## Zero-Fixing Extractors for Sub-Logarithmic Entropy

by Gil Cohen and Igor Shinkar

Towards proving the lower bounds, the authors introduce two types of sources, which may be of independent interest.

The first type, called "zero-fixing source", is a special case of a bit-fixing source in which all fixed bits are fixed to zero. That is a $(n,k)$-zero-fixing source in a source in which $n-k$ of the bits are set to zero, and the other $k$ bits are perfectly random (i.e., uniformly distributed over $\{0,1\}^k$). Such sources may arise in some applications that are modeled by bit-fixing sources; that is, in applications in which the fixed bit model defected devices, but these defects cause a default value rather than an arbitrary one (modeled by a worst-case choice).

As for the second type, I am presenting a version of it that is obtained from their notion of a $(n,k,w)$-fixed-weight source by letting $k=n$ and letting $w$ be a random variable. Specifically, for a class of distributions $D$ over $[n]$, an $(n,D)$-weight source is obtained by selecting $w$ according to some $W$ in $D$, and then letting the output be selected uniformly among all $n$-bit strings of Hamming weight $w$.

#### The original abstract

An $(n,k)$-bit-fixing source is a distribution on $n$ bit strings, that is fixed on $n-k$ of the coordinates, and jointly uniform on the remaining $k$ bits. Explicit constructions of bit-fixing extractors by Gabizon, Raz and Shaltiel [SICOMP 2006] and Rao [CCC 2009], extract $(1-o(1)) \cdot k$ bits for $k = polylog(n)$, almost matching the probabilistic argument. Intriguingly, unlike other well-studied sources of randomness, a result of Kamp and Zuckerman [SICOMP 2006] shows that, for any $k$, some small portion of the entropy in an $(n,k)$-bit-fixing source can be extracted. Although the extractor does not extract all the entropy, it does extract $(1/2 - o(1)) \cdot \log(k)$ bits.

In this paper we prove that when the entropy $k$ is small enough compared to $n$, this exponential entropy-loss is unavoidable. More precisely, one cannot extract more than $\log(k)/2 + O(1)$ bits from $(n,k)$-bit-fixing sources. The remaining entropy is inaccessible, information theoretically. By the Kamp-Zuckerman construction, this negative result is tight.

Our impossibility result also holds for what we call zero-fixing sources. These are bit-fixing sources where the fixed bits are set to $0$. We complement our negative result, by giving an explicit construction of an $(n,k)$-zero-fixing extractor, that outputs $\Omega(k)$ bits, even for $k = polyloglog(n)$. Furthermore, we give a construction of an $(n,k)$-bit-fixing extractor, that outputs $k-O(1)$ bits, for entropy $k = (1+o(1)) \cdot \log\log{n}$, with running-time $n^{O(( \log{\log{n}})^2)}$.

See ECCC TR14-160.

Back to list of Oded's choices.