To provide a communication system which enables a transmission side to determine access by a reception side for safe communication.

The transmission side 50 defines an access parameter such as the number of times a message can be deciphered, an expiration time/date of the message, and/or a contingent event that triggers expiration of the message. A transmission environment setting part 50 defines a ciphering method, a cipher key length, and control parameters. A key management part 52 functions to implement the transmission environment setting parameters during a communication. When a message is received, a reception part 54 contacts a management part 52 to request a decipher key. The key management part 52 sends back the decipher key or a message showing denial. Once the message is deciphered by the reception side 54, the key and the deciphered original are deleted from computer on the reception side 54. The message is displayed in a bit-mapped window.