In WW2 German spy Wulf Schmidt used a simple encryption method based on a crossword puzzle.
In the British National Archives I found a small codebook from World War I. Apparently, it contains a German code that was reverse-engineered by British codebreakers. Many details about the origin of this codebook are unclear.
The Handycipher is an encryption algorithm that doesn’t require a computer program or a machine. But is it secure?