In this paper, we propose a new zero-knowledge identification protocol. While the protocol consists of only two message flows, it does not rely on any underlying signature or encryption scheme. Its zero-knowledge property is preserved under concurrent composition and reset settings. It is secure under the strongest attack model which incorporates concurrent attacks, active-intruder attacks and reset attacks. Meanwhile its performance in computation and communication is close to that of the most efficient identification protocols not based on signature or encryption systems, most of which are insecure in this strong attack model.
In this paper, we propose a new zero-knowledge identification protocol. While the protocol consists of only two message flows, it does not rely on any underlying signature or encryption scheme. Its zero-knowledge property is preserved under concurrent composition and reset settings. It is secure under the strongest attack model which incorporates concurrent attacks, active-intruder attacks and reset attacks. Meanwhile its performance in computation and communication is close to that of the most efficient identification protocols not based on signature or encryption systems, most of which are insecure in this strong attack model.
The concept of zero-knowledge (ZK) has become of fundamental importance in cryptography. However, in a setting where entities are modeled by quantum computers, classical arguments for proving ZK fail to hold since, in the quantum setting, the concept of rewinding is not generally applicable. Moreover, known classical techniques that avoid rewinding have various shortcomings in the quantum setting. We propose new techniques for building quantum zero-knowledge (QZK) protocols, which remain secure even under (active) quantum attacks. We obtain computational QZK proofs and perfect QZK arguments for any NP language in the common reference string model. This is based on a general method converting an important class of classical honest-verifier ZK (HVZK) proofs into QZK proofs. This leads to quite practical protocols if the underlying HVZK proof is efficient. These are the first proof protocols enjoying these properties, in particular the first to achieve perfect QZK. As part of our construction, we propose a general framework for building unconditionally hiding (trapdoor) string commitment schemes, secure against quantum attacks, as well as concrete instantiations based on specific (believed to be) hard problems. This is of independent interest, as these are the first unconditionally hiding string commitment schemes withstanding quantum attacks. Finally, we give a partial answer to the question whether QZK is possible in the plain model. We propose a new notion of QZK, non-oblivious verifier QZK, which is strictly stronger than honest-verifier QZK but weaker than full QZK, and we show that this notion can be achieved by means of efficient (quantum) protocols.
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.
This paper focuses on the robustness of concurrent non-malleable zero-knowledge.Existing concurrent non-malleable zero-knowledge protocols either apply non-black-box techniques,or employ black-box but incorporate a zero-knowledge sub-protocol.Hence when concurrently composed with other protocols,security of these protocols is not preserved or hard to argue.Following the well-known Feige-Shamir style,this paper presents a new construction for concurrent non-malleable zero-knowledge argument system,which is easily composed.This protocol takes the robust non-malleable commitment scheme introduced by Lin and Pass in STOC 2009 and specially designed witness indistinguishable proofs as basic components to achieve non-malleability and robustness.Moreover,this paper applies the oblivious simulation strategy to simulate the view of the adversary,and the proof technique introduced by Lin et al.in Crypto 2010 to analyze the indistinguishability of simulation.Since witness indistinguishability is closed under concurrent composition,the commitment sub-protocol is robust and the is oblivious,when concurrently composed with other protocols,this protocol is easier to work with and analyze.Based on the one-way function assumption,the round complexity of this protocol is super-logarithmic.
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].
We consider communication networks in which it is not possible to identify the source of a message which is broadcastes through the network. A natural question is whether it is possible for two users to identify each other concurrently, through a secure two-party protocol. We show that more than the existence of a secure Public Key Cryptosystem should be assumed in order to present a secure protocol for concurrent identification. We present two concurrent identification protocols: The first one relies on the existence of a center who has distributed "identification tags" two the users; while the second protocol relies on the distribution of "experimental sequences" by instances of a pre-protocol which have taken place between every two users.
Secure computation protocols inherently involve multiple rounds of interaction among the parties where, typically a party has to keep a state about what has happened in the protocol so far and then wait for the other party to respond. We study if this is inherent. In particular, we study the possibility of designing cryptographic protocols where the parties can be completely stateless and compute the outgoing message by applying a single fixed function to the incoming message (independent of any state). The problem of designing stateless secure computation protocols can be reduced to the problem of designing protocols satisfying the notion of resettable computation introduced by Canetti, Goldreich, Goldwasser and Micali (FOCS'01) and widely studied thereafter. The current start of art in resettable computation allows for construction of protocols which provide security only when a single predetermined party is resettable [15]. An exception is for the case of the zero-knowledge functionality for which a protocol in which both parties are resettable was recently obtained by Deng, Goyal and Sahai (FOCS'09). The fundamental question left open in this sequence of works is, whether fully-resettable computation is possible, when: 1) An adversary can corrupt any number of parties, and 2) The adversary can reset any party to its original state during the execution of the protocol and can restart the protocol. In this paper, we resolve the above problem by constructing secure protocols realizing any efficiently computable multi-party functionality in the plain model under standard cryptographic assumptions. First, we construct a Fully-Resettable Simulation Sound Zero-Knowledge (ss-rs-rZK) protocol. Next, based on these ss-rs-rZK protocols, we show how to compile any semi-honest secure protocol into a protocol secure against fully resetting adversaries. Next, we study a seemingly unrelated open question: "Does there exist a functionality which, in the concurrent setting, is impossible to securely realize using BB simulation but can be realized using NBB simulation?". We resolve the above question in the affirmative by giving an example of such a (reactive) functionality. Somewhat surprisingly, this is done by making a connection to the existence of a fully resettable simulation sound zero-knowledge protocol.