Medieval cryptography
It was probably religiously motivated textual analysis of the Qur’an which led to the invention of the frequency analysis technique for breaking monoalphabetic substitution ciphers sometime around 1000 CE (Ibrahim Al-Kadi -1992). It was the most fundamental cryptanalytic advance until WWII. Essentially all ciphers remained vulnerable to this cryptanalytic technique until the invention of the polyalphabetic cipher by Alberti (ca 1465), and many remained so thereafter.
Although Alberti is usually considered the father of polyalphabetic cipher, Prof. Al-Kadi’s 1990 paper (ref- 3), reviewing Arabic contributions to cryptography reported knowledge of polyalphabetic ciphers 500 years before Alberti, based on a recently discovered manuscript).
It appears that Abu Yusuf Yaqub ibn Is-haq ibn as Sabbah ibn ‘omran ibn Ismail Al- Kindi, who wrote a book on cryptography called “Risalah fi Istikhraj al-Mu’amma” (Manuscript for the Deciphering Cryptographic Messages), circa 750 CE), may have described cryptanalysis techniques (including some for polyalphabetic ciphers), cipher classification, Arabic Phonetics and Syntax, and, most importantly, described the use of several statistical techniques for cryptanalysis. [This book seems to be the first post-classical era reference by about 300 years.] It also contains probability and statistical work some 800 years before Pascal and Fermat.
Cryptography became (secretly) still more important as a consequence of political competition and religious revolution. For instance, in Europe during and after the Renaissance, citizens of the various Italian states, the Papal States and the Roman Catholic Church included, were responsible for rapid proliferation of cryptographic techniques, few of which reflect understanding (or even knowledge) of Alberti’s polyalphabetic advance. ‘Advanced ciphers’, even after Alberti, weren’t as advanced as their inventors / developers / users claimed (and probably even themselves