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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 cryptographer Alexander D'Agapeyeff in 1939:

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

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

Can you solve it?

see

https://en.wikipedia.org/wiki/D%27Agapeyeff_cipher

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    $\begingroup$ Hello, and welcome to Puzzling S.E. Though I am not terribly familiar with ciphers, puzzles of this sort (with a string of numbers and very few clues as to their meaning) tend to be poorly received. If you are able to add more information to give guidance to those that are working on the puzzle, you may receive a better reception. If you can be even more clever and add some scenario to the puzzle, you can create a truly remarkable puzzle. Again, please continue to contribute, and you will receive feedback to improve your puzzle-writing skills. $\endgroup$ Commented Mar 21, 2016 at 15:08
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    $\begingroup$ Seeing how the creator of the cipher cannot solve this - how will we know if we are correct? $\endgroup$ Commented Mar 21, 2016 at 15:13
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    $\begingroup$ Observations: 1. If we re-split the numbers in pairs instead of groups of 5, we can see that no pair begins with a number less than 6 and no pair ends with a number greater than 5, which would suggest a matrix arrangement, probably transposed. We could assign letter to those values. 2. However, due to the repetition pattern, there's a high chance the alphabet set is not English-based. 3. Alternate way: substract 5-columns (75628-28591). Then apply a aeiou-repetition detection to the result. 4. The dictionary may be the one in the actual book (Codes and Ciphers). $\endgroup$ Commented Jul 14, 2016 at 12:39
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    $\begingroup$ Wikipedia has an article on it here. Note that it suggests there is an error, but has a meaningful solution. "THE NEW PLAN OF ATTACK INCLUDES OPERATIONS BY THREE BOMBER SQUADRONS OVER FACTORY ARYA [AREA] SOUTHWEST OF THE RIVER" $\endgroup$ Commented Oct 26, 2016 at 18:57
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    $\begingroup$ @Angzuril: That appears to be the plaintext for the "worked example" cipher text begins with CDDBC ECBCE BBEBD ..., not for the cipher quoted above. $\endgroup$ Commented Dec 2, 2016 at 18:02

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