Excerpt
On the morning of Wednesday, 15 October 1586, Queen Mary entered the crowdedcourtroom at Fotheringhay Castle. Years of imprisonment and the onset of rheumatismhad taken their toll, yet she remained dignified, composed and indisputably regal.Assisted by her physician, she made her way past the judges, officials andspectators, and approached the throne that stood halfway along the long, narrowchamber. Mary had assumed that the throne was a gesture of respect towards her, butshe was mistaken. The throne symbolised the absent Queen Elizabeth, Mary's enemy andprosecutor. Mary was gently guided away from the throne and towards the opposite sideof the room, to the defendant's seat, a crimson velvet chair.
Mary Queen of Scots was on trial for treason. She had been accused of plotting toassassinate Queen Elizabeth in order to take the English crown for herself. SirFrancis Walsingham, Elizabeth's Principal Secretary, had already arrested the otherconspirators, extracted confessions, and executed them. Now he planned to prove thatMary was at the heart of the plot, and was therefore equally culpable and equallydeserving of death.
Walsingham knew that before he could have Mary executed, he would have to convinceQueen Elizabeth of her guilt. Although Elizabeth despised Mary, she had severalreasons for being reluctant to see her put to death. First, Mary was a Scottishqueen, and many questioned whether an English court had the authority to execute aforeign head of state. Second, executing Mary might establish an awkward precedent —if the state is allowed to kill one queen, then perhaps rebels might have fewerreservations about killing another, namely Elizabeth. Third, Elizabeth and Mary werecousins, and their blood tie made Elizabeth all the more squeamish about ordering herexecution. In short, Elizabeth would sanction Mary's execution only if Walsinghamcould prove beyond any hint of doubt that she had been part of the assassinationplot.
The conspirators were a group of young English Catholic noblemen intent on removingElizabeth, a Protestant, and replacing her with Mary, a fellow Catholic. It wasapparent to the court that Mary was a figurehead for the conspirators, but it was notclear that she had actually given her blessing to the conspiracy. In fact, Mary hadauthorised the plot. The challenge for Walsingham was to demonstrate a palpable linkbetween Mary and the plotters.
On the morning of her trial, Mary sat alone in the dock, dressed in sorrowful blackvelvet. In cases of treason, the accused was forbidden counsel and was not permittedto call witnesses. Mary was not even allowed secretaries to help her prepare hercase. However, her plight was not hopeless because she had been careful to ensurethat all her correspondence with the conspirators had been written in cipher. Thecipher turned her words into a meaningless series of symbols, and Mary believed thateven if Walsingham had captured the letters, then he could have no idea of themeaning of the words within them. If their contents were a mystery, then the letterscould not be used as evidence against her. However, this all depended on theassumption that her cipher had not been broken.
Unfortunately for Mary, Walsingham was not merely Principal Secretary, he was alsoEngland's spymaster. He had intercepted Mary's letters to the plotters, and he knewexactly who might be capable of deciphering them. Thomas Phelippes was the nation'sforemost expert on breaking codes, and for years he had been deciphering the messagesof those who plotted against Queen Elizabeth, thereby providing the evidence neededto condemn them. If he could decipher the incriminating letters between Mary and theconspirators, then her death would be inevitable. On the other hand, if Mary's cipherwas strong enough to conceal her secrets, then there was a chance that she mightsurvive. 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 fatherof history' according to the Roman philosopher and statesman Cicero. In TheHistories, Herodotus chronicled the conflicts between Greece and Persia in thefifth century bc, which he viewed as a confrontation between freedom and slavery,between the independent Greek states and the oppressive Persians. According toHerodotus, it was the art of secret writing that saved Greece from being conquered byXerxes, King of Kings, the despotic leader of the Persians.
The long-running feud between Greece and Persia reached a crisis soon after Xerxesbegan constructing a city at Persepolis, the new capital for his kingdom. Tributesand gifts arrived from all over the empire and neighbouring states, with the notableexceptions of Athens and Sparta. Determined to avenge this insolence, Xerxes beganmobilising a force, declaring that 'we shall extend the empire of Persia such thatits boundaries will be God's own sky, so the sun will not look down upon any landbeyond the boundaries of what is our own'. He spent the next five years secretlyassembling the greatest fighting force in history, and then, in 480 bc, he was readyto launch a surprise attack.
However, the Persian military build-up had been witnessed by Demaratus, a Greek whohad 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 amessage to warn the Spartans of Xerxes' invasion plan. The challenge was how todispatch the message without it being intercepted by the Persian guards. Herodotuswrote:
As the danger of discovery was great, there was only one way in which he couldcontrive to get the message through: this was by scraping the wax off a pair ofwooden 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, beingapparently blank, would cause no trouble with the guards along the road. When themessage reached its destination, no one was able to guess the secret, until, as Iunderstand, Cleomenes' daughter Gorgo, who was the wife of Leonides, divined and toldthe others that if they scraped the wax off, they would find something written on thewood underneath. This was done; the message was revealed and read, and afterwardspassed on to the other Greeks.
