Software Design Of Encryption System For Phone And Fax With Automatic Switch Circuit

With the rapid development of public communicating system, people pay more and more attention on the security of information. Till now, telephone and fax are still the most popular communicating tools. The encryption becomes necessary since some special occasions and the specialities of customers. In present thesis,an encrypting system for fax with switching function between decrypted and encrypted data is proposed and realized.Aiming at the safety of telephone communication, a telephone encrypting system is realized based on 3DES algorithm. The system adopts singlechip as the central control unit; uses FPGA to encrypt and decrypt data; connects to computer system via serial port. By the program which is compiled by C++ Builder, users can input and edit the key. The system key has the length of 256 bits, and can be changed according to appointed interval. By designing the switching mode between decrypted and encrypted data of telephone, the system extends application of encrypted telephone to dialdirect call, extension telephone, IP phone, 201 phone, etc. This system prevents wiretapping, and can be applied to many fields. By real testing, it's validated that the system has an outstanding design; steadily works; and meets requirements of secure telephone standards.The present thesis focuses on basic procedure and main communicating protocols of telephone and fax, also introduces main part of working mode and communicating procedure of Modem. Hardware structure of the system and developing of the software including whole structure and sub-modules are presented as well. The manual for the system is also attached.Up to now, this system has been realized. Under PSTN (Public Switched Telephone Network) testing, the system can transmit audio signals clearly, and fax text or image without losing definition. The key has the length of 256 bits, and changes every 8 seconds.