• Some ciphers have simple key schedules. For example, the block cipher
TEA splits the 128-bit key into four 32-bit pieces and uses them repeatedly in successive rounds. •
DES has a key schedule in which the 56-bit key is divided into two 28-bit halves; each half is thereafter treated separately. In successive rounds, both halves are rotated left by one or two bits (specified for each round), and then 48 round key bits are selected by
Permuted Choice 2 (PC-2) – 24 bits from the left half and 24 from the right. The rotations have the effect that a different set of bits is used in each round key; each bit is used in approximately 14 out of the 16 round keys. • To avoid simple relationships between the cipher key and the round keys, in order to resist such forms of
cryptanalysis as
related-key attacks and
slide attacks, many modern ciphers use more elaborate key schedules to generate an "expanded key" from which round keys are drawn. Some ciphers, such as
Rijndael (AES) and
Blowfish, use the same operations as those used in the data path of the cipher algorithm for their key expansion, sometimes initialized with some "
nothing-up-my-sleeve numbers". Other ciphers, such as
RC5, expand keys with functions that are somewhat or completely different from the encryption functions. == Notes ==