A secure satellite communications system for mobile users. The secure satellite communications system includes an orbiting communications satellite which performs the function of relaying communications to and from a plurality of earth stations. The secure satellite communications system provides a public key system for satellite communications systems to establish private communications. Communications messages are sent by a transmitting earth station encrypted by a public encryption procedure for a particular receiving earth station. The public encryption procedures are broadcast along with other system status and operational information on a common signaling broadcast channel. The receiving earth station utilizes a private decryption procedure to decrypt the received communications messages. The receiving earth station also validates the transmitted communications messages received by the receiving earth station by utilizing a cyclic redundancy check on the received communications messages.