Identity management systems enable users (i.e., provers) to authenticate and provide attributes to verifiers by using certified credentials obtained from an authority. To accept such a credential, verifiers require information on whether the presented credentials are still valid or if they have been revoked. Up-to-date revocation information can be obtained from a revocation database; however, this requires that the verifier or prover is online. The problem becomes more interesting in the offline case when the prover (e.g., citizen) and verifier (e.g., police officer) do not have an Internet connection to query the revocation status of the presented credential (e.g., digital driver's license). In this paper, we extend the Self-Sovereign Identity (SSI) model to support both revocation as well as offline-verification. Our concept introduces attestations of validity for a point in time, which are issued by the SSI network for credentials that have not been revoked, i.e., added by authorized entities to a revocation list. The concept aims to be generic so that it can be used for various use cases, e.g., by giving users the control over the frequency of re-attestation. To show our concept's feasibility and practicality, we developed and evaluated an implementation that includes an efficient and privacy-preserving showing of credentials using noninteractive zero-knowledge proofs, all while being offline.

Enhancing trust among service providers and end-users with respect to data protection is an urgent matter in the growing information society. In response, CREDENTIAL proposes an innovative cloud-based service for storing, managing, and sharing of digital identity information and other highly critical personal data with a demonstrably higher level of security than other current solutions. CREDENTIAL enables end-to-end confidentiality and authenticity as well as improved privacy in cloud-based identity management and data sharing scenarios. In this paper, besides clarifying the vision and use cases, we focus on the adoption of CREDENTIAL. Firstly, for adoption by providers, we elaborate on the functionality of CREDENTIAL, the services implementing these functions, and the physical architecture needed to deploy such services. Secondly, we investigate factors from related research that could be used to facilitate CREDENTIAL's adoption and list key benefits as convincing arguments.

Data sharing - and in particular sharing of identity information - plays a vital role in many online systems. While in closed and trusted systems security and privacy can be managed more easily, secure and privacy-preserving data sharing as well as identity management becomes difficult when the data are moved to publicly available and semi-trusted systems such as public clouds. CREDENTIAL is therefore aiming on the development of a secure and privacy-preserving data sharing and identity management platform which gives stronger security guarantees than existing solutions on the market. The results will be showcased close to market-readiness through pilots from the domains of eHealth, eBusiness, and eGovernment, where security and privacy are crucial. From a technical perspective, the privacy and authenticity guarantees are obtained from sophisticated cryptographic primitives such as proxy re-encryption and redactable signatures.