Unformatted text preview: ssuming that the unit of “time” for our computer is a microsecond, the computer can complete a constant algorithm in a microsecond, a linear algorithm in a second, and a quadratic algorithm in 11.6 days. It would take 32, 000 years to complete a cubic algorithm; not terribly practical, but a computer built to withstand the next ice age would deliver a solution eventually. Performing the exponential algorithm is futile, no matter how well you extrapolate computing power, parallel processing, or contact with superintelligent aliens. Look at the problem of a bruteforce attack against an encryption algorithm. The time complexity of this attack is proportional to the number of possible keys, which is an exponential function of the key length. If n is the length of the key, then the complexity of a bruteforce attack is O(2n). Section 12.3 discusses the controversy surrounding a 56bit key for DES instead of a 112bit key. The complexity of a bruteforce attack against a 56bit key is 256; against a 112bit key the complexity is 2112. The former is possible; the latter isn’t. 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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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:
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 Complexity of Problems
Complexity theory also classifies the inherent complexity of problems, not just the complexity of particular algorithms used to solve problems...
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This note was uploaded on 10/18/2010 for the course MATH CS 301 taught by Professor Aliulger during the Fall '10 term at Koç University.
 Fall '10
 ALIULGER
 Cryptography

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