The aim of this project has been to develop the assembly language functions needed to allow easy implementation in real-time of a secure speech channel. The theory of security systems is introduced and developed. Encryption algorithms are described. A library of multi-precision arithmetic routines has been written for use on the TMS320C25 digital signed processor. These routines are compatible with code produced by the TMS320C25 C Compiler. Multi-precision arithmetic is used in public key encryption which requires large number arithmetic for security and which also has real-time operation requirements. An overview of DSP use in this kind of application is given, the design, implementation and test of these routines is described and some application examples and timings are shown.