Patent ID: 8290146

Claim:
A ciphertext generating apparatus that generates a ciphertext C to notify n recipient (n being an integer greater than or equal to 1) of a plaintext M comprising: a storage device for storing information; a processing device for processing information; an encryption parameter storage unit; a recipient identification input unit; a plaintext input unit; a ciphertext body generating unit; a hash value calculation unit; a ciphertext verification text generating unit; and a ciphertext combining unit, wherein the encryption parameter storage unit stores a public encryption parameter by using the storage device, the recipient identification input unit inputs n recipient identification information ID i (i being an integer greater than or equal to 1 and less than or equal to n) for identifying the n recipient respectively, by using the processing device, the plaintext input unit inputs the plaintext M by using the processing device, the ciphertext body generating unit generates n ciphertext body CB i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient, based on the public encryption parameter stored by the encryption parameter storage unit, the n recipient identification information ID i input by the recipient identification input unit, and the plaintext M input by the plaintext input unit, by using the processing device, the hash value calculation unit generates a combined ciphertext by combining the n ciphertext body CB i generated by the ciphertext body generating unit, by using the processing device, and calculates a hash value H based on the combined ciphertext generated, by using the processing device, the ciphertext verification text generating unit generates a ciphertext verification text CC, based on the public encryption parameter stored by the encryption parameter storage unit and the hash value H calculated by the hash value calculation unit, by using the processing device, and the ciphertext combining unit makes one ciphertext C by combining the n ciphertext body CB i generated by the ciphertext body generating unit and the ciphertext verification text CC generated by the ciphertext verification text generating unit, by using the processing device.