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SUNRISE Scientific Publication

Aggregate Signatures with Versatile Randomization and Issuer-Hiding Multi-Authority Anonymous Credentials

SUNRISE partner AIT published a paper in the proceedings of The 30th ACM Conference on Computer and Communications Security (ACM CCS 2023) titled, ‘Aggregate Signatures with Versatile Randomization and Issuer-Hiding Multi-Authority Anonymous Credentials’


Anonymous credentials (AC) have emerged as a promising privacy-preserving solution for user-centric identity management. They allow users to authenticate in an anonymous and unlinkable way such that only required information (i.e., attributes) from their credentials are revealed. With the increasing push towards decentralized systems and identity, e.g., self-sovereign identity (SSI) and the concept of verifiable credentials, this also necessitates the need for suit- able AC systems. For instance, when relying on existing AC systems, obtaining credentials from different issuers requires the presentation of independent credentials, which can become cumbersome. Consequently, it is desirable for AC systems to support the so-called multi-authority (MA) feature. It allows a compact and efficient showing of multiple credentials from different is- suers. Another important property is called issuer hiding (IH). This means that showing a set of credentials is not revealed which issuer has issued which credentials but only whether a verifier- defined policy on the acceptable set of issuers is satisfied. This issue becomes particularly acute in the context of MA, where a user could be uniquely identified by the combination of issuers in their showing. Unfortunately, there are no AC schemes that satisfy both these properties simultaneously. To close this gap, we introduce the concept of Issuer-Hiding Multi-Authority Anonymous Credentials (IhMA). Our proposed solution involves the development of two new signature primitives with versatile randomization features which are independent of interest: 1) Aggregate Sig- natures with Randomizable Tags and Public Keys (AtoSa) and 2) Aggregate Mercurial Signatures (ATMS), which extend the functionality of AtoSa to additionally support the randomization of messages and yield the first instance of an aggregate (equivalence-class) structure-preserving sig- nature. These primitives can be elegantly used to obtain IhMA with different trade-offs but have applications beyond. We formalize all notations and provide rigorous security definitions for our proposed primitives. We present provably secure and efficient instantiations of the two primitives as well as corresponding IhMA systems. Finally, we provide benchmarks based on an implementation to demonstrate the practical efficiency of our constructions.

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