In 1939, the Russian-born Briton Alexander D'Agapeyeff published a book named "Codes and Ciphers". This book contains a ciphertext that was meant as an exercise. So far, nobody has been able to decrypt this ciphertext. In later editions of his book, this exercise was omitted – possibly because he had forgotten the solution himself. D'Agapeyeff may have made a mistake during the encryption procedure, which makes it now difficult to find the cleartext.