Weakening the perfect encryption assumption in Dolev-Yao adversaries

We consider secrecy and authentication in a simple process calculus with cryptographic primitives. The standard Dolev-Yao adversary is enhanced so that it can guess the key required to decrypt an intercepted message. We borrow from the computational complexity approach the assumptions that guessing succeeds with a given negligible probability and that the resources available to adversaries are polynomially bounded. Under these hypotheses we prove that the standard Dolev-Yao adversary is as powerful as the enhanced one.