One of them is called the public key, and the other the private key. In most public key systems, it also works the other way, so if the private key is used to encrypt the message, then only the public key can decrypt it. Email encryption is much less common. They only verify that the key you generate corresponds to your email address, by sending a confirmation email. One problem with both of these email encryption solutions is that they leave a lot of information out in the clear.