Permutation Cipher

Requires a block length $n$, typically small, and a permutation $\sigma \in S_n$ as the secret key.

Encrypts the plaintext HELLO as ciphertext LHLEO.

Attacking Permutation Ciphers

A character frequency analysis matching the standard distribution exactly reveals that a substitution cipher, not a permutation cipher, was used, since permutation ciphers preserve single-character frequencies.