International Association
for Cryptologic Research

Discrete-logarithm authentication protocols are known to present two interesting features: The first is that the prover's commitment, $x=g^r$, claims most of the prover's computational effort. The second is that $x$ does not depend on the challenge and can hence be computed in advance. Provers exploit this feature by pre-loading (or pre-computing) ready to use commitment pairs $r_i,x_i$. The $r_i$ can be derived from a common seed but storing each $x_i$ still requires 160 to 256 bits when implementing DSA or Schnorr.

This paper proposes a new concept called {\it slow motion zero-knowledge} (SM-ZK). SM-ZK allows the prover to slash commitment size (by a factor of 4 to 6) by combining classical zero-knowledge and a timing side-channel. We pay the conceptual price of requiring the ability to measure time but, in exchange, obtain communication-efficient protocols.