C denotes a set called the ciphertext space . C consists of strings of symbols from an alphabet of definition, which may differ from that of M . An element of C is called ciphertext . D denotes a decryption function or decryption transformation . Each element d of K determines a bijection from C to M .
E denotes an encryption function or encryption transformation .
Each element e of K determines a bijection from M to C . K denotes a set called the key space . An element of K is called a key . M denotes a set called the message space . M consists of strings of symbols from an alphabet of definition.