What do we mean by entity authentication?

The design of authentication protocols has proven to be surprisingly error-prone. We suggest that this is partly due to a language problem. The objectives of entity authentication are usually given in terms of human encounters while we actually implement message passing protocols. We propose various translations of the high-level objectives into a language appropriate for communication protocols. In addition, protocols are often specified at too low a level of abstraction. We argue that encryption should not be used as a general primitive as it does not capture the specific purpose for using a cryptographic function in a particular protocol.