Patent ID: 7194090

Claim:
An encryption apparatus based on a common key encryption system in which a plurality of expanded keys are used in a predetermined order in a data randomizing process for encryption and in a reversed order in a data randomizing process for decryption, the apparatus comprising: a plurality of round processing circuits connected in series, the round processing circuit of a first stage receiving a common key and subjecting the received common key to a round function to output a sub key and the round processing circuit of subsequent stages receiving the sub key output from the round processing circuit of a previous stage and subjecting the sub key to a round function to output a sub key, the sub key output from the round processing circuit of a last stage being the common key; and a plurality of expanded key generating circuits configured to receive the sub keys output from at least a part of said round processing circuits and output expanded keys based on all or some bits of the received sub keys, wherein the plurality of round processing circuits comprise a first portion including at least two round processing circuits having first and second round functions and a second portion which follows the first portion, the second portion including at least two round processing circuits having third and fourth round functions, the third round function being an inverse of the second round function, and the fourth round function being an inverse of the first round function.