CIPHERING METHOD, DECIPHERING METHOD, CIPHER COMMUNICATION SYSTEM, PROGRAM, AND RECORDING MEDIUM

摘要

PROBLEM TO BE SOLVED: To provide a ciphering method and a deciphering method for an open key ciphering system which can improve safety. SOLUTION: A cipherer 3 generates a ciphertext of a product sum type by selecting an arbitrary open key out of prepared open keys by divided plaintexts obtained by dividing a plain text to be ciphered by a plaintext divider 2 and scrambling a plurality of components constituting the selected open key after adding a random number component. When this ciphertext is constituted on a finite field, those components and random number components are coefficients of a polynomial.