Summit (12–14): crack a keyword cipher with Kasiski’s trick →
Where this idea comes from
Counting letters (frequency analysis)
The scholar al-Kindi, working in Baghdad in the 800s, wrote the oldest known description of breaking a code by counting how often each letter turns up, in a book now called A Manuscript on Deciphering Cryptographic Messages.
One thing to hold: A tally: one mark for every time a letter appears.
Why it matters: A code that swaps letters hides each letter but not how often it is used. In English, E is the commonest, so the commonest letter in a long coded message is a good first guess for E.
What we don’t know: His book was lost for a long time; a copy was found in 1987 in the Süleymaniye Library in Istanbul. Exactly when in the 800s he wrote it is not known.