US Patent 6907123 Secure voice communication system

A secure real time voice communication system 70 is provided that allows for the secure transmission of voice communications between a sending device 72 and a receiving device 78 through the public switch telephone network 76. The device 72 uses an encryption decryption engine 30 which is capable of executing a number of encryption algorithms which are selected using an encryption selection table 80. An encryption key can be calculated from a periodic key value and a public variable key value. Further, the encryption algorithm used can be periodically changed during a voice communication session so that multiple encryption techniques can be used within the same communication session.