Relationships among the Computational Powers of Breaking Discrete Log Cryptosystems

Abstract

We investigate the complexity of breaking cryptosystems of which security is based on the discrete logarithm problem. We denote the algorithms of breaking the Diffie-Hellman’s key exchange scheme by DH, the Bellare-Micali’s non-interactive oblivious transfer scheme by BM, the ElGamal’s public-key cryptosystem by EG, the Okamoto’s conference- key sharing scheme by CONF, and the Shamir’s 3-pass key-transmission scheme by 3PASS, respectively. We show a relation among these cryp- tosystems that 3PASS ≤FPm CONF ≤FPm EG ≡FPm DH, where ≤FPm denotes the polynomial-time functionally many-to-one re- ducibility, i.e. a function version of the ≤pm -reducibility. We further give some condition in which these algorithms have equivalent difficulty. Namely, 1. If the complete factorization of p − 1 is given, i.e. if the the dis- crete logarithm problem is a certified one, then these cryptosystems are equivalent w.r.t. expected polynomial-time functionally Turing reducibility. 2.If the underlying group is the Jacobian of an elliptic curve over Z p with a prime order, then these cryptosystems are equivalent w.r.t. polynomial-time functionally many-to-one reducibility.We also discuss the complexity of several languages related to those computing problems.