Verifiable Credentials Status List v2021

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 In a case where a few hundred credentials have been revoked, the size of the list is less than a few hundred bytes while providing privacy in a herd of 100,000 individuals.

1.1 Conceptual Framework

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.

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.

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.

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.

5.1 Bitstring Encoding

This section is non-normative.

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 2.2 StatusList2021Credential, 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.

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.

This section is non-normative.

There are a number of accessibility considerations implementers should be aware of when processing data described in this specification. As with any web standards or protocols implementation, ignoring accessibility issues makes this information unusable to a large subset of the population. It is important to follow accessibility guidelines and standards, such as [WCAG21], to ensure all people, regardless of ability, can make use of this data. This is especially important when establishing systems utilizing cryptography, which have historically created problems for assistive technologies.

This section details the general accessibility considerations to take into account when utilizing this data model.

This section is non-normative.

There are a number of internationalization considerations implementers should be aware of when publishing data described in this specification. As with any web standards or protocols implementation, ignoring internationalization makes it difficult for data to be produced and consumed across a disparate set of languages and societies, which would limit the applicability of the specification and significantly diminish its value as a standard.

This section outlines general internationalization considerations to take into account when utilizing this data model.