ABSTRACT
Two of the main applications of cryptography are the provisions of secrecy and authentication for messages. Consider the model with three parties. A transmitter communicates a sequence of distinct source states from a set S to a receiver by encoding them using one from a set of encoding rules E . Each encoding rule is an injective mapping from S to the set of messagesM . The receiver recovers the source states from the received messages by determining their (unique) preimages under the agreed encoding rule. The receiver accepts a message as authentic if it lies in the image of the agreed encoding rule. An opponent observes the resulting sequence of messages and attempts to determine the information about the corresponding source states, thereby compromising their secrecy, or attempts to determine another message which will be accepted by the receiver as authentic, thereby deceiving the receiver. The encoding rule may be regarded as the cryptographic transformation corresponding to a secret key.
