Paper 2005/324

Extracting bits from coordinates of a point of an elliptic curve

Nicolas Gürel

Abstract

In the classic Diffie-Hellman protocol based on a generic group $\G$, Alice and Bob agree on a common secret $K_{AB}$ (master secret) which is indistinguishable from another element of $\G$ but not from a random bits-string of the same length. In this paper, we present a new deterministic method to extract bits from $K_{AB}$ when $\G$ is an elliptic curve defined over a quadratic extension of a finite field. In the last section, we show that it is also possible to extract a few bits when $\G$ is an elliptic curve defined over a prime field.