An Introduction to Cryptography

(named, you guessed it, for its inventors), and DSA, the Digital Signature. Algorithm, (invented by David Kravitz). Because conventional cryptography was once ...
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June 2004

An Introduction to Cryptography

Release Information An Introduction to Cryptography; released June 8, 2004. Copyright Information Copyright © 1991–2004 by PGP Corporation. All Rights Reserved. No part of this document can be reproduced or transmitted in any form or by any means, electronic or mechanical, for any purpose, without the express written permission of PGP Corporation. Trademark Information PGP and Pretty Good Privacy are registered trademarks, and the PGP logo is a trademark, of PGP Corporation in the U.S. and other countries. IDEA is a trademark of Ascom Tech AG. All other registered and unregistered trademarks in this document are the sole property of their respective owners. Licensing and Patent Information The IDEA cryptographic cipher described in U.S. patent number 5,214,703 is licensed from Ascom Tech AG. The CAST encryption algorithm is licensed from Northern Telecom, Ltd. PGP Corporation may have patents and/or pending patent applications covering subject matter in this software or its documentation; the furnishing of this software or documentation does not give you any license to these patents. Acknowledgments The compression code in PGP is by Mark Adler and Jean-Loup Gailly, used with permission from the free Info-ZIP implementation. Export Information Export of this software and documentation may be subject to compliance with the rules and regulations promulgated from time to time by the Bureau of Export Administration, U.S. Department of Commerce, which restrict the export and re-export of certain products and technical data. Limitations The software provided with this documentation is licensed to you for your individual use under the terms of the End User License Agreement provided with the software. The information in this document is subject to change without notice. PGP Corporation does not warrant that the information meets your requirements or that the information is free of errors. The information may include technical inaccuracies or typographical errors. Changes may be made to the information and incorporated in new editions of this document, if and when made available by PGP Corporation. About PGP Corporation The recognized worldwide leader in secure messaging and information storage, PGP Corporation develops, markets, and supports products used by a broad installed base of enterprises, businesses, governments, individuals, and cryptography experts to secure proprietary and confidential information. During the past 13 years, PGP technology has built a global reputation for open and trusted security products. PGP solutions are used by thousands of corporate/government users and millions of individual users worldwide, including many of the world’s largest and most security-sensitive enterprises, government agencies, individuals, and cryptography experts. Contact PGP Corporation at www.pgp.com.

Table of Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Who should read this guide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 How to use this guide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 Recommended readings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 Chapter 1: The Basics of Cryptography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Encryption and decryption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9 What is cryptography? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9 Conventional cryptography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 Public-key cryptography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12 How PGP works . . . . . . . . . . . . . . . . . . . . . . .