Near-optimal extractors against quantum storage

Abstract

We show that Trevisan's extractor and its variants \cite{T99,RRV99} are secure against bounded quantum storage adversaries. One instantiation gives the first such extractor to achieve an output length $\Theta(K-b)$, where $K$ is the source's entropy and $b$ the adversary's storage, together with a poly-logarithmic seed length. Another instantiation achieves a logarithmic key length, with a slightly smaller output length $\Theta((K-b)/K^\gamma)$ for any $\gamma>0$. In contrast, the previous best construction \cite{TS09} could only extract $(K/b)^{1/15}$ bits. Some of our constructions have the additional advantage that every bit of the output is a function of only a polylogarithmic number of bits from the source, which is crucial for some cryptographic applications. Our argument is based on bounds for a generalization of quantum random access codes, which we call \emph{quantum functional access codes}. This is crucial as it lets us avoid the local list-decoding algorithm central to the approach in \cite{TS09}, which was the source of the multiplicative overhead.