The Code Book Read online

  Figure 1 Mary Queen of Scots.(photo credit 1.1)

  The conspirators were a group of young English Catholic noblemen intent on removing Elizabeth, a Protestant, and replacing her with Mary, a fellow Catholic. It was apparent to the court that Mary was a figurehead for the conspirators, but it was not clear that she had actually given her blessing to the conspiracy. In fact, Mary had authorized the plot. The challenge for Walsingham was to demonstrate a palpable link between Mary and the plotters.

  On the morning of her trial, Mary sat alone in the dock, dressed in sorrowful black velvet. In cases of treason, the accused was forbidden counsel and was not permitted to call witnesses. Mary was not even allowed secretaries to help her prepare her case. However, her plight was not hopeless because she had been careful to ensure that all her correspondence with the conspirators had been written in cipher. The cipher turned her words into a meaningless series of symbols, and Mary believed that even if Walsingham had captured the letters, then he could have no idea of the meaning of the words within them. If their contents were a mystery, then the letters could not be used as evidence against her. However, this all depended on the assumption that her cipher had not been broken.

  Unfortunately for Mary, Walsingham was not merely Principal Secretary, he was also England’s spymaster. He had intercepted Mary’s letters to the plotters, and he knew exactly who might be capable of deciphering them. Thomas Phelippes was the nation’s foremost expert on breaking codes, and for years he had been deciphering the messages of those who plotted against Queen Elizabeth, thereby providing the evidence needed to condemn them. If he could decipher the incriminating letters between Mary and the conspirators, then her death would be inevitable. On the other hand, if Mary’s cipher was strong enough to conceal her secrets, then there was a chance that she might survive. Not for the first time, a life hung on the strength of a cipher.

  The Evolution of Secret Writing

  Some of the earliest accounts of secret writing date back to Herodotus, “the father of history” according to the Roman philosopher and statesman Cicero. In The Histories, Herodotus chronicled the conflicts between Greece and Persia in the fifth century B.C., which he viewed as a confrontation between freedom and slavery, between the independent Greek states and the oppressive Persians. According to Herodotus, it was the art of secret writing that saved Greece from being conquered by Xerxes, King of Kings, the despotic leader of the Persians.

  The long-running feud between Greece and Persia reached a crisis soon after Xerxes began constructing a city at Persepolis, the new capital for his kingdom. Tributes and gifts arrived from all over the empire and neighboring states, with the notable exceptions of Athens and Sparta. Determined to avenge this insolence, Xerxes began mobilizing a force, declaring that “we shall extend the empire of Persia such that its boundaries will be God’s own sky, so the sun will not look down upon any land beyond the boundaries of what is our own.” He spent the next five years secretly assembling the greatest fighting force in history, and then, in 480 B.C., he was ready to launch a surprise attack.

  However, the Persian military buildup had been witnessed by Demaratus, a Greek who had been expelled from his homeland and who lived in the Persian city of Susa. Despite being exiled he still felt some loyalty to Greece, so he decided to send a message to warn the Spartans of Xerxes’ invasion plan. The challenge was how to dispatch the message without it being intercepted by the Persian guards. Herodotus wrote:

  As the danger of discovery was great, there was only one way in which he could contrive to get the message through: this was by scraping the wax off a pair of wooden folding tablets, writing on the wood underneath what Xerxes intended to do, and then covering the message over with wax again. In this way the tablets, being apparently blank, would cause no trouble with the guards along the road. When the message reached its destination, no one was able to guess the secret, until, as I understand, Cleomenes’ daughter Gorgo, who was the wife of Leonidas, divined and told the others that if they scraped the wax off, they would find something written on the wood underneath. This was done; the message was revealed and read, and afterward passed on to the other Greeks.

  As a result of this warning, the hitherto defenseless Greeks began to arm themselves. Profits from the state-owned silver mines, which were usually shared among the citizens, were instead diverted to the navy for the construction of two hundred warships.

