The D'Agapeyeff Cipher

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, the ciphertext is:  75628 28591  62916  48164  91748  58464  74748  28483  81638  18174 74826  26475  83828  49175  74658  37575  75936  36565  81638  17585 75756  46282  92857  46382  75748  38165  81848  56485  64858  56382 72628  36281  81728  16463  75828  16483  63828  58163  63630  47481 91918  46385  84656  48565  62946  26285  91859  17491  72756  46575 71658  36264  74818  28462  82649  18193  65626  48484  91838  57491 81657  27483  83858  28364  62726  26562  83759  27263  82827  27283 82858  47582  81837  28462  82837  58164  75748  58162  92000  

It was not included in later editions, and D'Agapeyeff is said to have admitted later to having forgotten how he had encrypted it.