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With the rapid development of public information systems , information security has drawn increasing attention. Telephones and fax machines are still currently the most common communication tool , in view of certain special occasions and some of the special nature of the work , encryption has become a demand . In this regard , this paper proposes and implements a kind of plaintext , ciphertext encrypted fax telephone switching system . For telephone and fax security problems in the design and implementation of a telephone fax 3DES algorithm based encryption system . System uses a microcontroller as the central control unit ; voice signals using FPGA encryption and decryption ; serial communication interface with a computer connected to the system , PC machine running with windows program written in C Builder , users can enter this program , modify the key . System Key length is 256bit, can follow a predetermined time interval hopping . For the current range of phone types , such as direct dial telephone , extension phones, IP phones , 201 phone , the phone system will be designed to be part of plaintext ciphertext switching mode , expanding the encrypted phone applications. With this system, to solve the phone tapping information leakage caused by other issues, can be used in various fields confidential conversation. Through practical application, we prove the system design is reasonable, reliable, and meet the requirements of the telephone security products . This paper summarizes the research and telephone communications and fax communication the basic process and major communication protocols , a key part of the system introduced Modem works and the communication process. This paper briefly introduces the hardware structure and details of their part of the design and development of software , including software modules and the overall structure of the concrete implementation. Currently, the system has been designed , via the public switched telephone network (PSTN) test, the clear voice transmission , facsimile graphic clarity is not decreased , and the key is 256 bits , once every 8 seconds hopping .
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