The Kerberos-One-Time protocol is a key distribution protocol promoted for use with Javacards to provide secure communication over the GSM mobile phone network. From inspection we suspected a replay attack was possible on the protocol. To check this, we formally specified the protocol using Object-Z and then analysed its behaviour in the presence of an attacker using the Symbolic Analysis Laboratory?s model checker. To produce accurate results efficiently, our formalism included an abstraction of the protocol?s data structures that captured just those characteristics that we believed made the protocol vulnerable. Ultimately, the model checker?s analysis confirmed our suspicions about the protocol?s weakness.