Towards a standardized model for privacy-preserving Verifiable Credentials

Lack of standardization and the subsequent difficulty of integration has been one of the main reasons for the scarce adoption of privacy-preserving Attribute-Based Credentials (p-ABC). Integration with the W3C's Verifiable Credentials (VC) specification would help by encouraging homogenization between different p-ABC schemes and bringing them all closer to other digital credentials. What is more, p-ABCs can help to solve privacy issues that have been identified in applications of VCs to use cases like vaccination passports. However, there has not been much work focusing on the collaboration between p-ABCs and VCs. We address this topic by establishing initial steps for extra standardization of elements that will help with the integration of p-ABCs into the standard. Namely, we propose a data model for predicates, which are a staple of p-ABC systems, and tools and guidelines to ease the adaptation process like a validation meta-schema. These ideas have been applied in a proof-of-concept implementation of the OLYMPUS distributed p-ABC scheme paired with serialization following the VC data model.