In classical cryptography, a permutation cipher is a transposition cipher in which the key is a permutation.
To apply a cipher, a random permutation of size e is generated (the larger the value of e the more secure the cipher). The plaintext is then broken into segments of size e and the letters within that segment are permuted according to this key.
In theory, any transposition cipher can be viewed as a permutation cipher where e is equal to the length of the plaintext; this is too cumbersome a generalisation to use in actual practice, however.
Read more about Permutation Cipher: Identifying The Cipher, Breaking The Cipher
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