Non-malleable Extractors, Two-Source Extractors and Privacy Amplification

In [1], Dodis and Wichs introduced the notion of a non-malleable extractor. A non-malleable extractor is a much stronger version of a seeded extractor. Dodis and Wichs showed that such an object can be used to give optimal privacy amplification protocols with an active adversary. Previously, there are only two known constructions of nonmalleable extractors [2], [3]. Both constructions only work for (n, k)-sources with k >; n/2. Interestingly, both constructions are also two-source extractors. In this paper, we present a strong connection between nonmalleable extractors and two-source extractors. The first part of the connection shows that non-malleable extractors can be used to construct two-source extractors. This partially explains why previous constructions of non-malleable extractors only work for entropy rate >; 1/2, and why explicit non-malleable extractors for small min-entropy may be hard to get. The second part of the connection shows that certain two-source extractors can be used to construct non-malleable extractors. Using this connection, we obtain the first construction of non-malleable extractors for k <; n/2. Finally, despite the lack of explicit non-malleable extractors for arbitrarily linear entropy, we give the first 2-round privacy amplification protocol with asymptotically optimal entropy loss and communication complexity for (n, k) sources with k = αn for any constant α >; 0. This dramatically improves previous results and answers an open problem in [2].