On the eve of the Japanese attack on Pearl Harbor in December 1941, an apparently innocent weather forecast, ‘East wind rain, north wind cloudy, west wind clear’, alerted Japanese diplomats around the world that war was imminent.
The message is one of the simplest forms of code – a prearranged message which had a special meaning to those who received it.
Similar messages were broadcast by the BBC during the Second World War to the French Resistance. A sentence such as ‘Romeo embraces Juliet’ or ‘Benedictine is a sweet liqueur’ might convey prearranged information about the dropping of agents or supplies. The first line of a poem by the French writer Paul Verlaine (‘The long sobs of the violins of autumn’) told the Resistance that the D-Day landings were about to begin.
More complex codes replace words or whole phrases with other words or phrases. Alternatively, groups of unconnected letters may be used to create a whole dictionary of words and phrases. For example, the order ‘Provide supporting fire’ might be conveyed by the letters GYPHC. Long military reports can be transmitted in these five-letter groups – only intelligible to someone who can look them up in the correct codebook.
If a codebook falls into enemy hands, however, vital information can be intercepted without the sender’s knowledge. In the First World War the German naval codebook was recovered from the wreck of the light cruiser Magdeburg. Consequently many of the German High Seas Fleet’s most sensitive orders were read by the British. Even when the German Admiralty discovered its loss, it took weeks before it could supply every German ship with a new codebook.
The other major method of sending secret information is with ciphers. A cipher substitutes letters, numbers or symbols for the real letters of the alphabet. The Morse code is actually a cipher, conveying each letter by combinations of short and long signals which can be sent by radio bleeps, telegraph or signal lamps. The letter E, for example, is a single dot, while Q is dash, dash, dot, dash (- – . -).
Another common, and simple, form of cipher is worked out on a grid called a cipher box. The message ‘Enemy troops embark December first’ could be written in a grid of, say, six columns, alternately writing left to right and right to left.
The person who receives the message uses a similar grid to decipher it.
A weakness of the system is that the frequency of letters and combinations of letters remains the same as in normal language. E, for example, is the most commonly used letter in the English language, and Z is one of the least common. The combinations EE and OO occur frequently. Someone trying to break the cipher can assume the most-often occurring letter represents E, and so on.
So immensely complicated ciphers, involving numbers as well as letters, have been developed by mathematicians.
During the Second World War the German government used a cipher machine called Enigma. However often a particular letter was keyed, it would never repeat the same cipher letter. Each day a new basic setting was made, according to a schedule known only to the Germans.
A team of university mathematicians and linguists in Britain eventually unraveled Enigma’s ciphers in 1940. Their work played a major part in winning the war by giving Allied Headquarters an up-to-the-minute picture of the German plans in the North African campaign and the air war.
With the advent of computers, codes have become far more complicated and difficult to break. Complex programs use thousands of calculations, and without knowing the sequence of key commands, they could take thousands of years to decode.
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