The D'Agapeyeff cipher is an unsolved

cipher In cryptography, a cipher (or cypher) is an algorithm for performing encryption or decryption—a series of well-defined steps that can be followed as a procedure. An alternative, less common term is ''encipherment''. To encipher or encode i ...

that appears in the first edition of ''Codes and Ciphers'', an elementary book on cryptography published by the Russian-born English

cryptographer Cryptography, or cryptology (from "hidden, secret"; and ''graphein'', "to write", or '' -logia'', "study", respectively), is the practice and study of techniques for secure communication in the presence of adversarial behavior. More gen ...

and

cartographer Cartography (; from , 'papyrus, sheet of paper, map'; and , 'write') is the study and practice of making and using maps. Combining science, aesthetics and technique, cartography builds on the premise that reality (or an imagined reality) can ...

Alexander D'Agapeyeff in 1939. Offered as a "challenge cipher" at the end of the book, the ciphertext is:

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

Use of nulls in ciphertext

It is possible that not all the ciphertext characters are used in decryption and that some characters are nulls. Evidence for this is given by the author on p. 111 of the text under the sub-section heading ''Military Codes and Ciphers'':

"The cipher is of course easily made out, but if every third, fourth, or fifth letter, as may be previously arranged, is a dummy inserted after a message has been put into cipher, it is then extremely difficult to decipher unless you are in the secret."

While the

index of coincidence In cryptography, coincidence counting is the technique (invented by William F. Friedman) of putting two texts side-by-side and counting the number of times that identical letters appear in the same position in both texts. This count, either as a r ...

for the D'Agapeyeff cipher is 1.812 when taken in pairs horizontally (e.g., '75' '62' '82'), the

letter frequency Letter frequency is the number of times letters of the alphabet appear on average in written language. Letter frequency analysis dates back to the Arab mathematician Al-Kindi (c. AD 801–873), who formally developed the method to break ciph ...

distribution is too flat for a 196 character message written in English. Additionally, D'Agapeyeff left two ciphers for the reader to solve. Each are approximately 100 characters in length and have an index of coincidence much higher than what is expected for English plaintext.

Use of Polybius square methods in ''Codes and Ciphers''

The structure of the D'Agapeyeff Cipher has similarities to the

Polybius square The Polybius square, also known as the Polybius checkerboard, is a device invented by the ancient Greeks Cleoxenus and Democleitus, and made famous by the historian and scholar Polybius. The device is used for fractionating plaintext characters s ...

, which the author used as examples in his book. He explicitly solves an example of a Polybius square based cipher from a friend in his

cryptanalysis Cryptanalysis (from the Greek ''kryptós'', "hidden", and ''analýein'', "to analyze") refers to the process of analyzing information systems in order to understand hidden aspects of the systems. Cryptanalysis is used to breach cryptographic se ...

section of the book. This worked example consisted of 178 characters:

When deciphered with a Polybius square, the plaintext of this exercise contains a mistake (based on mis-encoding "E" as "BE" rather than "CE"), but reads:

"THE NEW PLAN OF ATTACK INCLUDES OPERATIONS BY THREE BOMBER SQUADRONS OVER FACTORY ARYA
REA REA or Rea may refer to: Places * Rea, Lombardy, in Italy * Rea, Missouri, United States * Rea River, in Fiordland, New Zealand * River Rea, a river in Birmingham, England * River Rea, Shropshire, a river in Shropshire, England * Rea, Hunga ...
SOUTHWEST OF THE RIVER"

{, class="wikitable" , - ! ! A !! B !! C !! D !! E , - ! A , S , , D , , U , , M , , I/J , - ! B , F , , W , , A , , O , , Y , - ! C , V , , N , , * , , T , , E , - ! D , L , , H , , R , , C , , Q , - ! E , B , , P , , K , , * , , *

References

* Shulman, David (nom: Ab Struse). "The D'Agapeyeff Cryptogram: A Challenge"
The Cryptogram
April/May 1952: 39-40, 46. * Shulman, David (nom: Ab Struse). "D'Agapeyeff Cipher: Postscript"
The Cryptogram
March/April 1959: 80-81. * Barker, Wayne G (1978). "The Unsolved D'Agapeyeff Cipher", Cryptologia, 2(2): 144-147

maintained by cryptographer

Elonka Dunin Elonka Dunin (; born December 29, 1958) is an American video game developer and cryptologist. Dunin worked at Simutronics Corp. in St. Louis, Missouri from 1990–2014, and in 2015 was Senior Producer at Black Gate Games in Nashville, Tennes ...

* D'Agapeyeff, Alexander. ''Codes and Ciphers'', 1939, Oxford University Press, p. 158

External links

Description of properties and potential decryption strategies for D'Agapeyeff cipher
by Robert Matthews

by

Nick Pelling Nick Pelling is a British people, British-born computer programmer and Investigative journalism, investigative writer best known as the creator of the 1984 game ''Frak!''."Desert Island Disks: Nick Pelling". ''Retro Gamer'' 26, pp. 82–85. Ga ...

Discussion of possible causes of failure to solve D'Agapeyeff cipher
at the

American Cryptogram Association The American Cryptogram Association (ACA) is an American non-profit organization devoted to the hobby of cryptography, with an emphasis on types of codes, ciphers, and cryptograms that can be solved either with pencil and paper, or with computers ...

website 1939 in literature Undeciphered historical codes and ciphers