#20005
Most people sending e-mail nowadays take no steps to prevent their messages from being intercepted. That's fine for many types of messages, but just as there are written messages that you wouldn't want to put on a postcard and would prefer to have protected by an envelope, there's a need for encryption in electronic communication. Besides, encryption can do more than keep things secret. The concepts on which encryption is based can be difficult, and most of the complication is handled behind the scenes by software. Nevertheless, it's useful to have a general understanding of how encryption works. Encryption software (often part of a Web browser or server, e-mail client, or other program) is built around the use of a special number, called a key, to convert information into a form that can be read only by someone who has the key needed to decrypt it.
Ivey, Keith C. Editorial Eye, The (1997). Articles>Writing>Email>Security