applied cryptography - protocols, algorithms, and source code in c

He can communicate with both alice and bob and

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Unformatted text preview: for example), it would be easy to cut and paste a valid signature from one document to another document. The mere presence of such a signature means nothing. Second, computer files are easy to modify after they are signed, without leaving any evidence of modification. Previous Table of Contents Next Products | Contact Us | About Us | Privacy | Ad Info | Home Use of this site is subject to certain Terms & Conditions, Copyright © 1996-2000 EarthWeb Inc. All rights reserved. Reproduction whole or in part in any form or medium without express written permission of EarthWeb is prohibited. Read EarthWeb's privacy statement. To access the contents, click the chapter and section titles. Applied Cryptography, Second Edition: Protocols, Algorthms, and Source Code in C (cloth) Go! Keyword Brief Full Advanced Search Search Tips (Publisher: John Wiley & Sons, Inc.) Author(s): Bruce Schneier ISBN: 0471128457 Publication Date: 01/01/96 Search this book: Go! Previous Table of Contents Next ----------- Signing Documents with Symmetric Cryptosystems and an Arbitrator Alice wants to sign a digital message and send it to Bob. With the help of Trent and a symmetric cryptosystem, she can. Trent is a powerful, trusted arbitrator. He can communicate with both Alice and Bob (and everyone else who may want to sign a digital document). He shares a secret key, KA, with Alice, and a different secret key, KB, with Bob. These keys have been established long before the protocol begins and can be reused multiple times for multiple signings. (1) Alice encrypts her message to Bob with KA and sends it to Trent. (2) Trent decrypts the message with KA. (3) Trent takes the decrypted message and a statement that he has received this message from Alice, and encrypts the whole bundle with KB. (4) Trent sends the encrypted bundle to Bob. (5) Bob decrypts the bundle with KB. He can now read both the message and Trent’s certification that Alice sent it. How does Trent know that the message is from Alice...
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