A method and system for a secure telephone protocol are disclosed, which can be implemented using current Voice over IP (VoIP) protocols, Session Initiation Protocol (SIP, as specified in the Request for Comment (RFC) 3261 from the Internet Engineering Task Force (IETF)), Real Time Transport Protocol (RTP, as specified in RFC 3550), and Secure RTP (SRTP, as specified in RFC 3711). The secure telephone protocol can include a shared secret value that is cached and then re-used later to authenticate a long series of session keys to be used for numerous separate secure phone calls over a long period of time, thereby providing cryptographic key continuity without the need for voice authentication. In an embodiment, the secure telephone protocol can utilize the Diffie-Hellman key exchange during call setup, and AES for encrypting the voice stream.