TIL that the Arab polymath al-Kindi invented frequency analysis & wrote the first cryptanalysis book ‘Manuscript on Deciphering Cryptographic Messages’, in the 9th century.
If you're interested in this sort of thing you'd probably like David Kahn's The Codebreakers. It's pretty much the authoritative source of what's publicly known about the history of cryptography. I believe he had some trouble getting it published at first, actually, because he made what was considered sensitive material more accessible.
