International Association
for Cryptologic Research

Recently, the problem of privacy amplification with an active adversary has received a lot of attention. Given a shared $n$-bit weak random source $X$ with min-entropy $k$ and a security parameter $s$, the main goal is to construct an explicit 2-round privacy amplification protocol that achieves entropy loss $O(s)$. Dodis and Wichs \\cite{DW09} showed that optimal protocols can be achieved by constructing explicit \\emph{non-malleable extractors}. However, the best known explicit non-malleable extractor only achieves $k=0.49n$ \\cite{Li12b} and evidence in \\cite{Li12b} suggests that constructing explicit non-malleable extractors for smaller min-entropy may be hard. In an alternative approach, Li \\cite{Li12} introduced the notion of a non-malleable condenser and showed that explicit non-malleable condensers also give optimal privacy amplification protocols.

In this paper, we give the first construction of non-malleable condensers for arbitrary min-entropy. Using our construction, we obtain a 2-round privacy amplification protocol with optimal entropy loss for security parameter up to $s=\\Omega(\\sqrt{k})$. This is the first protocol that simultaneously achieves optimal round complexity and optimal entropy loss for arbitrary min-entropy $k$. We also generalize this result to obtain a protocol that runs in $O(s/\\sqrt{k})$ rounds with optimal entropy loss, for security parameter up to $s=\\Omega(k)$. This significantly improves the protocol in \\cite{ckor}. Finally, we give a better non-malleable condenser for linear min-entropy, and in this case obtain a 2-round protocol with optimal entropy loss for security parameter up to $s=\\Omega(k)$, which improves the entropy loss and communication complexity of the protocol in \\cite{Li12b}.