Patent ID: 6917684

Claim:
An encryption method for use by an encryption apparatus that encrypts plaintext data composed of a plurality of blocks, the encryption method comprising: a block obtaining step for obtaining the plaintext data one block at a time in order from outside the encryption apparatus; a selecting step for selecting either a first mode or a second mode for a current block obtained in the block obtaining step according to how many blocks have been obtained; a key generating step for generating (1) a first group composed of a predetermined number n of different subkeys when the first mode is selected, and (2) a second group composed of less than n different subkeys when the second mode is selected; and an encrypting step for encrypting the current block by subjecting the current block to n conversion processes in order, wherein in the first mode, each of the n conversion processes is associated with a different subkey in the first group and is performed using the associated subkey, and in the second mode, the n conversion processes are associated with subkeys in the second group and are each performed using the associated subkey.