Dr. Dobb's Journal - October 2008 - (Page 51)

D10deja_p5db 8/15/08 10:07 AM Page 51 supply of at least one of the letters in allice.cod is consumed), the appropriate message is generated and the file allice.pos is unchanged. Bob should add another book to allice.cod using the command bkadd allice myotherbook.txt 1 and repeat the bkcode command. To decode a message, Allice uses the command bkdecode allice message1.cry, producing the message1.txt file. Figure 3 shows the complete process. In a two-way communication, by using different books, Allice should generate the file bob.cod and use it to encode messages to Bob. Eve is welcome to intercept any of the .cry files but without knowledge of the books used, she is clueless even if her other name is “Susan Fletcher.” Figure 3: Programs and results of coding and decoding the message using the Book cipher. How Strong Is It? Cryptanalysts mostly agree that the Book cipher, if used properly, is practically unbreakable; nearly as good as the one-time pad. Why isn’t it used every day? Maybe because of that “if used properly” clause— the complete algorithm is somehow “private.” The next time you bury a treasure, you can describe its location within an encrypted message and be reasonably sure that it will not be decoded for the next 150 years, but if you have to organize a secure correspondence for a web of spies all over the world, finding, deploying, and protecting adequate books might prove very difficult. By implementing the Book cipher in your applications, you don’t meddle with powers you cannot comprehend—you leave the meddling to users of your software. The average user will probably go to www .gutenberg.org, download the first book, and use it as a key without even bothering to delete the copyright message (which is basically the same for every book on that site). On the other hand, if users of your software contribute ideas of their own—using texts from the Internet instead of books, using different languages, pre-encoding messages using other algorithms, and so on—then potential attackers would be facing many groups of short messages encrypted using (at least slightly) different algorithms, which might present the ultimate challenge. DDJ Get data right AddressObject API Prevent errors before they occur to ensure the integrity of your database. Use AddressObject API to verify, cleanse and format customer data at the point of entry or in batch. Address Object easily integrates with Perl, PHP Java and .NET. , - offering a better way to build-in data validation. right from the start Get a FREE demo at: MelissaData.com/dobbs 1.800.MELISSA 51 October 2008 l www.ddj.com l Dr. Dobb’s Journal http://www.gutenberg.org http://www.gutenberg.org http://www.abxsoft.com http://www.abxsoft.com http://MelissaData.com/dobbs http://MelissaData.com/dobbs http://www.ddj.com

Table of Contents Feed for the Digital Edition of Dr. Dobb's Journal - October 2008

Dr. Dobb's Journal - October 2008 Contents Friday Night Fish Fry Alia Vox Developer Diaries Developer’s Notebook Is Your Next Language COBOL? Conversations Safe Coding Practices Code Signing in Adobe AIR OpenID Single Sign-On The Book Cipher Algorithm Indexing and Searching Image files Extending Continuous Integration Into ALM The Agile Edge Effective Concurrency Swaine’s Flames

Dr. Dobb's Journal - October 2008

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