The Muslim scholar al-Kindī (who lived around 850 AD) was called ‘the philosopher of the Arabs’. He was great at maths and philosophy.
He was also a code breaker. He invented a way to crack secret codes called ‘frequency analysis’. Al-Kindi worked out that each letter would occur the same number of times in normal writing as its symbol in a secret code. That meant he could crack the code just by counting how many times letters appeared in both.
The most common letter in English is the letter E, so whatever letter was most common in a simple code is likely to stand for E too.
Al-Kindi loved all kinds of subjects: he wrote about medicine and music as well as maths and philosophy. We call people who are interested in everything like this a ‘renaissance’ man or woman. Leonardo Da Vinci is the most famous, but al Kindi was a ‘renaissance man’ 500 years before Leonardo was born.
Message that matters: clever codes can be frequently broken