Cryptography · a Light

Codebreaking was invented by counting letters in Arabic

Al-Kindi, the philosopher of Baghdad who died around 873, wrote a short treatise on deciphering encrypted messages. In it he explains that if you know the language a message is written in, you take a long ordinary text in that language, count how often each letter appears, then count the symbols in the cipher and match the most frequent to the most frequent. That is frequency analysis, and it broke every simple substitution cipher for the next seven hundred years. The manuscript was lost for centuries and was rediscovered in 1987 in the Ottoman archive in Istanbul. Scholars of the Qur'an had already been counting letters and word frequencies, which is where the habit came from.

Al-Kindi · Baghdad, died c. 873 CE