With the growth of electronic communications, privacy has become a significant issue on the Internet. There is a newsgroup alt.privacy , a vast literature on anonymous email , and commercial services like The Anonymizer that offer anonymous email and anonymous World-Wide Web surfing. Average computer users would likely be satisfied with the capability of putting their email in an envelope.
Private email is now widely and freely available through public-key cryptography .
To use classical ciphers, the sender and recipient must exchange a common key ahead of time. The idea behind public-key cryptography is that one key can be used to encode a message, and a different key can be used to decode the message. A user can make public one of a pair of complementary keys to enable the world to send enciphered messages that can be deciphered only by that user. Also, the user can broadcast enciphered messages that everyone can decipher and verify as coming from that user. Thus, public-key cryptography simultaneously meets the goals of privacy and of authentication.