A permutation of a set `M`

is a bijection `f:M→M`

.
This is commonly used in a cipher to achieve diffusion.

Consider the set `M={1, 2, 3}`

.
The function `f(x)=x`

is a permutation.
The function `f={(1,3), (2,1), (3,2)}`

is also a permutation.
The function `f={(1,2), (2,3), (3,1)}`

is the inverse permutation of the previous permutation.

A. Menzes, P. van Oorschot, and S. Vanstone, *Handbook of Applied Cryptography.* CRC Press, 1996.

Copyright © 2014 Barry Watson. All rights reserved.