PGP has the obvious problem that few people know what it is, although it is the standard manner in which to verify digital identity. (And does a nice bit of cryptography at that) Of course, the instant someone is able to factorise large number instantaneously the whole system goes down the drain. For the benifit of those who don't know. PGP is a method of encryption which uses two keys. One key is called the public key, and can be used to encrypt but not decrypt things, whereas the other is the private key which can decrypt. The private key is kept secret, and consists of two very large prime numbers, whereas the public key is the product of those prime numbers and can be distribute freely. While it may seem easy to find the private key when given the public key it isn't, and although its possible it takes an exceedingly long time, as the only real way to go about it is by trial and error. (The numbers are large enough that my the time this has happened no one will care what the original message said anyway.) Yet how does this relate to proving identity? You see, the clever thing is that it can work in the opposite direction as well. It Tom were to encrypt the message 'I am Tom I am' using his PRIVATE key, then the PUBLIC key can be used to decrypt it. And the only way Tom can generate that message is if he had his private key. Now I've simplified things slightly, as in reality the keys aren't used to encrypt the message but rather a second key, which in turn encrypts the mesage. This is as encryting with the public key is very slow for long messages, and so it is used to encrypt a shorter random encryption key, which in turn encrypts the message.