Abu Yusuf Yaqub ibn Ishaq al-Kindi (Basra, c.801 – Baghdad, c.873), philosopher and polymath of Baghdad's House of Wisdom, writes c.850 the Risala fi Istikhraj al-Mu'amma (Manuscript on Deciphering Cryptographic Messages), the first systematic cryptanalysis treatise in history. Its central method is frequency analysis: in any language, letters appear with characteristic, predictable frequencies; by comparing the frequency of symbols in the ciphered text with the known statistical distribution of the original language, it is possible to deduce the substitution without knowing the key. This method breaks any monoalphabetic substitution cipher, including the Caesar cipher. The original manuscript was rediscovered in the Istanbul State Archive in 1987 by researcher Ibrahim al-Kadi. Western historiography of cryptography had attributed frequency analysis to 16th-century European cryptographers without mentioning Al-Kindi, even though his work precedes them by more than six centuries.