We provide a zero-knowledge argument for arithmetic circuit satisfiability with a communication complexity that grows logarithmically in the size of the circuit. The round complexity is also logarithmic and for an arithmetic circuit with fan-in 2 gates the computation of the prover and verifier is linear in the size of the circuit. The soundness of our argument relies solely on the well-established discrete logarithm assumption in prime order groups. At the heart of our new argument system is an efficient zero-knowledge argument of knowledge of openings of two Pedersen multicommitments satisfying an inner product relation, which is of independent interest. The inner product argument requires logarithmic communication, logarithmic interaction and linear computation for both the prover and the verifier. We also develop a scheme to commit to a polynomial and later reveal the evaluation at an arbitrary point, in a verifiable manner. This is used to build an optimized version of the constant round square root complexity argument of Groth (CRYPTO 2009), which reduces both communication and round complexity.Informally, a zero-knowledge argument involves two parties, the prover and the verifier, and allows the prover to prove to the verifier that a particular statement is true, without revealing anything else about the statement itself. Statements are of the form u ∈ L, where L is a language in NP. We call w a witness for a statement u if (u, w) ∈ R, where R is a polynomial time decidable binary relation associated with L. We require the zero-knowledge argument to be complete, sound and zero-knowledge. Completeness:A prover with a witness w for u ∈ L can convince the verifier of this fact. Soundness: A prover cannot convince a verifier when u / ∈ L. Zero-knowledge: The interaction should not reveal anything to the verifier except that u ∈ L. In particular, it should not reveal the prover's witness w.Our goal is to build an efficient argument system for the satisfiability of an arithmetic circuit, i.e., a circuit that consists of addition and multiplication gates over a finite field Z p . Moreover we want to base the security of this argument solely on the discrete logarithm assumption: this will provide both strong security guarantees and good efficiency since there exists no known attacks better than generic ones for well-chosen elliptic curve subgroups.The most efficient zero-knowledge arguments solely based on the discrete logarithm assumption are Groth's protocol based on linear algebra [Gro09b] and its variant by Seo [Seo11]. Both of these protocols have a communication complexity that is proportional to the square root of the circuit size. This square root complexity has since then appeared as a (perhaps fundamental) barrier for discrete logarithm-based arguments for circuit satisfiability. Our ContributionsWe provide an honest verifier zero-knowledge argument for arithmetic circuit satisfiability based on the discrete logarithm assumption that only requires a logarithmic communication complexity. Our argument has ...

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Group signatures allow members of a group to anonymously sign on behalf of the group. Membership is administered by a designated group manager. The group manager can also reveal the identity of a signer if and when needed to enforce accountability and deter abuse. For group signatures to be applicable in practice, they need to support fully dynamic groups, i.e., users may join and leave at any time. Existing security definitions for fully dynamic group signatures are informal, have shortcomings, and are mutually incompatible. We fill the gap by providing a formal rigorous security model for fully dynamic group signatures. Our model is general and is not tailored toward a specific design paradigm and can therefore, as we show, be used to argue about the security of different existing constructions following different design paradigms. Our definitions are stringent and when possible incorporate protection against maliciously chosen keys. We consider both the case where the group management and tracing signatures are administered by the same authority, i.e., a single group manager, and also the case where those roles are administered by two separate authorities, i.e., a group manager and an opening authority. We also show that a specialization of our model captures existing models for static and partially dynamic schemes. In the process, we identify a subtle gap in the security achieved by group signatures using revocation lists. We show that in such schemes new members achieve a slightly weaker notion of traceability. The flexibility of our security model allows to capture such relaxation of traceability.

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