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cipher alphabet, and yet it is effectively impossible for the enemy to check all possible keys by the so-called brute-force attack. The simplicity of the key is important, because the sender and receiver have to share knowledge of the key, and the simpler the key, the less the chance of a misunderstanding.
    In fact, an even simpler key is possible if the sender is prepared to accept a slight reduction in the number of potential keys. Instead of randomly rearranging the plain alphabet to achieve the cipher alphabet, the sender chooses a keyword or keyphrase . For example, to use JULIUS CAESAR as a keyphrase, begin by removing any spaces and repeated letters (JULISCAER), and then use this as the beginning of the jumbled cipher alphabet. The remainder of the cipher alphabet is merely the remaining letters of the alphabet, in their correct order, starting where the keyphrase ends. Hence, the cipher alphabet would read as follows.

    The advantage of building a cipher alphabet in this way is that it is easy to memorize the keyword or keyphrase, and hence the cipher alphabet. This is important, because if the sender has to keep the cipher alphabet on a piece of paper, the enemy can capture the paper, discover the key, and read any communications that have been encrypted with it. However, if the key can be committed to memory it is less likely to fall into enemy hands. Clearly the number of cipher alphabets generated by keyphrases is smaller than the number of cipher alphabets generated without restriction, but the number is still immense, and it would be effectively impossible for the enemy to unscramble a captured message by testing all possible keyphrases.
    This simplicity and strength meant that the substitution cipher dominated the art of secret writing throughout the first millennium A.D . 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 onus had fallen upon the codebreakers, those who were attempting to crack the substitution cipher. Was there any way for an enemy interceptor to unravel an encrypted message? Many ancient scholars considered that the substitution cipher was unbreakable, thanks to the gigantic number of possible keys, and for centuries this seemed to be true. However, codebreakers would eventually find a shortcut to the process of exhaustively searching all keys. Instead of taking billions of years to crack a cipher, the shortcut could reveal the message in a matter of minutes. The breakthrough occurred in the East, and required a brilliant combination of linguistics, statistics and religious devotion.

The Arab Cryptanalysts

    At the age of about forty, Muhammad began regularly visiting an isolated cave on Mount Hira just outside Mecca. This was a retreat, a place for prayer, meditation and contemplation. It was during a period of deep reflection, around A.D . 610, that he was visited by the archangel Gabriel, who proclaimed that Muhammad was to be the messenger of God. This was the first of a series of revelations which continued until Muhammad died some twenty years later. The revelations were recorded by various scribes during the Prophet’s life, but only as fragments, and it was left to Abū Bakr, the first caliph of Islam, to gather them together into a single text. The work was continued by Umar, the second caliph, and his daughter Hafsa, and was eventually completed by Uthmān, the third caliph. Each revelation became one of the 114 chapters of the Koran.
    The ruling caliph was responsible for carrying on the work of the Prophet, upholding his teachings and spreading his word. Between the appointment of Abū Bakr in 632 to the death of the fourth caliph, Alī, in 661, Islam spread until half of the known world was under Muslim rule. Then in 750, after a century of consolidation, the start of the Abbasid caliphate (or dynasty) heralded the golden age of Islamic civilization.