Security: Public key email encryption

While traditional cryptographic systems can be thought of as safes -- secure places where one can put or retrieve things with the help of the appropriate key -- public key cryptographic systems are more akin to mailboxes, where practically anybody can drop things in, but only someone with a key can retrieve them. The key (no pun intended) difference is that public key cryptography uses two different keys. One key, known as the public key, encrypts files, and another, the secret key, decrypts them.