The Black Chamber

The monoalphabetic substitution cipher seemed uncrackable, because of the huge number of possible keys. There was, however, a shortcut that would undermine its security. This section tells the story of how this code breaking technique was invented, explains how it works and provides you with a tool that will help you to crack ciphers.

The cracking of the substitution cipher marks the birth of cryptanalysis (code breaking). This occurred during the golden age of the Islamic civilization, when many ancient foreign manuscripts were being brought to Baghdad to be added to the great Arab libraries. Some of these manuscripts were encrypted, which motivated the code breakers to crack the ciphers and reveal the secrets within. The picture shows the first page of al-Kindi's manuscript "On Deciphering Cryptographic Messages", containing the oldest known description of cryptanalysis by frequency analysis.

The letters "a" and "I" are the most common in Arabic. In English, E, then T, then A are the most common letters.

If a message is enciphered so that every letter is substituted for a different letter, then the new letter will take on all the attributes of the old letter, including how common it is.

So if the most common letter in an encoded English message is W, then W probably represents E. If there are lots of Gs, then G might represent T. And so on.

The links in the menu give more informationon on how to crack substitution ciphers, including an interactive tool that will help you to crack enciphered messages.

The letters "a" and "I" are the most common in Arabic. In English, E, then T, then A are the most common letters.

Although it is not known who first realized that the variation in the frequencies of letters could be exploited in order to break ciphers, the earliest known description of the technique is by the 9th century scientist Abu Yusuf Ya 'qub ibn Is-haq ibn as-Sabbah ibn 'omran ibn Ismail al-Kindi. Known as the philosopher of the Arabs', al-Kindi was the author of 290 books on medicine, astronomy, mathematics, linguistics and music, but his greatest treatise, which was only rediscovered in 1987 in the Sulaimaniyyah Ottoman Archive in Istanbul, is entitled "A Manuscript on Deciphering Cryptographic Messages."