Non-Malleable Extractors and Codes, with their Many Tampered Extensions

摘要

Randomness extractors and error correcting codes are fundamental objects in computer science. Recently, there have been several natural generalizations of these objects, in the context and study of tamper resilient cryptography. These are emph{seeded non-malleable extractors}, introduced by Dodis and Wichs cite{DW09}; emph{seedless non-malleable extractors}, introduced by Cheraghchi and Guruswami cite{CG14b}; and emph{non-malleable codes}, introduced by Dziembowski, Pietrzak and Wichs cite{DPW10}. Besides being interesting on their own, they also have important applications in cryptography. For example, seeded non-malleable extractors are closely related to privacy amplification with an active adversary, non-malleable codes are related to non-malleable secret sharing, and seedless non-malleable extractors provide a universal way to construct explicit non-malleable codes.However, explicit constructions of non-malleable extractors appear to be hard, and the known constructions are far behind their non-tampered counterparts. Indeed, the best known seeded non-malleable extractor requires min-entropy rate at least 0 49 cite{Li12b}; while explicit constructions of non-malleable two-source extractors were not known even if both sources have full min-entropy, and was left as an open problem in cite{CG14b}. In addition, current constructions of non-malleable codes in the information theoretic setting only deal with the situation where the codeword is tampered once, and may not be enough for certain applications.In this paper we make progress towards solving the above problems. Our contributions are as follows.egin{itemize} item We construct an explicit seeded non-malleable extractor for min-entropy k log 2 n . This dramatically improves all previous results and gives a simpler 2-round privacy amplification protocol with optimal entropy loss, matching the best known result in cite{Li15b}.item We construct the first explicit non-malleable two-source extractor for min-entropy k n ? n (1) , with output size n (1) and error 2 ? n (1) .item We motivate and initiate the study of two natural generalizations of seedless non-malleable extractors and non-malleable codes, where the sources or the codeword may be tampered many times. For this, we construct the first explicit non-malleable two-source extractor with tampering degree t up to n (1) , which works for min-entropy k n ? n (1) , with output size n (1) and error 2 ? n (1) . We further show that we can efficiently sample uniformly from any pre-image. By the connection in cite{CG14b}, we also obtain the first explicit non-malleable codes with tampering degree t up to n (1) , relative rate n (1) n , and error 2 ? n (1) . end{itemize}