We use properties of observational equivalence for a probabilistic process calculus to prove an authentication property of a cryptographic protocol. The process calculus is a form of π-calculus, with probabilistic scheduling instead of nondeterminism, over a term language that captures probabilistic polynomial time. The operational semantics of this calculus gives priority to communication over private channels, so that the presence of private communication does not affect the observable probability of visible actions. Our definition of observational equivalence involves asymptotic comparison of uniform process families, only requiring equivalence to within vanishing error probabilities. This definition differs from previous notions of probabilistic process equivalence that require equal probabilities for corresponding actions; asymptotics fit our intended application and make equivalence transitive, thereby justifying the use of the term “equivalence.” Our security proof uses a series of lemmas about probabilistic observational equivalence that may well prove useful for establishing correctness of other cryptographic protocols.