Two-Source Randomness Extractors for Elliptic Curves for Authenticated Key Exchange

Abstract

This paper studies the task of two-sources randomness extractors for elliptic curves defined over finite fields K, where K can be a prime or a binary field. In fact, we introduce new constructions of functions over elliptic curves which take in input two random points from two different subgroups. In other words, for a given elliptic curve E defined over a finite field $\mathbb {F}_q$ and two random points $P \in \mathcal {P}$ and $Q\in \mathcal {Q}$, where $\mathcal {P}$ and $\mathcal {Q}$ are two subgroups of $E(\mathbb {F}_q)$, our function extracts the least significant bits of the abscissa of the point $P\oplus Q$ when q is a large prime, and the k-first $\mathbb {F}_p$ coefficients of the abscissa of the point $P\oplus Q$ when $q = p^n$, where p is a prime greater than 5. We show that the extracted bits are close to uniform.

Our construction extends some interesting randomness extractors for elliptic curves, namely those defined in [7, 9, 10], when $\mathcal {P} = \mathcal {Q}$. The proposed constructions can be used in any cryptographic schemes which require extraction of random bits from two sources over elliptic curves, namely in key exchange protocol, design of strong pseudo-random number generators, etc.