P-box

Tags

cryptography

A [see page 27, P-Box] permutation. This reorders the input bits in a predictable way, but doesn't manipulate or otherwise alter the number of 0s or 1s in the input block.

See the [see page 27, DES] example, where the first and last bits of a block are swapped with the last and first bits of the adjacent blocks. With the first and last blocks wrapping-around their first and last bits to each other, as if they were adjacent neighbours.

Consider a Transposition Cipher as an example of a P-box implementation, such a cipher takes successive blocks of the same sized input and re-orders them in a consistent way. For example with the input ABCDE and the order key \( \{2,1,5,3,4\} \) we'd get a cipher-text of BAECD.

We call the [see page 3, permutation] of a round as the mapping of bits, from the input to the round, to the output of it. In the example before the permutation is:

| Input  | A B C D E |
| Output | B A E C D |