The secret world of code-breaking

The secret world of code-breaking
Published: 17 May 2007
Independent UK

Codes are as old as language. Language allows you to communicate with anybody; a secure code lets you pick and choose the cipient of the information you want to impart. From the beginnings of recorded history we find examples of people communicating in code, that amuse or appal us. And codes have lost none of their importance in the 21st century.

Tomorrow will see the release of a film based on a real-life example of a code being used to torment and manipulate. Zodiac is a fictionalised version of the story of a serial killer who taunted the California police with coded messages which, he claimed, would reveal his identity. Those sections of the messages that were decoded revealed a sick mind, but not the killer's identity. Perhaps they were never meant to.

Zodiac is not the first recent film predicated on the challenge of codes - The Da Vinci Code is another well-known example - but it is perhaps a better illustration of why codes matter. The Zodiac's activities were an example of "cryptology", which is the art of concealing the meaning of a message. The cryptologist lets you know that he is sending a message, but unless you have the key to the code, it does not mean anything.

There is a whole other branch of the science of codemaking, called steganography, which is about passing messages in such a way that the message itself is concealed. Understanding the message is not hard; the challenge is to find it. This general category includes clever dodges that we would not call codes, although we may marvel at their ingenuity. Histaiaeus, in ancient Greece, wanted to send a message to Aristagorus of Miletus, inciting him to revolt against the Persians, but needed to be very sure that the Persians did not suspect anything. So he shaved a messenger's head, wrote the message on his pate, and dispatched him to Miletus as soon as his hair had grown again.

A speedier and much commoner form of steganography involves concealing a message within an innocent-looking piece of text, for instance by making pinpricks in a book under selected letters which spell out a message. This was popular in the days when it was expensive to post a letter in Britain, but the Post Office delivered newspapers free. Families would communicate through newspapers covered in barely visible pinpricks.

A variant, if you are the person writing the original text, is to conceal the message in the words themselves. The journalist Stephen Pollard, best known as the author of a biography that helped destroy the career of David Blunkett, also very nearly destroyed his own career with a steganographic message.

On his last day working as a leader writer at the Daily Express, six years ago, Mr Pollard wrote a cleverly crafted editorial, in which the first word of the first sentence began with the letter "f", the first word of the second sentence began with "u", and - well, you see where this is going. It was a not-so-fond farewell directed at the newspaper's proprietor, Richard Desmond. Unfortunately for Mr Pollard, his new employer, The Times, failed to find it funny and ripped up his contract.

One pleasing example of this kind of coded message is in the St James version of the Bible. The 46th word of Psalm 46 in the St James Bible is "shake". The 46th word counting backwards from the end of Psalm 46 is "spear". The St James Bible came out in 1610, the year that William Shakespeare celebrated his 46th birthday. England's finest scholars, set to work producing this translation of the Bible, were required to include a frontispiece paying a grovelling tribute to England's much despised king, James I. Evidently, someone decided to conceal an acknowledgement to a person they genuinely admired. There is also the case of Rosslyn Chapel, near Edinburgh, in whose carvings cryptographers claim to have discovered an encoded piece of music.

The ancients generally had more reason to use concealed messages than cryptographic puzzles. A message in jumbled writing, or numbers, or symbols will arouse suspicion and could get a person into trouble even if its secrets could not be unlocked. Samuel Pepys wrote his dairies in a cryptographic code, but he was not afraid someone would find out that he was keeping a diary; he just did not want the idly curious to know what it said.

Cryptography took off with the invention of the radio. From the First World War onwards, commanders have communicated with their juniors by messages sent across the airwaves, which the enemy can easily pick up. The codemakers' task was to ensure that the message was quickly and easily understood by its legitimate recipient, and no one else. This produced a new form of warfare between codemakers and codebreakers, which the codebreakers usually won.

The commonest form of cryptography is substitution, where letters are jumbled up according to a concealed pattern. This has usually proved to be an activity for men, despite the advice given in the Kama Sutra that women should learn it, so that they can conduct their love affairs in secret.

Hundreds of thousands of people have seen a quaint example of substitution cryptography when they watched Borat's film Cultural Leanings of America. As participants sang a song in English, up came what looked like Russian subtitles, which were gibberish to any Russian. The subtitles were decoded by a smart Moscow graduate writing a dissertation on Western culture. On a Russian keyboard, you find a Cyrillic letter that looks like the Latin "u" where we would expect to find a "q", a letter vaguely resmbling a "y" where we expect a "w" - and so on. The film-makers attached a Latin keyboard to a computer that was programmed to produce Cyrillic writing, and tapped the keys as if they were typing out the song in English. To decode the resulting nonsense, a Russian needed to attach a Cyrillic keyboard to a computer programmed to write Latin.

The weakness of any code based on an alphabet is that every codebreaker knows the relatively frequency with which different letters appear in a given language. In a passage written in English, every eighth letter, approximately, is an "e", one letter in 11 is a "t", and one in 1,000 is a "q". If you know the normal percentages for each letter, you can crack a mono-alphabetic code.

There is a film based on a novel by Robert Harris about the Enigma machine, a German computer which instead of substituting one alphabet for another, used 17,576 substitutions simultaneously. It was easy to read a message sent out by Enigma, if you had your own Enigma machine and an up-to-date codebook that told you the right settings for that day. Without either, the messages should have been impossible to read. Yet the codebreakers in Bletchley Park did it. Their success was an enormous contributor to the Allied victory.

Today's codemakers and codemakers use immensely more sophisticated methods. Enigma encryption was a piece of cake compared with 128-bit encryption. But today's codebreakers also have at their disposal computers vastly quicker and more powerful than the Bletchley boffins. The battle continues.

But don't worry, you don't need a super computer to crack the Independent code. All you need is a brain and a bit of time.