Key schedule
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In cryptography, the so-called product ciphers are a certain kind of ciphers, where the (de-)ciphering of data is done in so-called "rounds", where the general setup of each round is the same, except for some hard-coded parameters and a part of the cipher key, called a subkey. A key scheduler is an algorithm that, given the key, calculates the subkey for these rounds.
Some types of key schedulers
- Some ciphers have simple key schedules. For example, the block cipher TEA simply splits the 128-bit key into four 32-bit pieces and uses them repeatedly in successive rounds. RC5, Rijndael (AES), and Blowfish have much more elaborate key-schedules.
- DES uses a key scheduler where 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 subkey bits are selected by Permuted Choice 2 (PC-2) — 24 bits from the left half, and 24 from the right. The rotations (denoted by "<<<" in the diagram) mean that a different set of bits is used in each subkey; each bit is used in approximately 14 out of the 16 subkeys.
Notes
Knudsen and Mathiassen (2004) give some experimental evidence that indicate that the key-schedule plays a part in providing strength against linear and differential cryptanalysis. For toy Feistel ciphers, it was observed that those with complex and well-designed key schedules can reach a uniform distribution for the probabilities of differentials and linear hulls faster than those with poorly-designed key schedules.
See also
References
- Lars R. Knudsen and John Erik Mathiassen, On the Role of Key Schedules in Attacks on Iterated Ciphers, ESORICS 2004, pp322–334.
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