DES Permutation
DES permutations are bit-rearrangement operations — the initial/final permutations (IP/IP^-1), the expansion permutation (E), and others — that spread bit positions across the DES stages.
Purpose
They rearrange bits across 64/32/48-bit blocks and, together with the S-boxes, achieve diffusion so each output bit depends on many input bits.
Mechanism
IP reorders the 64-bit input by a fixed table; E expands the 32-bit half-block to 48 bits (duplicating some bits) for XOR with the round key; the P permutation reorders the 32 bits after the S-boxes; IP^-1 restores the order at the end.
Use cases
- Bit diffusion in DES encryption
- Permutation stages of 3DES
- Feistel network components