**Locally Computable UOWHF with Linear Shrinkage**

*Benny Applebaum and Yoni Moses*

**Abstract: **We study the problem of constructing locally computable Universal One-Way Hash Functions (UOWHFs) $\mathcal{H}:\{0,1\}^n \rightarrow \{0,1\}^m$. A construction with constant \emph{output locality}, where every bit of the output depends only on a constant number of bits of the input, was established by [Applebaum, Ishai, and Kushilevitz, SICOMP 2006]. However, this construction suffers from two limitations: (1) It can only achieve a sub-linear shrinkage of $n-m=n^{1\epsilon}$; and (2) It has a super-constant \emph{input locality}, i.e., some inputs influence a large super-constant number of outputs. This leaves open the question of realizing UOWHFs with constant output locality and linear shrinkage of $n-m= \epsilon n$, or UOWHFs with constant input locality and minimal shrinkage of $n-m=1$.

We settle both questions simultaneously by providing the first construction of UOWHFs with linear shrinkage, constant input locality, and constant output locality. Our construction is based on the one-wayness of ``random'' local functions -- a variant of an assumption made by Goldreich (ECCC 2000). Using a transformation of [Ishai, Kushilevitz, Ostrovsky and Sahai, STOC 2008], our UOWHFs give rise to a digital signature scheme with a minimal \emph{additive} complexity overhead: signing $n$-bit messages with security parameter $\kappa$ takes only $O(n+\kappa)$ time instead of $O(n\kappa)$ as in typical constructions. Previously, such signatures were only known to exist under an \emph{exponential} hardness assumption. As an additional contribution, we obtain new locally-computable hardness amplification procedures for UOWHFs that preserve linear shrinkage.

**Category / Keywords: **foundations / hash functions, NC0, input locality, output locality

**Publication Info: **An extended abstract of this work appears in Eurocrypt 2013

**Date: **received 28 Jun 2013

**Contact author: **benny applebaum at gmail com

**Available format(s): **PDF | BibTeX Citation

**Version: **20130702:185715 (All versions of this report)

**Short URL: **ia.cr/2013/423

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