Patent ID: 7415616

Claim:
A cipher strength evaluation apparatus for evaluating strength on ciphertext outputted by a Feistel encryption apparatus having a plurality of steps of accepting unstirred text, stirring with an extended key, and calculating stirred text for encrypting plaintext step by step, the cipher strength evaluation apparatus comprising: an estimated plaintext calculating part for accepting predetermined-step stirred text being stirred text at a predetermined step, calculating an estimated parameter A, estimated as a parameter A, determined from a predetermined-step extended key being an extended key at a predetermined step, and calculating estimated plaintext based on the predetermined-step stirred text and the estimated parameter A; an encryption control part for using and allowing an encryption apparatus to calculate estimated ciphertext based on the estimated plaintext calculated by the estimated plaintext calculating part; a key verification part for formulating an encryption equation with higher order differences based on the predetermined-step stirred text accepted by the estimated plaintext calculating part and the estimated ciphertext calculated under the control of the encryption control part, processing it by an algebraic technique to try to calculate a last-step estimated extended key estimated as an extended key at a last step, verifying the parameter A, to be right by detecting that the last-step estimated extended key can be calculated, calculating a right last-step estimated extended key with a predetermined probability, and outputting a calculation impossible signal when detecting that calculation is impossible; and a decryption control part for accepting the calculation impossible signal, and controlling the estimated plaintext calculating part, the encryption control part, and the key verification part to allow the last-step estimated extended key to be calculated.