I don't know how' much you savvy public-key encryption, but it's a slick little system that's been around for nigh on eighty years now. Everyone who uses the system has two key codes (both 70 bits long, in this implementation, equivalent to a 22-digit number): a private key that he tells no one and a public key that he can tell all and sundry, or even publicize. The way the system is most commonly used today in 2056, if Adolf wants to send a secret message to Barney, Adolf encrypts the message using two keys: his own private key and Barney's public key. To decrypt the message, Barney uses two keys: his private key and Adolf's public key. Theoretically, only Barney can read the message, since only Barney knows his private key. (Well, duh.) As an added bonus, he knows it had to come from Adolf—or, at least, that it had to have been encrypted using Adolf's private key—otherwise it wouldn't have decrypted properly. Clear as mud? Good, then we'll continue.