Secure speech and data communication over the public switching telephone network

We describe a communication terminal called Tiche. Tiche was designed for achieving privacy when fax or speech are transmitted over the public switching telephone network, PSTN. It is composed of three main functional blocks: a speech coder, an encryption algorithm and a digital modem. When Tiche is connected to a telephone set and to the PSTN, the modem tries to communicate at the higher rate possible for the given channel and selects the speech coder according to that, it selects one of the bit rates available with the standard V32 (4800 or 9600 bps). When it transmits fares Tiche detects the communication standard: V27, V29 or V17 and selects the corresponding modem. The ciphering system is based on both public key and private key algorithms. The overall system is digital and was programmed using DSP