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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 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:
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'':
While the index of coincidence for the D'Agapeyeff cipher is 1.74 when taken in pairs horizontally (e.g., '75' '62' '82'), the letter frequency 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「D'Agapeyeff cipher」の詳細全文を読む スポンサード リンク
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