  Xerxes had lost the vital element of surprise and, on September 23, 480 B.C., when the Persian fleet approached the Bay of Salamis near Athens, the Greeks were prepared. Although Xerxes believed he had trapped the Greek navy, the Greeks were deliberately enticing the Persian ships to enter the bay. The Greeks knew that their ships, smaller and fewer in number, would have been destroyed in the open sea, but they realized that within the confines of the bay they might outmaneuver the Persians. As the wind changed direction the Persians found themselves being blown into the bay, forced into an engagement on Greek terms. The Persian princess Artemisia became surrounded on three sides and attempted to head back out to sea, only to ram one of her own ships. Panic ensued, more Persian ships collided and the Greeks launched a full-blooded onslaught. Within a day, the formidable forces of Persia had been humbled.

  Demaratus’ strategy for secret communication relied on simply hiding the message. Herodotus also recounted another incident in which concealment was sufficient to secure the safe passage of a message. He chronicled the story of Histaiaeus, who wanted to encourage Aristagoras of Miletus to revolt against the Persian king. To convey his instructions securely, Histaiaeus shaved the head of his messenger, wrote the message on his scalp, and then waited for the hair to regrow. This was clearly a period of history that tolerated a certain lack of urgency. The messenger, apparently carrying nothing contentious, could travel without being harassed. Upon arriving at his destination he then shaved his head and pointed it at the intended recipient.

  Secret communication achieved by hiding the existence of a message is known as steganography, derived from the Greek words steganos, meaning “covered,” and graphein, meaning “to write.” In the two thousand years since Herodotus, various forms of steganography have been used throughout the world. For example, the ancient Chinese wrote messages on fine silk, which was then scrunched into a tiny ball and covered in wax. The messenger would then swallow the ball of wax. In the sixteenth century, the Italian scientist Giovanni Porta described how to conceal a message within a hard-boiled egg by making an ink from a mixture of one ounce of alum and a pint of vinegar, and then using it to write on the shell. The solution penetrates the porous shell, and leaves a message on the surface of the hardened egg albumen, which can be read only when the shell is removed. Steganography also includes the practice of writing in invisible ink. As far back as the first century A.D., Pliny the Elder explained how the “milk” of the thithymallus plant could be used as an invisible ink. Although transparent after drying, gentle heating chars the ink and turns it brown. Many organic fluids behave in a similar way, because they are rich in carbon and therefore char easily. Indeed, it is not unknown for modern spies who have run out of standard-issue invisible ink to improvise by using their own urine.

  The longevity of steganography illustrates that it certainly offers a modicum of security, but it suffers from a fundamental weakness. If the messenger is searched and the message is discovered, then the contents of the secret communication are revealed at once. Interception of the message immediately compromises all security. A thorough guard might routinely search any person crossing a border, scraping any wax tablets, heating blank sheets of paper, shelling boiled eggs, shaving people’s heads, and so on, and inevitably there will be occasions when the message is uncovered.

  Hence, in parallel with the development of steganography, there was the evolution of cryptography, derived from the Greek word kryptos, meaning “hidden.” The aim of cryptography is not to hide the existence of a message, but rather to hide its meaning, a process known as enc
ryption. To render a message unintelligible, it is scrambled according to a particular protocol which is agreed beforehand between the sender and the intended recipient. Thus the recipient can reverse the scrambling protocol and make the message comprehensible. The advantage of cryptography is that if the enemy intercepts an encrypted message, then the message is unreadable. Without knowing the scrambling protocol, the enemy should find it difficult, if not impossible, to recreate the original message from the encrypted text.

  Although cryptography and steganography are independent, it is possible to both scramble and hide a message to maximize security. For example, the microdot is a form of steganography that became popular during the Second World War. German agents in Latin America would photographically shrink a page of text down to a dot less than 1 millimeter in diameter, and then hide this microdot on top of a full stop in an apparently innocuous letter. The first microdot to be spotted by the FBI was in 1941, following a tip-off that the Americans should look for a tiny gleam from the surface of a letter, indicative of smooth film. Thereafter, the Americans could read the contents of most intercepted microdots, except when the German agents had taken the extra precaution of scrambling their message before reducing it. In such cases of cryptography combined with steganography, the Americans were sometimes able to intercept and block communications, but they were prevented from gaining any new information about German spying activity. Of the two branches of secret communication, cryptography is the more powerful because of this ability to prevent information from falling into enemy hands.

