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Incryptography,key whitening is a technique intended to increase the security of aniterated block cipher. It consists of steps that combine the data with portions of thekey.
The most common form of key whitening isxor–encrypt–xor – using a simpleXOR before the first round and after the last round ofencryption.
The first block cipher to use a form of key whitening isDES-X, which simply uses two extra 64-bit keys for whitening, beyond the normal 56-bit key ofDES. This is intended to increase the complexity of abrute-force attack, increasing the effective size of the key without major changes in the algorithm. DES-X's inventor,Ron Rivest, named the techniquewhitening.
The cipherFEAL (followed byKhufu and Khafre) introduced the practice of key whitening using portions of the same key used in the rest of the cipher. This offers no additional protection from brute-force attacks, but it can make other attacks more difficult. In aFeistel cipher or similar algorithm, key whitening can increase security by concealing the specific inputs to the first and last round functions. In particular, it is not susceptible to ameet-in-the-middle attack. This form of key whitening has been adopted as a feature of many later block ciphers, includingAES,MARS,RC6, andTwofish.
