A Randomized Algorithm to Finite Automata Public Key Cryptosystem

In this paper we give a randomized algorithm as a ciphertext attack against Finite Automata Public Key Cryptosystem(FAPKC). The finite automaton used to encrypt in FAPKC is composed of a nonlinear finite automaton and a linear finite automaton. The running time of our algorithm to find out the plaintext with high probability depends heavily on the increasing ranks of the linear finite automaton. Hence we give a way to specify the safe linear finite automaton used in FAPKC. According to our analysis, the parameter τ (delay) should be enlarged and the increasing ranks of the linear finite automaton should be chosen carefully.