A zero-knowledge protocol allows a prover to convince a verifier of the correctness of a statement without disclosing any other information to the verifier. It is a basic tool and widely used in many other cryptographic applications. However, when stand-alone zero-knowledge protocols are used in complex environments, e.g., the Internet, the basic properties may not be sufficient. This is why researchers considered security of zero-knowledge protocols under concurrent composition and manin-the-middle attacks. Moreover, it is very likely that an adversary might break computers that run the protocol and get internal information of the parties. It is thus necessary to take account of the security of zero-knowledge protocols when adaptive corruptions are allowed. Previous adaptively secure zero-knowledge protocols work either in a stand-alone setting, or in a concurrent setting with trusted setup assumptions. In this paper, we study adaptive security of zero-knowledge protocols under both concurrent self composition and man-in-the-middle attacks in the plain model (i.e., without any set-up assumptions). We provide a construction of adaptively secure concurrent non-malleable zero-knowledge proof/argument for every language in NP.
Recently, Goyal (STOC’13) proposed a new non-black-box simulation techniques for fully concurrent zero knowledge with straight-line simulation. Unfortunately, so far this technique is limited to the setting of concurrent zero knowledge. The goal of this paper is to study what can be achieved in the setting of concurrent secure computation using non-black-box simulation techniques, building upon the work of Goyal. The main contribution of our work is a secure computation protocol in the fully concurrent setting with a straight-line simulator, that allows us to achieve several new results: • We give first positive results for concurrent blind signatures and verifiable random functions in the plain model as per the ideal/real world security definition. Our positive result is somewhat surprising in light of the impossibility result of Lindell (STOC’03) for black-box simulation. We circumvent this impossibility using non-black-box simulation. This gives us a quite natural example of a functionality in concurrent setting which is impossible to realize using black-box simulation but can be securely realized using non-black-box simulation.
We introduce the notion of Resettable Zero-Knowledge (rZK), a new security measure for cryptographic protocols which strengthens the classical notion of zero-knowledge.In essence, an rZK protocol is one that remains zero knowledge even if an adversary can interact with the prover many times, each time resetting the prover to its initial state and forcing it to use the same random tape.All known examples of zero-knowledge proofs and arguments are trivially breakable in this setting.Moreover, by definition, all zero-knowledge proofs of knowledge are breakable in this setting.Under general complexity assumptions, which hold for example if the Discrete Logarithm Problem is hard, we construct: • Resettable Zero-Knowledge proof-systems for NP with non-constant number of rounds.* Five-round Resettable Witness-Indistinguishable proofsystems for NP. e Four-round Resettabie Zero-Knowledge arguments for NP in the public key model: where verifiers have fixed, public keys associated with them.In addition to shedding new light on what makes zero knowledge possible (by constructing ZK protocols that use randomness in a dramatically weaker way than before), rZK has great relevance to applications.Firstly, rZK protocols are closed under parallel and concurrent execution and thus are guaranteed to be secure when implemented in fully asynchronous networks, even if an adversary schedules the arrival of every message sent so as to foil security.Secondly, rZK protocols enlarge the range of physical ways in which provers of ZK protocols can be securely implemented, including devices which cannot reliably toss coins on line, nor keep state *A subset of this work is included in patent application [21].