A self-service terminal (10) is described. The terminal (10) includes a plurality of modules (14) arranged in a network (16) so that the modules are operable to communicate using the network (16). Each module (14) has storage means (34) for storing data and cipher means (32) for encrypting and decrypting communications, whereby the cipher means (32) is operable to encrypt data prior to sending or receiving a communication, and subsequently to decrypt a received encrypted communication by applying a Boolean function to the encrypted data and to the received encrypted communication. A module (14) for use in an SST (10) and a method of encrypting a communication for transmission between interconnected modules (14) in a self-service terminal (10) are also described.