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. 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. . .