The Cracking Code Book. Simon Singh

wrapped them in a leather packet, which was then hidden inside a hollow bung used to seal a barrel of beer. The brewer would deliver the barrel to Chartley Hall, whereupon one of Mary’s maids would open the bung and take the contents to her mistress. The process worked equally well for getting messages out of Chartley Hall.

      Meanwhile, unknown to Mary, a plan to rescue her was being hatched in the taverns of London. At the centre of the plot was Catholic Anthony Babington, only twenty-four years old but already well known in the city as a handsome, charming and witty man about town. What his many admirers and close friends failed to appreciate was that Babington deeply resented the government, which had persecuted him, his family and his faith. The state’s anti-Catholic policies had reached new heights of horror, with priests being accused of treason, and anybody caught harbouring them punished by the rack, mutilation and disembowelling while still alive. The Catholic Mass was officially banned, and families who remained loyal to the Pope were forced to pay extremely high taxes. Babington’s resentment was fuelled by the death of Lord Darcy, his great-grandfather, who was beheaded for his involvement in the Pilgrimage of Grace, a Catholic uprising against Henry VIII.

      The conspiracy began one evening in March 1586, when Babington and six confidants gathered in The Plough, an inn outside Temple Bar. As the historian Philip Caraman observed, “He drew to himself by the force of his exceptional charm and personality many young Catholic gentlemen of his own standing, gallant, adventurous and daring in defence of the Catholic faith in its day of stress; and ready for any arduous enterprise whatsoever that might advance the common Catholic cause.” Over the next few months an ambitious plan emerged to free Mary Queen of Scots, assassinate Queen Elizabeth and incite a rebellion supported by an invasion from abroad.

      The conspirators were agreed that the Babington Plot, as it became known, could not proceed without the blessing of Mary, but there was no apparent way to communicate with her. Then, on July 6, 1586, Gifford arrived on Babington’s doorstep. He delivered a letter from Mary, explaining that she had heard about Babington via her supporters in Paris and looked forward to hearing from him. In reply, Babington compiled a detailed letter in which he outlined his scheme, including a reference to the excommunication of Elizabeth by Pope Pius V in 1570, which he believed legitimized her assassination.

      Myself with ten gentlemen and a hundred of our followers will undertake the delivery of your royal person from the hands of your enemies. For the dispatch of the usurper, from the obedience of whom we are by the excommunication of her made free, there be six noble gentlemen, all my private friends, who for the zeal they bear to the Catholic cause and your Majesty’s service will undertake that tragical execution.

      As before, Gifford used his trick of putting the message in the bung of a beer barrel in order to sneak it past Mary’s guards. This can be considered a form of steganography, because the letter was being hidden. As an extra precaution, Babington enciphered his letter so that even if it was intercepted by Mary’s jailer, it would be indecipherable and the plot would not be uncovered. He used a cipher that was not a simple monoalphabetic substitution, but rather a nomenclátor, as shown in Figure 7. It consisted of twenty-three symbols that were to be substituted for the letters of the alphabet (excluding j, v and w), along with thirty-five symbols representing words or phrases. In addition, there were four nulls (

) and a symbol (σ) signifying that the next symbol represents a double letter (“dowbleth”).

      Gifford was still a youth, even younger than Babington, and yet he conducted his deliveries with confidence and cleverness. His aliases, such as Mr Colerdin, Pietro and Cornelys, enabled him to travel the country without suspicion, and his contacts within the Catholic community provided him with a series of safe houses between London and Chartley Hall. However, each time Gifford travelled to or from Chartley Hall, he would make a detour. Although Gifford was apparently acting as an agent for Mary, he was actually a double agent. Back in 1585, before his return to England, Gifford had written to Sir Francis Walsingham, principal secretary to Queen Elizabeth, offering his services. Gifford realized that his Catholic background would act as a perfect mask for infiltrating plots against Queen Elizabeth. In the letter to Walsingham, he wrote, “I have heard of the work you do and I want to serve you. I have no scruples and no fear of danger. Whatever you order me to do I will accomplish.”

      

      Figure 7 The nomenclator of Mary Queen of Scots, consisting of a cipher alphabet and codewords.

      Walsingham was Elizabeth’s most ruthless minister, a spymaster who was responsible for the security of the monarch. He had inherited a small network of spies, which he rapidly expanded into the Continent, where many of the plots against Elizabeth were being hatched. After his death it was discovered that he had been receiving regular reports from twelve locations in France, nine in Germany, four in Italy and four in Spain, as well as having informants in Constantinople, Algiers and Tripoli.

      Walsingham recruited Gifford as a spy, and in fact it was Walsingham who ordered Gifford to approach the French embassy and offer himself as a courier. Each time Gifford collected a message to or from Mary, he would first take it to Walsingham. The vigilant spymaster would then pass it to his counterfeiters, who would break the seal on each letter, make a copy, and reseal the original letter with an identical stamp before handing it back to Gifford. The apparently untouched letter could then be delivered to Mary or her correspondents, who remained oblivious to what was going on.

      When Gifford handed Walsingham a letter from Babington to Mary, the first objective was to decipher it. Walsingham employed Thomas Phelippes as his cipher secretary, a man “of low stature, slender every way, dark yellow haired on the head, and clear yellow bearded, eaten in the face with smallpox, of short sight, thirty years of age by appearance.” Phelippes was a linguist who could speak French, Italian, Spanish, Latin and German, and, more importantly, he was one of Europe’s finest cryptanalysts.

      Upon receiving any message to or from Mary, Phelippes devoured it. He was a master of frequency analysis, and it would be merely a matter of time before he found a solution. He established the frequency of each character, and tentatively proposed values for those that appeared most often. When a particular approach hinted at absurdity, he would backtrack and try alternative substitutions. Gradually he would identify the nulls, the cryptographic red herrings, and put them to one side. Eventually all that remained were the handful of codewords, whose meaning could be guessed from the context.

      When Phelippes deciphered Babington’s message to Mary, which clearly proposed the assassination of Elizabeth, he immediately forwarded the damning text to his master. At this point Walsingham could have pounced on Babington, but he wanted more than the execution of a handful of rebels. He bided his time in the hope that Mary would reply and authorize the plot, thereby incriminating herself. Walsingham had long wished for the death of Mary Queen of Scots, but he was aware of Elizabeths reluctance to execute her cousin. However, if he could prove that Mary was endorsing an attempt on the life of Elizabeth, then surely his queen would permit the execution of her Catholic rival. Walsingham’s hopes were soon fulfilled.

      On July 17, Mary replied to Babington, effectively signing her own death warrant. She explicitly wrote about the “design”, showing particular concern that she should be released simultaneously with, or before, Elizabeth’s assassination, otherwise news might reach her jailer, who might then murder her. Before reaching Babington, the letter made the usual detour to Phelippes. Having cryptanalyzed the earlier message, he deciphered this one with ease, read its contents and marked it with a П – the sign of the gallows.

      Walsingham had all the evidence he needed to arrest Mary and Babington, but still he was not satisfied. In order to destroy the conspiracy completely, he needed the names of all those involved. He asked Phelippes to forge a postscript to Mary’s letter, which would entice Babington to name names. One of Phelippes’s additional talents was forgery, and it was said that he had the ability “to write any man’s hand, if he had once seen it, as if the