Speech compression and encryption (Secure Phone)

Hi folks, we want to start a project for a computer science class at the University of Karlsruhe. We want to write software to use a workstation/PC with a modem, speaker, and microphone as a secure Phone with encryption and authentication. Therefore we need speech compression. Can anyone give me hints to books, articles, algorithms or software for speech compression? Is CELP public available? Thanks a lot Hadmut