Post-Whitening
Post-whitening XORs the output with an independent key after the final round function, symmetric to pre-whitening.
Purpose
It introduces extra key material after the round function so the final ciphertext depends on more key bits, raising the security margin.
Mechanism
ciphertext = round_output XOR K_post, with K_post independent of the round keys and the pre-whitening key. The first decryption step XORs K_post to recover the round output.
Use cases
- Twofish encryption
- Block-cipher hardening
- Effective key-length extension