Chernoff-type direct product theorems

Russell Impagliazzo, Ragesh Jaiswal, and Valentine Kabanets

Abstract

Consider a challenge-response protocol where the probability of a correct response is at least α for a legitimate user, and at most β < α for an attacker. One example is a CAPTCHA challenge, where a human should have a significantly higher chance of answering a single challenge (e.g., uncovering a distorted letter) than an attacker. Another example would be an argument system without perfect completeness. A natural approach to boost the gap between legitimate users and attackers would be to issue many challenges, and accept if the response is correct for more than a threshold fraction, for the threshold chosen between α and β. We give the first proof that parallel repetition with thresholds improves the security of such protocols. We do this with a very general result about an attacker's ability to solve a large fraction of many independent instances of a hard problem, showing a Chernoff-like convergence of the fraction solved incorrectly to the probability of failure for a single instance.

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