Ciphers
Type a message, encrypt it, then break it with frequency analysis — the technique Al-Kindi invented in 850 CE Baghdad. For nine centuries, this was the most powerful tool in cryptanalysis.
E(x) = (x + k) mod 26 · frequency analysis · Kasiski examination
In approximately 850 CE, the Arab polymath Abū Yūsuf Ya‘qūb ibn Isḥāq al-Kindī wrote A Manuscript on Deciphering Cryptographic Messages in Baghdad. In it, he described the first known technique for breaking substitution ciphers: count how often each letter appears in the ciphertext, then match those frequencies against the known frequencies of the language.
The insight is elegant: encryption hides the identity of each letter, but it cannot hide how often each letter appears. In English, E accounts for roughly 12.7% of all letters, T for 9.1%, A for 8.2%. These statistical signatures survive encryption and betray the cipher.
This technique remained the fundamental weapon in cryptanalysis for nearly nine hundred years — from the Abbasid caliphate through the Renaissance courts of Europe, until the development of polyalphabetic ciphers and, eventually, mechanical encryption in the twentieth century.