In order to obtain an appropriate, high level of security, a number of architectural elements for secnre downloading of software to a software defined radio (SDR) terminal have been pointed out. They include four different cryptographic techniques and employment of tamper resistant hardware. The cryptographic techniques employed are: (a) a secret key encryption technique; (b) a public key encryption technique; (c) a technique for cryptographic hashing and (d) a technique for signature. Particularly, a protocol for exchanging clyptographic components in an automatic manner without any assistance fmm the user, is proposed. Implementation characteristics of certain cryptographic components are also discussed.