The Code Book: The Secret History of Codes and Code-breaking. Simon Singh

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СКАЧАТЬ will be locked in a cell under guard at night, and permitted to walk the battlements during the day with a mask.

      This was an explicit reference to a masked prisoner at Pignerole, and a sufficiently serious crime, with dates that seem to fit the myth of the Man in the Iron Mask. Does this solve the mystery? Not surprisingly, those favouring more conspiratorial solutions have found flaws in Bulonde as a candidate. For example, there is the argument that if Louis XIV was actually attempting to secretly imprison his unacknowledged twin, then he would have left a series of false trails. Perhaps the encrypted letter was meant to be deciphered. Perhaps the nineteenth-century codebreaker Bazeries had fallen into a seventeenth-century trap.

      The Black Chambers

      Reinforcing the monoalphabetic cipher by applying it to syllables or adding homophones might have been sufficient during the 1600s, but by the 1700s cryptanalysis was becoming industrialised, with teams of government cryptanalysts working together to crack many of the most complex monoalphabetic ciphers. Each European power had its own so-called Black Chamber, a nerve centre for deciphering messages and gathering intelligence. The most celebrated, disciplined and efficient Black Chamber was the Geheime Kabinets-Kanzlei in Vienna.

      It operated according to a rigorous timetable, because it was vital that its nefarious activities should not interrupt the smooth running of the postal service. Letters which were supposed to be delivered to embassies in Vienna were first routed via the Black Chamber, arriving at 7 a.m. Secretaries melted seals, and a team of stenographers worked in parallel to make copies of the letters. If necessary, a language specialist would take responsibility for duplicating unusual scripts. Within three hours the letters had been resealed in their envelopes and returned to the central post office, so that they could be delivered to their intended destination. Mail merely in transit through Austria would arrive at the Black Chamber at 10 a.m., and mail leaving Viennese embassies for destinations outside Austria would arrive at 4 p.m. All these letters would also be copied before being allowed to continue on their journey. Each day a hundred letters would filter through the Viennese Black Chamber.

      The copies were passed to the cryptanalysts, who sat in little kiosks, ready to tease out the meanings of the messages. As well as supplying the emperors of Austria with invaluable intelligence, the Viennese Black Chamber sold the information it harvested to other powers in Europe. In 1774 an arrangement was made with Abbot Georgel, the secretary at the French Embassy, which gave him access to a twice-weekly package of information in exchange for 1,000 ducats. He then sent these letters, which contained the supposedly secret plans of various monarchs, straight to Louis XV in Paris.

      The Black Chambers were effectively making all forms of monoalphabetic cipher insecure. Confronted with such professional cryptanalytic opposition, cryptographers were at last forced to adopt the more complex but more secure Vigenère cipher. Gradually, cipher secretaries began to switch to using polyalphabetic ciphers. In addition to more effective cryptanalysis, there was another pressure that was encouraging the move towards securer forms of encryption: the development of the telegraph, and the need to protect telegrams from interception and decipherment.

      Although the telegraph, together with the ensuing telecommunications revolution, came in the nineteenth century, its origins can be traced all the way back to 1753. An anonymous letter in a Scottish magazine described how a message could be sent across large distances by connecting the sender and receiver with 26 cables, one for each letter of the alphabet. The sender could then spell out the message by sending pulses of electricity along each wire. For example, to spell out hello, the sender would begin by sending a signal down the h wire, then down the e wire, and so on. The receiver would somehow sense the electrical current emerging from each wire and read the message. However, this ‘expeditious method of conveying intelligence’, as the inventor called it, was never constructed, because there were several technical obstacles that had to be overcome.

      For example, engineers needed a sufficiently sensitive system for detecting electrical signals. In England, Sir Charles Wheatstone and William Fothergill Cooke built detectors from magnetised needles, which would be deflected in the presence of an incoming electric current. By 1839, the Wheatstone-Cooke system was being used to send messages between railway stations in West Drayton and Paddington, a distance of 29 km. The reputation of the telegraph and its remarkable speed of communication soon spread, and nothing did more to popularise its power than the birth of Queen Victoria’s second son, Prince Alfred, at Windsor on 6 August 1844. News of the birth was telegraphed to London, and within the hour The Times was on the streets announcing the news. It credited the technology that had enabled this feat, mentioning that it was ‘indebted to the extraordinary power of the Electro-Magnetic Telegraph’. The following year, the telegraph gained further fame when it helped capture John Tawell, who had murdered his mistress in Slough, and who had attempted to escape by jumping on to a London-bound train. The local police telegraphed Tawell’s description to London, and he was arrested as soon as he arrived at Paddington.

      Meanwhile, in America, Samuel Morse had just built his first telegraph line, a system spanning the 60 km between Baltimore and Washington. Morse used an electromagnet to enhance the signal, so that upon arriving at the receiver’s end it was strong enough to make a series of short and long marks, dots and dashes, on a piece of paper. He also developed the now familiar Morse code for translating each letter of the alphabet into a series of dots and dashes, as given in Table 6. To complete his system he designed a sounder, so that the receiver would hear each letter as a series of audible dots and dashes.

      Back in Europe, Morse’s approach gradually overtook the Wheatstone-Cooke system in popularity, and in 1851 a European form of Morse Code, which included accented letters, was adopted throughout the Continent. As each year passed, Morse code and the telegraph had an increasing influence on the world, enabling the police to capture more criminals, helping newspapers to bring the very latest news, providing valuable information for businesses, and allowing distant companies to make instantaneous deals.

      However, guarding these often sensitive communications was a major concern. The Morse code itself is not a form of cryptography, because there is no concealment of the message. The dots and dashes are merely a convenient way to represent letters for the telegraphic medium; Morse code is effectively nothing more than an alternative alphabet. The problem of security arose primarily because anyone wanting to send a message would have to deliver it to a Morse code operator, who would then have to read it in order to transmit it. The telegraph operators had access to every message, and hence there was a risk that one company might bribe an operator in order to gain access to a rival’s communications. This problem was outlined in an article on telegraphy published in 1853 in England’s Quarterly Review:

      Means should also be taken to obviate one great objection, at present felt with respect to sending private communications by telegraph – the violation of all secrecy – for in any case half-a-dozen people must be cognisant of every word addressed by one person to another. The clerks of the English Telegraph Company are sworn to secrecy, but we often write things that it would be intolerable to see strangers read before our eyes. This is a grievous fault in the telegraph, and it must be remedied by some means or other.

      Table 6 International Morse Code symbols.

      The solution was to encipher a message before handing it to the telegraph operator. The operator would then turn the ciphertext into Morse code before transmitting it. As well as preventing the operators from seeing sensitive material, encryption also stymied the efforts of any spy who might be tapping the telegraph wire. The polyalphabetic Vigenère cipher was clearly the best way to ensure secrecy for important business communications. It was considered unbreakable, and became known as le chiffre indéchiffrable. Cryptographers had, for the time being at least, a clear lead over the cryptanalysts.

      Mr Babbage Versus the Vigenère Cipher

      The СКАЧАТЬ