  In turn, cryptography itself can be divided into two branches, known as transposition and substitution. In transposition, the letters of the message are simply rearranged, effectively generating an anagram. For very short messages, such as a single word, this method is relatively insecure because there are only a limited number of ways of rearranging a handful of letters. For example, three letters can be arranged in only six different ways, e.g., cow, cwo, ocw, owc, wco, woc. However, as the number of letters gradually increases, the number of possible arrangements rapidly explodes, making it impossible to get back to the original message unless the exact scrambling process is known. For example, consider this short sentence. It contains just 35 letters, and yet there are more than 50,000,000,000,000,000,000,000,000,000,000 distinct arrangements of them. If one person could check one arrangement per second, and if all the people in the world worked night and day, it would still take more than a thousand times the lifetime of the universe to check all the arrangements.

  A random transposition of letters seems to offer a very high level of security, because it would be impractical for an enemy interceptor to unscramble even a short sentence. But there is a drawback. Transposition effectively generates an incredibly difficult anagram, and if the letters are randomly jumbled, with neither rhyme nor reason, then unscrambling the anagram is impossible for the intended recipient, as well as an enemy interceptor. In order for transposition to be effective, the rearrangement of letters needs to follow a straightforward system, one that has been previously agreed by sender and receiver, but kept secret from the enemy. For example, schoolchildren sometimes send messages using the “rail fence” transposition, in which the message is written with alternate letters on separate upper and lower lines. The sequence of letters on the lower line is then tagged on at the end of the sequence on the upper line to create the final encrypted message. For example:

  The receiver can recover the message by simply reversing the process. There are various other forms of systematic transposition, including the three-line rail fence cipher, in which the message is first written on three separate lines instead of two. Alternatively, one could swap each pair of letters, so that the first and second letters switch places, the third and fourth letters switch places, and so on.

  Another form of transposition is embodied in the first ever military cryptographic device, the Spartan scytale, dating back to the fifth century B.C. The scytale is a wooden staff around which a strip of leather or parchment is wound, as shown in Figure 2. The sender writes the message along the length of the scytale, and then unwinds the strip, which now appears to carry a list of meaningless letters. The message has been scrambled. The messenger would take the leather strip, and, as a steganographic twist, he would sometimes disguise it as a belt with the letters hidden on the inside. To recover the message, the receiver simply wraps the leather strip around a scytale of the same diameter as the one used by the sender. In 404 B.C. Lysander of Sparta was confronted by a messenger, bloody and battered, one of only five to have survived the arduous journey from Persia. The messenger handed his belt to Lysander, who wound it around his scytale to learn that Pharnabazus of Persia was planning to attack him. Thanks to the scytale, Lysander was prepared for the attack and repulsed it.

  Figure 2 When it is unwound from the sender’s scytale (wooden staff), the leather strip appears to carry a list of random letters; S, T, S, F,.… Only by rewinding the strip around another scytale of the correct diameter will the message reappear.

  The alternative to transposition is substitution. One of the earliest descriptions of encryption by substitution appears in the Kāma-Sūtra, a text written in the fourth century A.D. by the Brahmin scholar Vātsyāyana, but based on manuscripts dating back to the fourth century B.C. The Kāma-Sūtra recommends that women should study 64 arts, such as cooking, dressing, massage and the preparation of perfumes. The list also includes some less obvious arts, namely conjuring, chess, bookbinding and carpentry. Number 45 on the list is mlecchita-vikalpā, the art of secret writing, advocated in order to help women conceal the details of their liaisons. One of the recommended techniques is to pair letters of the alphabet at random, and then substitute each letter in the original message with its partner. If we apply the principle to the Roman alphabet, we could pair letters as follows:

  Then, instead of meet at midnight, the sender would write CUUZ VZ CGXSGIBZ. This form of secret writing is called a substitution cipher because each letter in the plaintext is substituted for a different letter, thus acting in a complementary way to the transposition cipher. In transposition each letter retains its identity but changes its position, whereas in substitution each letter changes its identity but retains its position.

