In Proceedings of Crypto 2009, vol. 5677 of LNCS. pages 108–125. Springer-Verlag, Aug. 2009.
We construct an efficient delegatable anonymous credential system. Users can anonymously and unlinkably obtain credentials from any authority, delegate their credentials to other users, and prove possession of a credential L levels away from the given authority. The size of the proof (and time to compute it) is O(Lk), where k is the security parameter. The only other construction of delegatable anonymous credentials (Chase and Lysyanskaya, Crypto 2006) relies on general non-interactive proofs for NP-complete languages of size kΩ(2L).
We revise the entire approach to constructing anonymous credentials and identify randomizable zero-knowledge proof of knowledge systems as the key building block. We formally define the notion of randomizable non-interactive zero-knowledge proofs, and give the first construction by showing how to appropriately rerandomize Groth and Sahai (Eurocrypt 2008) proofs. We show that such proof systems, in combination with an appropriate authentication scheme and a few other protocols, allow us to construct delegatable anonymous credentials. Finally, we instantiate these building blocks under appropriate assumptions about groups with bilinear maps.