B sends the nonce to A, which decrypts it and returns an unencrypted message to B containing the numbers in the nonce. If the return message matches the one it first sent, B knows it is, in fact, communicating with A, because only A could have decrypted the message with its private key. In a few more steps, the two computers may be ready to transfer a book or carry out some other transaction. Although not all trusted systems use a challenge-response protocol, most use encryption for exchanging digital works. They may also incorporate other security features.
Some systems contain tamperproof clocks to prevent a user from exercising expired rights.
Others have secure memories for recording billing transactions. Still others must be connected to an on-line financial clearinghouse during transactions. Trusted systems can place identifying watermarks that make it possible to track down unauthorized duplications or alterations. Watermarks maintain a record of each work, the name of the purchaser and a code for the devices on which they are being played. Publishers would still need to be watchful for unlicensed distribution of their property.