The field of protocol analysis is one area in which CSP has proven particularly successful, and several techniques have been proposed that use CSP to reason about security properties such as confidentiality and authentication. In this paper we describe one such approach, based on theorem-proving, that uses the idea of a rank function to establish the correctness of protocols. This description is motivated by the consideration of a simple, but flawed, authentication protocol. We show how a rank function analysis can be used to locate this flaw and prove that a modified version of the protocol is correct.