The Cracking Code Book. Simon Singh

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      As well as being invulnerable to frequency analysis, the Vigenère cipher has an enormous number of keys. The sender and receiver can agree on any word in the dictionary or any combination of words, or even fabricate words. A cryptanalyst would be unable to crack the message by searching all possible keys because the number of options is simply too great.

      The traditional forms of substitution cipher, those that existed before the Vigenère cipher, were called monoalphabetic substitution ciphers because they used only one cipher alphabet per message. In contrast, the Vigenère cipher belongs to a class known as polyalphabetic because it employs several cipher alphabets per message.

      In 1586 Vigenère published his work in A Treatise on Secret Writing. Although some people continued to use traditional ciphers (Appendix D), use of the Vigenère cipher spread during the seventeenth and eighteenth centuries, and the arrival of the telegraph in the nineteenth century suddenly made it popular within the business community.

      The polyalphabetic Vigenère cipher was clearly the best way to ensure secrecy for important business communications that were transmitted via a telegraph operator, who would otherwise be able to read the contents of the message. The cipher was considered unbreakable, and became known as le chiffre indéchiffrable: the uncrackable cipher. Cryptographers had, for the time being at least, a clear lead over the cryptanalysts.

       MR BABBAGE VERSUS THE VIGENÈRE CIPHER

      The most intriguing figure in nineteenth-century cryptanalysis is Charles Babbage, the eccentric British genius best known for developing the blueprint for the modern computer. He was born in 1791, the son of Benjamin Babbage, a wealthy London banker. When Charles married without his father’s permission, he no longer had access to the Babbage fortune, but he still had enough money to be financially secure, and he pursued the life of a roving scholar, applying his mind to whatever problem tickled his fancy. His inventions include the speedometer and the cowcatcher, a device that could be fixed to the front of steam locomotives to clear cattle from railway tracks. In terms of scientific breakthroughs, he was the first to realize that the width of a tree ring depended on that year’s weather, and he deduced that it was possible to determine past climates by studying ancient trees. He was also intrigued by statistics, and as a diversion he drew up a set of mortality tables, a basic tool for today’s insurance industry.

      The turning point in Babbage’s scientific career came in 1821, when he and the astronomer John Herschel were examining a set of mathematical tables, the sort used as the basis for astronomical, engineering and navigational calculations. The two men were disgusted by the number of errors in the tables, which in turn would generate flaws in important calculations. One set of tables, the Nautical Ephemeris for Finding Latitude and Longitude at Sea, contained over a thousand errors. Indeed, many shipwrecks and engineering disasters were blamed on faulty tables.

      These mathematical tables were calculated by hand, and the mistakes were simply the result of human error, causing Babbage to exclaim, “I wish to God these calculations had been executed by steam!” This marked the beginning of an extraordinary endeavour to build a machine capable of faultlessly calculating the tables to a high degree of accuracy. In 1823 Babbage designed Difference Engine No. 1, a magnificent calculator consisting of twenty-five thousand precision parts, to be built with government funding. Although Babbage was a brilliant innovator, he was not a great implementer. After ten years of toil, he abandoned Difference Engine No. 1, cooked up an entirely new design and set to work building Difference Engine No. 2.