The key schedule of DES ("<<<" denotes a left rotation), showing the calculation of each round key ("Subkey").
Incryptography, the so-calledproduct ciphers are a certain kind of cipher, where the (de-)ciphering of data is typically done as an iteration ofrounds. The setup for each round is generally the same, except for round-specific fixed values called around constant, and round-specific data derived from thecipher key called around key. Akey schedule is an algorithm that calculates all the round keys from the key.
Some ciphers have simple key schedules. For example, the block cipherTEA 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 byPermuted 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 ofcryptanalysis asrelated-key attacks andslide attacks, many modern ciphers use more elaborate key schedules to generate an "expanded key" from which round keys are drawn. Some ciphers, such asRijndael (AES) andBlowfish, 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 asRC5, expand keys with functions that are somewhat or completely different from the encryption functions.
Knudsen and Mathiassen (2004) give some experimental evidence that indicate that the key schedule plays a part in providing strength againstlinear anddifferential cryptanalysis. For toyFeistel ciphers, it was observed that those with complex and well-designed key schedules can reach a uniform distribution for the probabilities ofdifferentials andlinear hulls faster than those with poorly designed key schedules.
