The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography

by Simon Singh

It has been said that the First World War was the chemists’ war, because mustard gas and chlorine were employed for the first time, and that the Second World War was the physicists’ war, because the atom bomb was detonated. Similarly, it has been argued that the Third World War would be the mathematicians’ war, because mathematicians will have control over the next great weapon of war—information.

In a code, a word or phrase is replaced with a word, number or symbol.

The alternative to a code is a cipher, a technique that acts at a more fundamental level, by replacing letters rather than whole words.

More often than not, though, crypto-jargon is quite transparent: for example, plaintext is the message before encryption, and ciphertext is the message after encryption.

First, Mary was a Scottish queen, and many questioned whether an English court had the authority to execute a foreign 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 fewer reservations about killing another, namely Elizabeth.

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.”

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.

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 encryption.

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.

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 transposition each letter retains its identity but changes its position, whereas in substitution each letter changes its identity but retains its position.

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.”

Instead of randomly rearranging the plain alphabet to achieve the cipher alphabet, the sender chooses a keyword or keyphrase.

Codemakers had evolved a system for guaranteeing secure communication, so there was no need for further development-without necessity, there was no need for further invention.

The Abbasid caliphs were less interested than their predecessors in conquest, and instead concentrated on establishing an organized and affluent society. Lower taxes encouraged businesses to grow and gave rise to greater commerce and industry, while strict laws reduced corruption and protected the citizens.

The monoalphabetic substitution cipher is the general name given to any substitution cipher in which the cipher alphabet consists of either letters or symbols, or a mix of both.

They in fact invented cryptanalysis, the science of unscrambling a message without knowledge of the key.

Cryptanalysis could not be invented until a civilization had reached a sufficiently sophisticated level of scholarship in several disciplines, including mathematics, statistics and linguistics.

At the same time as acquiring knowledge, the Islamic civilization was able to disperse it, because it had procured the art of papermaking from the Chinese.

The theologians were interested in establishing the chronology of the revelations, which they did by counting the frequencies of words contained in each revelation. The theory was that certain words had evolved relatively recently, and hence if a revelation contained a high number of these newer words, this would indicate that it came later in the chronology.

This was done by studying the etymology of words and the structure of sentences, to test whether particular texts were consistent with the linguistic patterns of the Prophet.

Al-Kindī’s technique, known as frequency analysis, shows that it is unnecessary to check each of the billions of potential keys. Instead, it is possible to reveal the contents of a scrambled message simply by analyzing the frequency of the characters in the ciphertext.

In general, short texts are likely to deviate significantly from the standard frequencies, and if there are less than a hundred letters, then decipherment will be very difficult.

If O represents a vowel it should appear before and after most of the other letters, whereas if it represents a consonant, it will tend to avoid many of the other letters.

In the English language, the letter h frequently goes before the letter e (as in the, then, they, etc.), but rarely after e.

Between A.D. 800 and 1200, Arab scholars enjoyed a vigorous period of intellectual achievement. At the same time, Europe was firmly stuck in the Dark Ages.

However, this may have been a ploy to lull the Vatican cryptographers into a false sense of security-Soro might have been reluctant to point out the weaknesses of the Papal cipher, because this would only have encouraged the Vatican to switch to a more secure cipher, one that Soro might not have been able to break.

Toward the end of the sixteenth century the French consolidated their codebreaking prowess with the arrival of François Viète, who took particular pleasure in cracking Spanish ciphers. Spain’s cryptographers, who appear to have been naive compared with their rivals elsewhere in Europe, could not believe it when they discovered that their messages were transparent to the French. King Philip II of Spain went as far as petitioning the Vatican, claiming that the only explanation for Viète’s cryptanalysis was that he was an “archfiend in league with the devil.” Philip argued that Viète should be tried before a Cardinal’s Court for his demonic deeds; but the Pope, who was aware that his own cryptanalysts had been reading Spanish ciphers for years, rejected the Spanish petition. News of the petition soon reached cipher experts in various countries, and Spanish cryptographers became the laughingstock of Europe.

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.

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.

In a polyalphabetic cipher, a plaintext letter will also be represented by different symbols, but, even more confusingly, these symbols will represent different letters during the course of an encipherment. Perhaps the fundamental reason why the homophonic cipher is considered monoalphabetic is that once the cipher alphabet has been established, it remains constant throughout the process of encryption.

Bazeries viewed the letters as the ultimate challenge, and he spent the next three years of his life attempting to decipher them.

Morse used an electromagnet to enhance the signal, so that upon arriving at the receiver’s end it was strong enough to make a series of short and long marks, dots and dashes, on a piece of paper.

Although Babbage was a brilliant innovator, he was not a great implementer.

When Babbage abandoned his first machine, the government lost confidence in him and decided to cut its losses by withdrawing from the project—it had already spent £17,470, enough to build a pair of battleships.

In fact, the Analytical Engine provided the template for modern computers. The design included a “store” (memory) and a “mill” (processor), which would allow it to make decisions and repeat instructions, which are equivalent to the “IF … THEN …” and “LOOP” commands in modern programming.

Unfortunately, as with many other of his grand plans, the book was never completed.

Being a keen statistician and compiler of mortality tables, Babbage was irritated by the lines “Every moment dies a man, Every moment one is born,” which are the last lines of the plaintext above.

He certainly had a habit of not finishing projects and not publishing his discoveries, which might suggest that this is just one more example of his lackadaisical attitude.

It is quite possible that British Intelligence demanded that Babbage keep his work secret, thus providing them with a nine-year head start over the rest of the world.

Morse operators could send plain English at speeds of up to 35 words per minute because they could memorize entire phrases and transmit them in a single burst, whereas the jumble of letters that make up a ciphertext was considerably slower to transmit, because the operator had to continually refer back to the sender’s written message to check the sequence of letters.

However, newspapers could be posted free of charge, and this provided a loophole for thrifty Victorians. Instead of writing and sending letters, people began to use pinpricks to spell out a message on the front page of a newspaper.

The public’s growing fascination with cryptographic techniques meant that codes and ciphers soon found their way into nineteenth-century literature.

In 1843, keen to exploit the interest he had generated, Poe wrote a short story about ciphers, which is widely acknowledged by professional cryptographers to be the finest piece of fictional literature on the subject.

A false line of attack will sometimes generate a few tantalizing words within a sea of gibberish, which then encourages the cryptanalyst to devise a series of caveats to excuse the gibberish.

According to Hammer, “the Beale ciphers have occupied at least 10% of the best cryptanalytic minds in the country. And not a dime of this effort should be begrudged. The work—even the lines that have led into blind alleys—has more than paid for itself in advancing and refining computer research.”

You might be surprised by the strength of the unbroken Beale ciphers, especially bearing in mind that when we left the ongoing battle between codemakers and codebreakers, it was the codebreakers who were on top.

Creating a keytext for a message is much more secure than using a key based on a published book, but it is practical only if the sender has the time to create the keytext and is able to convey it to the intended recipient, requirements that are not feasible for routine, day-to-day communications.

An alternative theory for explaining the indecipherability of the Beale ciphers is that the author of the pamphlet deliberately sabotaged them before having them published.

For example, according to the pamphlet, Beale’s letter, which was locked in the iron box and supposedly written in 1822, contains the word “stampede,” but this word was not seen in print until 1834. However, it is quite possible that the word was in common use in the Wild West at a much earlier date, and Beale could have learned of it on his travels.