Keyed-Verification Anonymous Credentials with Highly Efficient Partial Disclosure

• An anonymous credential (AC) system with partial disclosure lets users prove they have a credential while only sharing specific attributes in a privacy-friendly way. In KVAC systems, the issuer and verifier share a secret key.
• Current KVAC systems rely on heavy zero-knowledge proofs, and the size of the presentation grows with the number of attributes. This work introduces two new KVAC designs that remove the need for zero-knowledge proofs and keep presentation sizes constant.
• The first approach builds on ideas from Fuchsbauer et al. (JoC'19), using structure-preserving message authentication codes (SP-MAC-EQ) and designated-verifier set commitments (DVSC). This results in constant-size credentials (2 group elements) and presentations (4 group elements).
• The second approach skips bilinear groups and pairings by using a homomorphic MAC and a simplified DVSC. While credentials grow with the number of attributes, presentation sizes stay constant (2 group elements) in a pairing-free setup.
• Both designs are proven secure, with open-source implementations showing their practicality. Benchmarks also highlight big performance gains, especially with SP-MAC-EQ compared to the original SPS-EQ scheme.

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