An important element to the public key system is that the public and private keys are related in such a way that only the public key can be used to encrypt messages and only the corresponding private key can be used to decrypt them. Moreover, it is virtually impossible to deduce the private key if you know the public key.

Public-key systems, such as Pretty Good Privacy (PGP), are becoming popular for transmitting information via the Internet. They are extremely secure and relatively simple to use. The only difficulty with public-key systems is that you need to know the recipient's public key to encrypt a message for him or her. What's needed, therefore, is a global registry of public keys, which is one of the promises of the new LDAP technology.

Public key cryptography was invented in 1976 by Whitfield Diffie and Martin Hellman. For this reason, it is sometime called Diffie-Hellman encryption. It is also called asymmetric encryption because it uses two keys instead of one key (symmetric encryption).

Cryptography for encryption, digital signatures and authentication 
Contains links to information on cryptography, a tutorial on public key encryption for secrecy and a discussion of the debate regarding government control of cryptography.

NIST computer security resource clearinghouse 
National Institute of Standards and Technology's computer security resource clearinghouse. It contains information on numerous security topics, as well as alerts about viruses and other security threats.

PGP introduction
This site explains how PGP software works through the use of a public and private key pairs.

Public-key cryptology FAQ
USENET FAQ that focuses on public-key cryptology.