US Patent 7039802 Conditional access system for set-top boxes

A system conditionally establishes a communication channel between two devices only if one device is authenticated by the other device. Authentication of the second device by the first device involves sending a message to the second device; receiving, from the second device, the message encrypted using a private key of the second device and a digital certificate having a public key of the second device; decrypting the digital certificate to obtain the public key, using the public key to decrypt the message and comparing the decrypted message to the message originally sent to the second device.