An M4 Enigma from World War II has set a new world record price at a Christie’s auction in New York.
During the Thirty Years’ War emperor Ferdinand III (1608 -1657) used a secret writing system. It has never been solved.
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.