Dr. Dobb's Journal - October 2008 - (Page 46)

D10deja_p5db 8/15/08 10:02 AM Page 46 Core Technology by Dejan Ristanovic and Jelica Protic The Book Cipher Algorithm A simple—but safe—approach to security Unless you’re a professional cryptanalyst, writing cryptography code means meddling with Dejan is editor-inchief of PC Press, a personal computer magazine in Serbia and former Yugoslavia. He can be contacted at www.ristanovic .com. Jelica is a professor of computer engineering at the School of Electrical Engineering, University of Belgrade. She can be contacted at www.jeca.rs. “powers” you cannot fully comprehend, and seemingly insignificant slips can be fatal. During World War II, for instance, Polish and British mathematicians broke Germany’s Enigma code only because the same message-key was enciphered twice at the beginning of every message. The Germans did this to avoid mistakes caused by radio interference, but at the same time, it ruined their carefully planned cryptosystem. And how many slips are there in the code that multiply big numbers, look for 1000-digits primes, and encrypt the fixed header of your document? With the Book cipher algorithm, you’re safe from these kinds of errors because it is simple enough that you can code it in a few lines of C that are completely understandable, but still extremely secure. The socalled Beale ciphers (unmuseum.org/beal.htm), which point to a location of buried treasure somewhere in Bedford county, were coded in 1885, but still have not been decoded. This secret (or maybe hoax) has occupied some of the best cryptanalytic minds. Likewise, when Simon Singh gave 10 problems in the appendix of The Code Book, problem #5 (Book cipher) was the most difficult one for the winners of the £10,000 prize (www.simonsingh.com/Cipher_ Challenge.html). Still, the Book cipher has probably never been used in commercial software. ally called the “running key cipher.” In this case, text (usually from a book) is used to provide a very long key stream. The book used is agreed upon in advance, while the passage used is chosen randomly for each message and secretly indicated somewhere in a previous message. In this example, we agreed to use J.K. Rowling’s Harry Potter and the Order of the Phoenix and to start on page 335, line 28, with the sentence, “Hermione bit her lip and did not answer.” We write this text under the plaintext and use it as the running key. The particular message to send is “DRDOBBS.” We XOR the corresponding characters of the message and the running key to get the ciphertext 12 23 22 2 11 13 29. The running key cipher is much better than the famous Vigenère cipher because we do not repeat the key—a book is hopefully long enough to encode everything we have to say. However, security is still poor because the entropy per character of both the plaintext and the running key is low, and the combining operation is relatively easily inverted. By Book Cipher Algorithms Basically, the Book cipher algorithm uses letters of subsequent words in some text or book as a key to encode a message. Figure 1 is the simplest form, usu46 Dr. Dobb’s Journal l www.ddj.com l October 2008 http://www.ristanovic.com http://www.ristanovic.com http://www.jeca.rs http://unmuseum.org/beal.htm http://www.simonsingh.com/Cipher_Challenge.html http://www.simonsingh.com/Cipher_Challenge.html 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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