What a lovely hat

We study post-quantum zero-knowledge (classical) protocols that are sound against quantum resetting attacks. Our model is inspired by the classical model of resetting provers (Barak-Goldreich-Goldwasser-Lindell, FOCS `01), providing a malicious efficient prover with oracle access to the verifier's next-message-function, fixed to some initial random tape; thereby allowing it to effectively reset (or equivalently, rewind) the verifier. In our model, the prover has quantum access to the...

The introduction of a non-black-box simulation technique by Barak (FOCS 2001) has been a major landmark in cryptography, breaking the previous barriers of black-box impossibility. Barak's technique has given rise to various powerful applications and it is a key component in all known protocols with non-black-box simulation. We present the first non-black-box simulation technique that does not rely on Barak's technique (or on nonstandard assumptions). Invoking this technique, we obtain new...

The simulation paradigm, introduced by Goldwasser, Micali and Rackoff, is of fundamental importance to modern cryptography. In a breakthrough work from 2001, Barak (FOCS'01) introduced a novel non-black-box simulation technique. This technique enabled the construction of new cryptographic primitives, such as resettably-sound zero-knowledge arguments, that cannot be proven secure using just black-box simulation techniques. The work of Barak and its follow-ups, however, all require stronger...

The traditional notion of {\em program obfuscation} requires that an obfuscation $\tilde{f}$ of a program $f$ computes the exact same function as $f$, but beyond that, the code of $\tilde{f}$ should not leak any information about $f$. This strong notion of {\em virtual black-box} security was shown by Barak et al. (CRYPTO 2001) to be impossible to achieve, for certain {\em unobfuscatable function families}. The same work raised the question of {\em approximate obfuscation}, where the...

We initiate the formal study on authenticated key exchange (AKE) under bad randomness. This could happen when (1) an adversary compromises the randomness source and hence directly controls the randomness of each AKE session; and (2) the randomness repeats in different AKE sessions due to reset attacks. We construct two formal security models, Reset-1 and Reset-2, to capture these two bad randomness situations respectively, and investigate the security of some widely used AKE protocols in...

Code obfuscation is one of the most powerful concepts in cryptography. It could yield functional encryption, digital rights management, and maybe even secure cloud computing. However, general code obfuscation has been proven impossible and the research then focused on obfuscating very specific functions, studying weaker security definitions for obfuscation, and using tamper-proof hardware tokens to achieve general code obfuscation. Following this last line this work presents the first scheme...

A fundamental question in cryptography deals with understanding the role that randomness plays in cryptographic protocols and to what extent it is necessary. One particular line of works was initiated by Canetti, Goldreich, Goldwasser, and Micali (STOC 2000) who introduced the notion of resettable zero-knowledge, where the protocol must be zero-knowledge even if a cheating verifier can reset the prover and have several interactions in which the prover uses the same random tape. Soon...

Oblivious transfer (OT, for short) [RAB81] is a fundamental primitive in the foundations of Cryptography. While in the standard model OT constructions rely on public-key cryptography, only very recently Kolesnikov in [KOL10] showed a truly efficient string OT protocol by using tamper-proof hardware tokens. His construction only needs few evaluations of a block cipher and requires stateless (therefore resettable) tokens that is very efficient for practical applications. However, the protocol...

We show new lower bounds and impossibility results for general (possibly *non-black-box*) zero-knowledge proofs and arguments. Our main results are that, under reasonable complexity assumptions: 1. There does not exist a two-round zero-knowledge *proof* system with perfect completeness for an NP-complete language. The previous impossibility result for two-round zero knowledge, by Goldreich and Oren (J. Cryptology, 1994) was only for the case of *auxiliary-input* zero-knowledge proofs...

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 adeversary can interact with the prover many times, each time resetting the prover to its initial state and forcing him to use the same random tape. Under general complexity asumptions, which hold for example if the Discrete Logarithm Problem is hard,...