I'm surprised nobody's mentioned yet that Bruce Schneier solves this problem in chapters five and six of _Applied Cryptography._ See especially page 90, "Noninteractive Zer-Knowledge Proofs." b& -- Ben.Goren@asu.edu, Arizona State University School of Music net.proselytizing (write for info): Protect your privacy; oppose Clipper. Voice concern over proposed Internet pricing schemes. Stamp out spamming. Finger ben@tux.music.asu.edu for PGP 2.3a public key.