Dual-System Simulation-Soundness with Applications to UC-PAKE and More

We introduce a novel concept of dual-system simulation-sound non-interactive zero-knowledge (NIZK) proofs. Dual-system NIZK proof system can be seen as a two-tier proof system. As opposed to the usual notion of zero-knowledge proofs, dual-system defines an intermediate partialsimulation world, where the proof simulator may have access to additional auxiliary information about the potential language member, for example a membership bit, and simulation of proofs is only guaranteed if the membership bit is correct. Further, dual-system NIZK proofs allow a quasi-adaptive setting where the CRS can be generated based on language parameters. This allows for the further possibility that the partial-world CRS simulator may have access to additional trapdoors related to the language parameters. We show that for important hard languages like the Diffie-Hellman language, such dual-system proof systems can be given which allow unbounded partial simulation soundness, and which further allow transition between partial simulation world and single-theorem full simulation world even when proofs are sought on non-members. The construction is surprisingly simple, involving only two additional group elements for general linear-subspace languages in asymmetric bilinear pairing groups. As a direct application we give a short keyed-homomorphic CCA-secure encryption scheme. The ciphertext in this scheme consists of only six group elements (under the SXDH assumption) and the security reduction is tight. An earlier scheme of Libert et al based on their efficient unbounded simulation-sound QA-NIZK proofs only provided a loose security reduction, and further had ciphertexts almost twice as long as ours. We also show a single-round universally-composable password authenticated key-exchange (UC-PAKE) protocol which is secure under adaptive corruption in the erasure model. The single message flow only requires nine group elements under the SXDH assumption. This is the shortest known UC-PAKE even without considering adaptive corruption. The latest published scheme which considered adaptive corruption, by Abdalla et al [ABB13], required non-constant (more than 10 times the bit-size of the password) number of group elements.