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.
An element of M is called a plaintext message or simply a plaintext . Let M and C be the set of all strings of length five over A . The key e is chosen to be a permutation on A . In this example, key e is chosen to be the permutation which maps each letter to the one which is three positions to its right, like this. . . Once again, although the nature of this example is quite easy, very difficult encryption techniques follow the same basic ideas.