Free The Code Book by Simon Singh Page B

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. ( photo credit 1.4 )

2 Le Chiffre Indéchiffrable

    F or 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.

    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 l is enciphered as P, but the second l 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 l 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 l.
    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