Cryptanalysis

The so-called Vigenère cipher was thought for several centuries to be unbreakable. The idea of looking for repeats was introduced by Friedrich Kasiski in 1863. A statistical approach based on the fact that frequencies tend to be closer to uniform for longer keys was introduced by William Friedman in the 1920s. The methods described in the main text are fairly characteristic of the mixture between generality and detail that is typical in practical cryptanalysis.