Key Whitening
Key whitening is the technique of XORing extra key material before and after the round function, strengthening a cipher against brute force without complicating the round function.
Purpose
It extends the effective key length at minimal overhead, making ciphers more robust against known-plaintext attacks, and is adopted by Twofish, AES-256, and others.
Mechanism
An input-whitening key is XORed before the first round and an output-whitening key after the last, both independent of the round keys. Decryption applies the XORs in reverse.
Use cases
- Twofish input/output whitening
- AES-256 key utilization
- Block-cipher hardening