Encryption communication method and terminal device for encryption

Abstract

In a calling facsimile, if an operator sets an encryption communication mode, a random number sequence is generated by a random number generating section 15. A control section 11 produces an encryption key from the random number sequence, and transmits the random number sequence used for producing the encryption key to a telephone line 3 with the random number sequence included in NSS which is an option signal. At the same time, an encryption/decryption processing section 16 subjects information to be transmitted to encryption processing on the basis of the encryption key. The encrypted information is transmitted to the telephone line 3. On the other hand, in a called facsimile, the encryption key is reproduced on the basis of the random number sequence included in the NSS, and the received information is decrypted on the basis of the reproduced encryption key. Accordingly, the encryption key need not be determined in advance between a transmitter and a receiver. Further, the encryption key does not leak during transmission, thereby protecting the security of communication.