Accountable Many-to-One Signature with Short Verification Key for Self-Sovereign Identity

Self-Sovereign Identity (SSI) shifts identity management authority from central institutions to users, enhancing privacy protections. However, malicious identity providers may collude with users to issue credentials that pass verification but contain false information. While multi-signature schemes enable joint credential issuance by multiple identity providers to mitigate this risk, they result in a linear increase in the size of signatures within credentials and verification keys as the number of identity providers grows, leading to substantial storage overhead for both users and verifiers. Furthermore, malicious verifiers may leak users' credentials to third parties or unlawfully duplicate them, causing users to lose control over the distribution of their credentials. To address these challenges, we propose the Accountable Many-to-One Signature Scheme with Short Verification Key (ASVK-MOSS), a multi-signature scheme with designated verifiers and accountability that ensures fixed sizes for both the verification key and the signature. We prove the security of the proposed ASVK-MOSS under the random oracle model. Building on ASVK-MOSS, we design a novel SSI system, named MO-SSI, in which multiple identity providers jointly sign the user's personal information to generate a credential with fixed sizes. Service providers only need to store a minimal, fixed-size verification key to validate the credentials from any set of identity providers. Additionally, the designated verifier and accountability mitigate risks posed by malicious service providers and identity providers within MO-SSI. The theoretical analysis and experimental results demonstrate its effectiveness and feasibility.