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

trial was held in Fotheringhay Castle, a bleak, miserable place in the middle of the featureless fens of East Anglia. It began on Wednesday 15 October, in front of two chief justices, four other judges, the Lord Chancellor, the Lord Treasurer, Walsingham, and various earls, knights and barons. At the back of the courtroom there was space for spectators, such as local villagers and the servants of the commissioners, all eager to see the humiliated Scottish queen beg forgiveness and plead for her life. However, Mary remained dignified and composed throughout the trial. Mary’s main defence was to deny any connection with Babington. ‘Can I be responsible for the criminal projects of a few desperate men’, she proclaimed, ‘which they planned without my knowledge or participation?’ Her statement had little impact in the face of the evidence against her.

      Mary and Babington had relied on a cipher to keep their plans secret, but they lived during a period when cryptography was being weakened by advances in cryptanalysis. Although their cipher would have been sufficient protection against the prying eyes of an amateur, it stood no chance against an expert in frequency analysis. In the spectators’ gallery sat Phelippes, quietly watching the presentation of the evidence that he had conjured from the enciphered letters.

      The trial went into a second day, and Mary continued to deny any knowledge of the Babington Plot. When the trial finished, she left the judges to decide her fate, pardoning them in advance for the inevitable decision. Ten days later, the Star Chamber met in Westminster and concluded that Mary had been guilty of ‘compassing and imagining since June 1st matters tending to the death and destruction of the Queen of England’. They recommended the death penalty, and Elizabeth signed the death warrant.

      On 8 February 1587, in the Great Hall of Fotheringhay Castle, an audience of three hundred gathered to watch the beheading. Walsingham was determined to minimise Mary’s influence as a martyr, and he ordered that the block, Mary’s clothing, and everything else relating to the execution be burned in order to avoid the creation of any holy relics. He also planned a lavish funeral procession for his son-in-law, Sir Philip Sidney, to take place the following week. Sidney, a popular and heroic figure, had died fighting Catholics in the Netherlands, and Walsingham believed that a magnificent parade in his honour would dampen sympathy for Mary. However, Mary was equally determined that her final appearance should be a defiant gesture, an opportunity to reaffirm her Catholic faith and inspire her followers.

      While the Dean of Peterborough led the prayers, Mary spoke aloud her own prayers for the salvation of the English Catholic Church, for her son and for Elizabeth. With her family motto, ‘In my end is my beginning’, in her mind, she composed herself and approached the block. The executioners requested her forgiveness, and she replied, ‘I forgive you with all my heart, for now I hope you shall make an end of all my troubles’. Richard Wingfield, in his Narration of the Last Days of the Queen of Scots, describes her final moments:

      Then she laide herself upon the blocke most quietlie, & stretching out her armes & legges cryed out In manus tuas domine three or foure times, & at the laste while one of the executioners held her slightlie with one of his handes, the other gave two strokes with an axe before he cutt of her head, & yet lefte a little gristle behinde at which time she made verie small noyse & stirred not any parte of herself from the place where she laye … Her lipps stirred up & downe almost a quarter of an hower after her head was cutt of. Then one of her executioners plucking of her garters espied her little dogge which was crept under her clothes which could not be gotten forth but with force & afterwardes could not depart from her dead corpse, but came and laye betweene her head & shoulders a thing dilligently noted.

      Figure 10 The execution of Mary Queen of Scots.

       Scottish National Portrait Gallery, Edinburgh.

      2 Le Chiffre Indéchiffrable

      For centuries, the simple monoalphabetic substitution cipher had been sufficient to ensure secrecy. The subsequent development of frequency analysis, first in the Arab world and then in Europe, destroyed its security. The tragic execution of Mary Queen of Scots was a dramatic illustration of the weaknesses of monoalphabetic substitution, and in the battle between cryptographers and cryptanalysts it was clear that the cryptanalysts had gained the upper hand. Anybody sending an encrypted message had to accept that an expert enemy codebreaker might intercept and decipher their most precious secrets.

      The onus was clearly on the cryptographers to concoct a new, stronger cipher, something that could outwit the cryptanalysts. Although this cipher would not emerge until the end of the sixteenth century, its origins can be traced back to the fifteenth-century Florentine polymath Leon Battista Alberti. Born in 1404, Alberti was one of the leading figures of the Renaissance – a painter, composer, poet and philosopher, as well as the author of the first scientific analysis of perspective, a treatise on the housefly and a funeral oration for his dog. He is probably best known as an architect, having designed Rome’s first Trevi Fountain and having written De re aedificatoria, the first printed book on architecture, which acted as a catalyst for the transition from Gothic to Renaissance design.

      Sometime in the 1460s, Alberti was wandering through the gardens of the Vatican when he bumped into his friend Leonardo Dato, the pontifical secretary, who began chatting to him about some of the finer points of cryptography. This casual conversation prompted Alberti to write an essay on the subject, outlining what he believed to be a new form of cipher. At the time, all substitution ciphers required a single cipher alphabet for encrypting each message. However, Alberti proposed using two or more cipher alphabets, switching between them during encipherment, thereby confusing potential cryptanalysts.

      Plain alphabet a b c d e f g h i j k l m n o p q r s t u v w x y z

      Cipher alphabet 1 F Z B V K I X A Y M E P L S D H J O R G N Q C U T W

      Cipher alphabet 2 G O X B F W T H Q I L A P Z J D E S V Y C R K U H N

      For example, here we have two possible cipher alphabets, and we could encrypt a message by alternating between them. To encrypt the message hello, we would encrypt the first letter according to the first cipher alphabet, so that h becomes A, but we would encrypt the second letter according to the second cipher alphabet, so that e becomes F. To encrypt the third letter we return to the first cipher alphabet, and to encrypt the fourth letter we return to the second alphabet. This means that the first I is enciphered as P, but the second I is enciphered as A. The final letter, o, is enciphered according to the first cipher alphabet and becomes D. The complete ciphertext reads AFPAD. The crucial advantage of Alberti’s system is that the same letter in the plaintext does not necessarily appear as the same letter in the ciphertext, so the repeated I in hello is enciphered differently in each case. Similarly, the repeated A in the ciphertext represents a different plaintext letter in each case, first h and then I.

      Although he had hit upon the most significant breakthrough in encryption for over a thousand years, Alberti failed to develop his concept into a fully formed system of encryption. That task fell to a diverse group of intellectuals, who built on his initial idea. First came Johannes Trithemius, a German abbot born in 1462, then Giovanni Porta, an Italian scientist born in 1535, and finally Blaise de Vigenère, a French diplomat born in 1523. Vigenère became acquainted with the writings of Alberti, Trithemius and Porta when, at the age of twenty-six, he was sent to Rome on a two-year diplomatic mission. To start with, his interest in cryptography was purely practical and was linked to his diplomatic work. Then, at the age of thirty-nine, Vigenère decided that he had accumulated enough money for him to be able to abandon his career and concentrate on a life of study. It was only then that he examined in detail the ideas of Alberti, Trithemius and Porta, weaving them into a coherent and powerful new cipher.

      Figure 11 Blaise de Vigenère.

       Cliché Bibliothèque Nationale de France, Paris, France.

      Table 3 A Vigenère square.