CIPHERING METHOD AND CIPHERING SAFETY EVALUATING METHOD

摘要

PROBLEM TO BE SOLVED: To provide a new deciphering method for product sum open key ciphering and a safety evaluating method for product-sum open key ciphering. SOLUTION: A plaintext divider 2 divides a plaintext (m) into divided plaintexts, open keys of an entity (b) found according to the multiplication format of a secret key are read out of an open key storage part 3, and a ciphertext C is generated by a cipherer 4 by using those divided plaintexts and open keys and sent to the entity (b) through a communication line 1. A decipherer 5 of the entity (b) deciphers the ciphertext C into the original plaintext (m). At the time, some of the secret keys are found by LLL algorithm using two pairs of two successive open keys and some of the found secret keys are used to decipher the ciphertext C into the original plaintext (m).