  The first documented use of a substitution cipher for military purposes appears in Julius Caesar’s Gallic Wars. Caesar describes how he sent a message to Cicero, who was besieged and on the verge of surrendering. The substitution replaced Roman letters with Greek letters, rendering the message unintelligible to the enemy. Caesar described the dramatic delivery of the message:

  The messenger was instructed, if he could not approach, to hurl a spear, with the letter fastened to the thong, inside the entrenchment of the camp. Fearing danger, the Gaul discharged the spear, as he had been instructed. By chance it stuck fast in the tower, and for two days was not sighted by our troops; on the third day it was sighted by a soldier, taken down, and delivered to Cicero. He read it through and then recited it at a parade of the troops, bringing the greatest rejoicing to all.

  Caesar used secret writing so frequently that Valerius Probus wrote an entire treatise on his ciphers, which unfortunately has not survived. However, thanks to Suetonius’ Lives of the Caesars LVI, written in the second century A.D., we do have a detailed description of one of the types of substitution cipher used by Julius Caesar. He simply replaced each letter in the message with the letter that is three places further down the alphabet. Cryptographers often think in terms of the plain alphabet, the alphabet used to write the original message, and the cipher alphabet, the letters that are substituted in place of the plain letters. When the plain alphabet is placed above the cipher alphabet, as shown in Figure 3, it is clear that the cipher alphabet has been shifted by three places, and hence this form of substitution is often called the Caesar shift cipher, or simply the Caesar cipher. A cipher is the name given to any form of cryptographic substitution in which each letter is replaced by another letter o
r symbol.

  Figure 3 The Caesar cipher applied to a short message. The Caesar cipher is based on a cipher alphabet that is shifted a certain number of places (in this case three), relative to the plain alphabet. The convention in cryptography is to write the plain alphabet in lower-case letters, and the cipher alphabet in capitals. Similarly, the original message, the plaintext, is written in lower case, and the encrypted message, the ciphertext, is written in capitals.

  Although Suetonius mentions only a Caesar shift of three places, it is clear that by using any shift between 1 and 25 places it is possible to generate 25 distinct ciphers. In fact, if we do not restrict ourselves to shifting the alphabet and permit the cipher alphabet to be any rearrangement of the plain alphabet, then we can generate an even greater number of distinct ciphers. There are over 400,000,000,000,000,000,000,000,000 such rearrangements, and therefore the same number of distinct ciphers.

  Each distinct cipher can be considered in terms of a general encrypting method, known as the algorithm, and a key, which specifies the exact details of a particular encryption. In this case, the algorithm involves substituting each letter in the plain alphabet with a letter from a cipher alphabet, and the cipher alphabet is allowed to consist of any rearrangement of the plain alphabet. The key defines the exact cipher alphabet to be used for a particular encryption. The relationship between the algorithm and the key is illustrated in Figure 4.

  An enemy studying an intercepted scrambled message may have a strong suspicion of the algorithm, but would not know the exact key. For example, they may well suspect that each letter in the plaintext has been replaced by a different letter according to a particular cipher alphabet, but they are unlikely to know which cipher alphabet has been used. If the cipher alphabet, the key, is kept a closely guarded secret between the sender and the receiver, then the enemy cannot decipher the intercepted message. The significance of the key, as opposed to the algorithm, is an enduring principle of cryptography. It was definitively stated in 1883 by the Dutch linguist Auguste Kerckhoffs von Nieuwenhof in his book La Cryptographie militaire: “Kerckhoffs’ Principle: The security of a cryptosystem must not depend on keeping secret the crypto-algorithm. The security depends only on keeping secret the key.”