Patent ID: 7460665

Claim:
A cipher strength evaluation apparatus for evaluating strength on ciphertext obtained by an encryption apparatus having a plurality of steps of accepting unstirred text, stirring with an extended key calculated from a user key based on a key schedule, and outputting stirred text for encrypting plaintext step by step, the cipher strength evaluation apparatus comprising: a key relationship information storing part for storing key relationship information determined by the key schedule and showing a relationship between a segment bit pattern of the user key and a segment bit pattern of the extended key thereof; an estimated key information storing part for storing estimated key information about an estimated extended key estimated as equal to the extended key and an estimated user key estimated as equal to the user key; and an estimated unstirred text calculating part for calculating estimated unstirred text estimated as equal to unstirred text at a certain stirring step based on ciphertext or estimated stirred text estimated as equal to stirred text at that stirring step, wherein the estimated unstirred text calculating part includes: an estimated extended key calculating part for calculating an estimated extended key based on the key relationship information stored in the key relationship information storing part and the estimated key information stored in the estimated key information storing part and storing the estimated extended key in the estimated key information storing part, and an estimated unstirred text calculating part main body for calculating estimated unstirred text based on ciphertext or the estimated stirred text, and the estimated extended key.