#11: D’Agapeyeff Cipher (1939)
Russian-born cartographer Alexander D’Agapeyeff included a cipher puzzle in his 1939 book on codes and ciphers. Unfortunately, nobody has ever solved it—even modern supercomputers have failed. Some suggest D’Agapeyeff made a mistake in the encryption, making it unsolvable. Regardless, it remains one of the most famous unsolved published codes.
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