Patent ID: 7602906

Claim:
A cipher system for encrypting plaintext into ciphertext in an encryption direction and for decrypting ciphertext into plaintext in a decryption direction generally opposite the encryption direction, the cipher system comprising: a cipher; a first diffuser that provides good diffusion in the decryption direction and not-good diffusion in the encryption direction; and a second diffuser that provides good diffusion in the encryption direction and not-good diffusion in the decryption direction, wherein good diffusion in a particular direction in a particular diffuser is defined so that a relatively small change to an input of the particular diffuser in the particular direction produces a relatively large change to an output of the particular diffuser in the particular direction, the relatively large change of the good diffusion being larger as compared with the relatively small change of the good diffusion, wherein not-good diffusion in a particular direction in a particular diffuser is defined so that a relatively large change to an input of the particular diffuser in the particular direction produces a relatively small change to an output of the particular diffuser in the particular direction, the relatively large change of the not-good diffusion being larger as compared with the relatively small change of the not-good diffusion, wherein the first diffuser is provided with input data that is based on plaintext xorred with a sector key when the cipher system operates in the encryption direction, the sector key is derived from a key that comprises a number of bits, the sector key employs a first portion of the number of bits, and the cipher employs a second portion of the number of bits, wherein the second diffuser receives first diffused data from the first diffuser and provides second diffused data to the cipher when the cipher system operates in the encryption direction, and wherein the cipher encrypts the second diffused data into ciphertext when the cipher system operates in the encryption direction.