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

realised 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.’

      Walsingham was Elizabeth’s most ruthless minister. He was a machiavellian figure, 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, four in Spain and three in the Low Countries, 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 then 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 had originally encountered codes and ciphers while reading a book written by the Italian mathematician and cryptographer Girolamo Cardano (who, incidentally, proposed a form of writing for the blind based on touch, a precursor of Braille). Cardano’s book aroused Walsingham’s interest, but it was a decipherment by the Flemish cryptanalyst Philip van Marnix that really convinced him of the power of having a codebreaker at his disposal. In 1577, Philip of Spain was using ciphers to correspond with his half-brother and fellow Catholic, Don John of Austria, who was in control of much of the Netherlands. Philip’s letter described a plan to invade England, but it was intercepted by William of Orange, who passed it to Marnix, his cipher secretary. Marnix deciphered the plan, and William passed the information to Daniel Rogers, an English agent working on the Continent, who in turn warned Walsingham of the invasion. The English reinforced their defences, which was enough to deter the invasion attempt.

      Now fully aware of the value of cryptanalysis, Walsingham established a cipher school in London and 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 authorise the plot, thereby incriminating herself. Walsingham had long wished for the death of Mary Queen of Scots, but he was aware of Elizabeth’s 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 17 July, 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 cryptanalysed the earlier message, he deciphered this one with ease, read its contents, and marked it with a ‘

’ – 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 as a forger, and it was said that he had the ability ‘to write any man’s hand, if he had once seen it, as if the man himself had writ it’. Figure 9 shows the postscript that was added at the end of Mary’s letter to Babington. It can be deciphered using Mary’s nomenclator, as shown in Figure 8, to reveal the following plaintext:

      I would be glad to know the names and qualities of the six gentlemen which are to accomplish the designment; for it may be that I shall be able, upon knowledge of the parties, to give you some further advice necessary to be followed therein, as also from time to time particularly how you proceed: and as soon as you may, for the same purpose, who be already, and how far everyone is privy hereunto.

      Figure 9 The forged postscript added by Thomas Phelippes to Mary’s message. It can be deciphered by referring to Mary’s nomenclator (Figure 8).

       Public Record Office, London.

      The cipher of Mary Queen of Scots clearly demonstrates that a weak encryption can be worse than no encryption at all. Both Mary and Babington wrote explicitly about their intentions because they believed that their communications were secure, whereas if they had been communicating openly they would have referred to their plan in a more discreet manner. Furthermore, their faith in their cipher made them particularly vulnerable to accepting Phelippes’s forgery. Sender and receiver often have such confidence in the strength of their cipher that they consider it impossible for the enemy to mimic the cipher and insert forged text. The correct use of a strong cipher is a clear boon to sender and receiver, but the misuse of a weak cipher can generate a very false sense of security.

      Soon after receiving the message and its postscript, Babington needed to go abroad to organise the invasion, and had to register at Walsingham’s department in order to acquire a passport. This would have been an ideal time to capture the traitor, but the bureaucrat who was manning the office, John Scudamore, was not expecting the most wanted traitor in England to turn up at his door. Scudamore, with no support to hand, took the unsuspecting Babington to a nearby tavern, stalling for time while his assistant organised a group of soldiers. A short while later a note arrived at the tavern, informing Scudamore that it was time for the arrest. Babington, however, caught sight of it. He casually said that he would pay for the beer and meal and rose to his feet, leaving his sword and coat at the table, implying that he would return in an instant. Instead, he slipped out of the back door and escaped, first to St John’s Wood and then on to Harrow. He attempted to disguise himself, cutting his hair short and staining his skin with walnut juice to mask his aristocratic background. He managed to elude capture for ten days, but by 15 August Babington and his six colleagues were captured and brought to London. Church bells across the city rang out in triumph. Their executions were horrid in the extreme. In the words of the Elizabethan historian William Camden, ‘they were all cut down, their privities were cut off, bowelled alive and seeing, and quartered’.

      Meanwhile, on 11 August, Mary Queen of Scots and her entourage had been allowed the exceptional privilege of riding in the grounds of Chartley Hall. As Mary crossed the moors she spied some horsemen approaching, and immediately thought that these must be Babington’s men coming to rescue her. It soon became clear that these men had come to arrest her, not release her. Mary had been implicated in the Babington Plot, and was charged under the Act of Association, an Act of Parliament passed in 1584 specifically designed to convict anybody involved in a conspiracy