A-PoA: Anonymous Proof of Authorization for Decentralized Identity Management

Self-sovereign Identity Management (SSIM) pro-motes self-control of credentials without relying on external administration. However, the state-of-the-art SSIM based on Decentralized Identifiers and Verifiable Credentials (VCs) defined by the World Wide Web Consortium does not enable credential holders to verify whether a Credential Issuing Authority (CIA) legitimately issued a credential.As a remedy, our work constructs a secure authentication protocol, called A-PoA, to provide decentralized and anonymous authorization of CIAs. We leverage a cryptographic accumulator to enable the Root Authority (registering a Credential Schema) with the ability to authorize a CIA (registering a Credential Definition) to issue a credential. The proof of accumulator membership relies on a non-interactive zero-knowledge proof. This allows a credential holder or validator node to verify the validity of a CIA, while the CIA remains anonymous. Our security analysis shows the integrity and confidentiality of our protocol against hostile network participants and our experimental evaluation shows constant verification times independent of the number of authenticated CIAs. Hence, A-PoA introduces the missing building block to develop SSIM-capable and VC-compatible ecosystems acting as a drop-in replacement for traditional Public Key Infrastructure schemes.