A software defined radio device and a download server store a plurality of common keys in common key data. The download server arbitrarily determines a common key from the common key data and conveys information identifying the common key to be used to the software defined radio device. An authenticator of the software defined radio device identifies a common key from the common key data using the information identifying the common key, authenticates using the common key, and performs subsequent communications using the common key. When sending software, a hash value is attached for confirming the security. A device ID of the software defined radio device is also attached to data for confirming which software defined radio device receives the software. The software is securely downloaded by a common key encryption having smaller processing requirements than those of a public key encryption.