Permutation Box (P-Box)
A permutation box (P-box) is a linear operation in block ciphers that rearranges bit positions, paired with S-boxes to achieve diffusion.
Purpose
It spreads bits to wider positions so each output bit depends on many input bits, complementing the confusion of S-boxes to realize Shannon’s confusion and diffusion.
Mechanism
A P-box is an n→n bit bijective position rearrangement implemented with a fixed permutation table. Bit permutation is linear and provides no confusion on its own, so it must be cascaded with nonlinear S-boxes.
Use cases
- The DES P permutation
- AES ShiftRows
- Block-cipher diffusion layers