, while preserving the property that issuer's do not know which verifier's will be interested in dynamic state information associated with their credentials, the issuer includes the identifier for their Issuer's Credential State Resource in their credential claim sets, however the verifier rewrites this URL to be their Mediator's Credential State Resource before resolution. Editor note: is there a simpler solution here? + Mediator +<------>+ Issuer | '--------' +----------+ '-------' ]]>

Identifier While many different protocol schemes can be used to identify resources, to improve interoperability and reduce attack surface, this document requires credential state resources to be identified with https URLS, as described in . The following URI Templates, as described in are required to improve interoperability and reduce the chances of degrading the privacy properties through the inclusion of extraneos information in the identifiers embedded in credentials.

issuer MUST support internationalizaton considerations, as described in , for example: 🏛️.example
mediator MUST support internationalizaton considerations, as described in , for example: 🚛.example
resource-name MUST be a URN as described in , for example: urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6

Issuer's Credential State Resource

Issuer Credential State Resources

Mediator's Credential State Resource

Mediator Credential State Resources

Resources Credential state resources typically rely on a similar content type as the credentials that require them. Mixing different content types for credentials and their state, increases implementation costs and harms interoperability. The credential state resource MUST be secured with the same content type that was used to secure the digital credential that has dynamic state. For example, a JSON Web Token, based digital credential must rely on a based credential state resource. There are many different content types that can be used to secure digital credentials, this document does not require a specific content type to be used. The Accept header MUST be supported, and the application/cose content type SHOULD be supported.

Processing Credential State Resources Validation of the digital credential state MUST occur after verification. Validation of the digital credential validity period MUST occur before credential state checks. Implementers are cautioned that concepts like "suspended" or "revoked" are interpretted differently and used differently by issuers. All dynamic claims provided through credential state resources MUST be considered issuer defined, and cannot be interpretted globally. Interpretting the structure of the Issuer's Credential State Resource is outside the scope of this document. However, other documents describe this process in detail. provides guidance on processing resources that secure the content type application/vc+ld+json, such as application/cose. provides guidance on processing resources of the content type application/cwt, and application/jwt.

Techniques

CRL Distribution Points described a mechanism for verifiers to check the revocation status of attribute certificates.

Online Certificate Status Protocol described a protocol useful in determining the current status of a digital certificate without requiring CRLs.

Bitmaps In this approach, the size of the herd is the length of the bitmap, and the state of a digital credential claim is the value of the bit at a given index. Scaling this approach can be difficult, as a seperate list is needed for each dynamic claim in a digital credential. This scalling challenge can be partially addressed by consuming multiple bits at a given index, however, the resulting enumeration needs to be consistently understood. A common solution to consistent interpretation of enumerations is the establishment of a registry, however this can become impractical depending on the nature of the issuer's need to express dynamic state. Publishing a dictionary per issuer, or per sets of issuer's can help address these challenges for some use cases.

Cryptographic Accumulators describes an approach to expressing proofs of set membership.

Bloom Filters mentions an application of bloom filters, that can be applied to communicating credential state assuming the probabilistic nature of bloom filters is acceptable to the verifier.

Transparency Services Tree structures, such as described in can be used to provide advanced membership proofs, such as proving inclusion, consistency, non inclusion, and freshness.