hose eyes with permission to view it. Encryption is one aspect of a very old science called cryptography. Cryptography is the science of writing in code; its first known use dates to 1900 BC when an Egyptian scribe used nonstandard hieroglyphs to capture the private thoughts of a customer. Some historians feel that Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Protocols Chapter 2 cryptography first appeared as the natural result of the invention of the written word; its use in diplomatic messages, business strategy, and bat- tle plans certainly supports the theory. In data communications and telecommunications, encryption is required any time the information to be conveyed is sensitive and the possibility exists that the transmission medium is insecure. This can occur over any network, although the Internet is most commonly cited as being the most insecure of all networks. All secure networks require a set of specific characteristics if they are to be truly secure. The most important of them are listed here: Privacy/confidentiality: The ability to guarantee that no one can read the message except the intended recipient Authentication: The guarantee that the identity of the recipient can be verified with full confidence Message integrity: Assurance that the receiver can confirm that the message has not been changed in any way during its transit across the network Nonrepudiation: A mechanism to prove that the sender really sent this message and that it was not sent by someone pretending to be the sender Cryptographic techniques, including encryption, have two responsi- bilities: They ensure that the transmitted information is free from theft or any form of alteration and provide authentication for both senders and receivers. Today, three forms of encryption are most commonly employed: secret-key (or symmetric) cryptography, public-key (or as