ENCRYPTION SYSTEM, DECRYPTION METHOD, DECRYPTION HISTORY STORAGE SERVER, RECEIVING DEVICE, TRANSMISSION DEVICE, AND PROGRAM

PROBLEM TO BE SOLVED: To provide an encryption system capable of recording cryptogram identification information, receiving device identification information, and time in association with each other as decryption history.SOLUTION: A transmission device 100 transmits a cryptogram to a receiving device 200 and transmits session information and cryptogram identification information to a decryption history storage server 300. The decryption history storage server 300 records the two pieces of information in association with each other. The receiving device 200 transmits a request, cryptogram identification information, and receiving device identification information to the decryption history storage server 300. The decryption history storage server 300 acquires session information corresponding to the cryptogram identification information received from the receiving device 200. A response is calculated by using the session information and a secret key for proxy calculation. The decryption history storage server 300 records the cryptogram identification information, receiving device identification information, and time in association with each other. A decryption unit 230 of the receiving device 200 decrypts a cryptogram by using the response so as to be a plaintext.