Cryptography · a Light

Frequency analysis came out of Arabic dictionary work

Counting letters was a habit of Arabic linguistics before it was a weapon. Al-Khalil ibn Ahmad, who died around 786, built the first Arabic dictionary by working through the permutations of the roots, and is credited with a book on ciphers that has not survived. Scholars of the Qur'an counted words and letters as a matter of course. So when al-Kindi wrote his treatise on deciphering messages in the ninth century, he could assume readers who already thought of a language as a distribution: take a long ordinary text, count how often each letter occurs, count the symbols in the cipher, and match them in order. The tradition ran on for centuries through Ibn Adlan and Ibn al-Durayhim into the Ottoman chancery manuals. Al-Kindi's own treatise was thought lost until a copy turned up in an Istanbul archive in 1987.

Al-Khalil to Ibn al-Durayhim · 8th to 14th c.