Unformatted text preview: conveniently losing her private key. To protect against the former, Alice should buy herself a good guard dog; that kind of thing is beyond the scope of cryptography. Additional theory and applications of failstop signatures can be found in [1239, 1241, 730, 731]. 4.8 Computing with Encrypted Data
Alice wants to know the solution to some function f(x), for some particular value of x. Unfortunately, her computer is broken. Bob is willing to compute f(x) for her, but Alice isn’t keen on letting Bob know her x. How can Alice let Bob compute f(x) for her without telling him x? This is the general problem of computing with encrypted data, also called hiding information from an oracle. (Bob is the oracle; he answers questions.) There are ways to do this for certain functions; they are discussed in Section 23.6. 4.9 Bit Commitment
The Amazing Alice, magician extraordinaire, will now perform a mystifying feat of mental prowess. She will guess the card Bob will choose before he chooses it! Watch as Alice writes her prediction on a piece of paper. Marvel as Alice puts that piece of paper in an envelope and seals it shut. Thrill as Alice hands that sealed envelope to a random member of the audience. “Pick a card, Bob, any card.” He looks at it and shows it to Alice and the audience. It’s the seven of diamonds. Alice now takes the envelope back from the audience. She rips it open. The prediction, written before Bob chose his card, says “seven of diamonds”! Applause. 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 © 19962000 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)
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