The D'Agapeyeff cipher is an as-yet unbroken cipher that appears in the first edition of 'Codes and Ciphers', an elementary book on cryptography published by the Russian-born English cryptographer and cartographer Alexander D'Agapeyeff in 1939.

Offered as a "challenge cipher" at the end of the book, it was not included in later editions, and D'Agapeyeff is said to have admitted later to having forgotten how he had encrypted it. It has been argued that the failure of all attempts at decryption is due to D'Agapeyeff incorrectly encrypting the original text.

Details of the code can be found on the Wikipedia page [1], the Cipher Mysteries page [2] and also on [3] and elsewhere.