One of the oldest secret codes is the substitution cipher, which turns the original message into an enciphered message by substituting each letter with a different one. So every A in the original message might be replaced with P, every B with K, and so on. There are over 400 million million million million ways of substituting the letters of the alphabet, and so a codebreaker cannot hope to decipher an intercepted message by checking every possible set of substitutions - even if it was possible to check one set of substitutions every second, it would still take roughly a billion times the age of the universe to check them all.
For over a millennium, the substitution cipher was considered unbreakable, but in the 8th-century AD, Islamic scholars made a crucial observation. Theologians were interested in establishing the chronology of the revelations contained in the Koran, and they did this by counting the frequencies of words contained in each revelation. The theory was that certain words had evolved relatively recently, and so if a revelation contained a high number of these words, then it probably belonged to the latter part of the chronology.
Significantly, the religious scholars did not stop their scrutiny at the level of words. They also analysed letters and discovered that some are more common than others. This led to the first great breakthrough in codebreaking, as documented by the Arabic polymath Al-Kindi, "the philosopher of the Arabs".
In A Manuscript on Deciphering Cryptographic Messages, Al-Kindi advised codebreakers to look for the most common letter in the encrypted text. If the original message was in English, and if the most common letter in the encrypted text is W, then this probably represents the letter E, because E is the most common letter in English. Similarly, if the second most common letter in the encrypted text is Q, then this probably represents T, because T is the second most common letter in English. And so on.
This technique, known as frequency analysis, destroyed the security of the substitution cipher. However, it needs to be applied with some care, because the frequencies in some texts are highly unusual. In 1969, the French author Georges Perec wrote La Disparition, a 200-page novel that did not use words containing the letter "e".
Simon Singh is the author of `The Code Book - the Science of Secrecy from Ancient Egypt to Quantum Cryptography', Fourth Estate, pounds 16.99