The Atlantic | September 2002 | Homeland Insecurity | Mann
A Web-only Primer on Public-key Encryption

Public-key encryption, as noted in the profile of cryptographer Bruce Schneier, is complicated in detail but simple in outline. The article below is an outline of the principles of the most common variant of public-key cryptography, which is known as RSA, after the initials of its three inventors; a mathematically detailed explanation of RSA by the programmer Brian Raiter, understandable to anyone willing to spend a little time with paper and pencil, is available here.
http://www.theatlantic.com/issues/2002/09/mann_g.htm