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A key schedule is an algorithm that expands a relatively short master key to a relatively large expanded key for later use in an encryption and decryption algorithm.

A key schedule is an algorithm that expands a relatively short master key (typically between 40 and 256 bits long) to a relatively large expanded key (typically several hundred or thousand bits) for later use in an encryption and decryption algorithm. Key schedules are used in several ways:

  • To specify the round keys of a product cipher. DES [NBS77] uses its key schedule in this way, as do many other product ciphers.
  • To initialize some fixed elements of a cryptographic transform. Khufu [Mer91], Blowfish [Sch94], and SEAL [RC94] use a key schedule this way.
  • To initialize the state of a stream cipher prior to generating keystream. RC4 [Sch96] uses a key schedule in this way.
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