As a result of this warning, the hitherto defenceless Greeks began to arm themselves.Profits from the state-owned silver mines, which were usually shared among thecitizens, were instead diverted to the navy for the construction of two hundredwarships.
Xerxes had lost the vital element of surprise and, on 23 September 480 bc, when thePersian fleet approached the Bay of Salamis near Athens, the Greeks were prepared.Although Xerxes believed he had trapped the Greek navy, the Greeks were deliberatelyenticing 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 theyrealised that within the confines of the bay they might outmanoeuvre the Persians. Asthe wind changed direction the Persians found themselves being blown into the bay,forced into an engagement on Greek terms. The Persian princess Artemisia becamesurrounded on three sides and attempted to head back out to sea, only to ram one ofher own ships. Panic ensued, more Persian ships collided and the Greeks launched afull-blooded onslaught. Within a day, the formidable forces of Persia had beenhumbled.
Demaratus' strategy for secret communication relied on simply hiding the message.Herodotus also recounted another incident in which concealment was sufficient tosecure the safe passage of a message. He chronicled the story of Histaiaeus, whowanted to encourage Aristagoras of Miletus to revolt against the Persian king. Toconvey his instructions securely, Histaiaeus shaved the head of his messenger, wrotethe message on his scalp, and then waited for the hair to regrow. This was clearly aperiod of history that tolerated a certain lack of urgency. The messenger, apparentlycarrying nothing contentious, could travel without being harassed. Upon arriving athis 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 assteganography, derived from the Greek words steganos, meaning 'covered', andgraphein, meaning 'to write'. In the two thousand years since Herodotus,various forms of steganography have been used throughout the world. For example, theancient Chinese wrote messages on fine silk, which was then scrunched into a tinyball and covered in wax. The messenger would then swallow the ball of wax. In thefifteenth century, the Italian scientist Giovanni Porta described how to conceal amessage within a hard-boiled egg by making an ink from a mixture of one ounce of alumand a pint of vinegar, and then using it to write on the shell. The solutionpenetrates the porous shell, and leaves a message on the surface of the hardened eggalbumen, which can be read only when the shell is removed. Steganography alsoincludes the practice of writing in invisible ink. As far back as the first centuryad, Pliny the Elder explained how the 'milk' of the thithymallus plant could be usedas an invisible ink. Although transparent after drying, gentle heating chars the inkand turns it brown. Many organic fluids behave in a similar way, because they arerich in carbon and therefore char easily. Indeed, it is not unknown for modern spieswho have run out of standard-issue invisible ink to improvise by using their ownurine.
The longevity of steganography illustrates that it certainly offers a modicum ofsecurity, but it suffers from a fundamental weakness. If the messenger is searchedand the message is discovered, then the contents of the secret communication arerevealed at once. Interception of the message immediately compromises all security. Athorough guard might routinely search any person crossing a border, scraping any waxtablets, 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 ofcryptography, derived from the Greek word kryptos, meaning 'hidden'. The aimof cryptography is not to hide the existence of a message, but rather to hide itsmeaning, a process known as encryption. To render a message unintelligible, it isscrambled according to a particular protocol which is agreed beforehand between thesender and the intended recipient. Thus the recipient can reverse the scramblingprotocol and make the message comprehensible. The advantage of cryptography is thatif the enemy intercepts an encrypted message, then the message is unreadable. Withoutknowing the scrambling protocol, the enemy should find it difficult, if notimpossible, to recreate the original message from the encrypted text.
Although cryptography and steganography are independent, it is possible to bothscramble and hide a message to maximise security. For example, the microdot is a formof steganography that became popular during the Second World War. German agents inLatin America would photographically shrink a page of text down to a dot less than 1millimetre in diameter, and then hide this microdot on top of a full stop in anapparently 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 surfaceof a letter, indicative of smooth film. Thereafter, the Americans could read thecontents of most intercepted microdots, except when the German agents had taken theextra precaution of scrambling their message before reducing it. In such cases ofcryptography combined with steganography, the Americans were sometimes able tointercept and block communications, but they were prevented from gaining any newinformation about German spying activity. Of the two branches of secretcommunication, cryptography is the more powerful because of this ability to preventinformation from falling into enemy hands.
In turn, cryptography itself can be divided into two branches, known as transpositionand 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 ofrearranging a handful of letters. For example, three letters can be arranged in onlysix different ways, e.g. cow, cwo, ocw, owc, wco, woc. However, as the numberof letters gradually increases, the number of possible arrangements rapidly explodes,making it impossible to get back to the original message unless the exact scramblingprocess is known. For example, consider this short sentence. It contains just35 letters, and yet there are more than 50,000,000,000,000,000,000,000,000,000,000distinct 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 morethan a thousand times the lifetime of the universe to check all the arrangements.
Continues...
Excerpted from The Code Bookby Simon Singh Copyright © 2000 by Simon Singh. Excerpted by permission.
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