Finite State Verifiers I: The Power of Interaction

An investigation of interactive proof systems 1992 where the verifier is a 2-way probabilistic finite state automaton (2pfa) is initiated. In this model, it is shown: (1) IPSs in which the verifier uses private randomization are strictly more powerful than IPSs in which the random choices of the verifier are made public to the prover. (2) IPSs in which the verifier uses public randomization are strictly more powerful than 2pfa's alone, that is, without a prover. (3) Every language which can be accepted by some deterministic Turing machine in exponential time can be accepted by some IPS. Additional results concern two other classes of verifiers: 2pfa's that halt in polynomial expected time, and 2-way probabilistic pushdown automata that halt in polynomial time. In particular, IPSs with verifiers in the latter class are as powerful as IPSs where verifiers are polynomial-time probabilistic Turing machines. In a companion paper [7], zero knowledge IPSs with 2pfa verifiers are investigated. © 1992, ACM. All rights reserved.