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

Eve who is in the middle watching this protocol has

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Unformatted text preview: 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 ----------- Proofs of Membership Alice wants to prove to Bob that she is a member of some super-secret organization, but she does not want to reveal her identity. This problem is similar but different to proving identity, and has also been studied [887,906,907,1201,1445]. Some solutions are related to the problem of group signatures (see Section 4.6). 5.3 Blind Signatures An essential feature of digital signature protocols is that the signer knows what he is signing. This is a good idea, except when we want the reverse. We might want people to sign documents without ever seeing their contents. There are ways that a signer can almost, but not exactly, know what he is signing. But first things first. Completely Blind Signatures Bob is a notary public. Alice wants him to sign a document, but does not want him to have any idea what he is signing. Bob doesn’t care what the document says; he is just certifying that he notarized it at a certain time. He is willing to go along with this. (1) Alice takes the document and multiplies it by a random value. This random value is called a blinding factor. (2) Alice sends the blinded document to Bob. (3) Bob signs the blinded document. (4) Alice divides out the blinding factor, leaving the original document signed by Bob. This protocol only works if the signature function and multiplication are commutative. If they are not, there are other ways to modify the document other than by multiplying. Some relevant algorithms appear in Section 23.12. For now, assume that the operation is multiplic...
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