1© David Morgan 2011Japanese Naval Code JNJapanese Naval Code JN--2525David Morgan© David Morgan 2011A realA real--world secretworld secret--key substitution cipherkey substitution cipherJapanese Naval Code JNJapanese Naval Code JN--2525conceptual demonstrationconceptual demonstration in ciphertext a Japanese word appears as a number– a unique 5-decimal-digit number– called a “code group”[of digits] this ciphertext code group is generated asΣ two other code groups– one assigned to the word (constant hard mapping, belonging to the word)– the other corresponded to that word (variable, soft mapping)2© David Morgan 2011First code group for a wordFirst code group for a word drawn from the “codebook” amounts to a dictionary entries are fixedlike41712you64479I72084eat80514bananas95280apples39318WordCode groupEnglish demo example code book:© David Morgan 2011A codebookA codebookJapanese codebook from 1933 on display at Bletchley Park Museumhttp://www.mkheritage.co.uk/bpt/JapCDSCH1.html3© David Morgan 2011An exampleAn examplethe Zimmerman notethe Zimmerman noteA message enciphered througha word-to-number codebook“dictionary.” The numbers mapto words in German.(The note, from Germany urgingthat Mexico go to war with the US,was intercepted, decoded, and publicized.It catalyzed US entry into World War Isoon after.)© David Morgan 2011Second code group for a wordSecond code group for a word drawn from within an “additive book” of code groups at a place determined by an “indicator” in the ciphertext added to the word’s own code group by no-carry addition “poly-additive” similar to poly-alphabetic– same word could be transformed by different additives, variouslydemo example additive book:061509235522242690068421370122493567654321…4© David Morgan 2011Encoding a messageEncoding a messagelike41712you64479I72084eat80514bananas95280apples39318WordCode group061509235522242690068421370122493567654321Code book:Additive book:Given cleartext: “I eat apples”Chosen indicator: 4216630275631080923552224269006393188051472084message as code groups:additive groups, from 4th:no-carry sums; ciphertext:transmitted: 4 31080 02756 21663© David Morgan 2011Variation of indicator Variation of indicator (same message)(same message)like41712you64479I72084eat80514bananas95280apples39318WordCode group061509235522242690068421370122493567654321Code book:Additive book:Given cleartext: “I eat apples”Chosen indicator: 5354687286994226061509235522242393188051472084message as code groups:additive groups, from 5th:no-carry sums; ciphertext:transmitted: 5 94226 72869 354685© David Morgan 2011Variation of message Variation of message (same indicator)(same indicator)like41712you64479I72084eat80514bananas95280apples39318WordCode group061509235522242690068421370122493567654321Code book:Additive book:Given cleartext: “You eat bananas”Chosen indicator: 4875350275623475923552224269006952808051464479message as code groups:additive groups, from 4th:no-carry sums; ciphertext:transmitted: 4 23475 02756 87535© David Morgan 2011Decoding a messageDecoding a messagelike41712you64479I72084eat80514bananas95280apples39318WordCode group061509235522242690068421370122493567654321Code book:Additive book:Given ciphertext: “2 42106 25925 54286”952804171272084690068421370122542862592542106ciphertext as code groups:additive groups, from 2nd:diffs; cleartext codegroups:“I like bananas”indicator6© David Morgan 2011Starting with a Starting with a ciphertextciphertextcode groupcode group the cleartext code group follows– if you know the additive code group the additive code group follows– if you know the cleartext code groupciphertext = cleartext + additivethereforecleartext = ciphertext - additiveadditive = ciphertext - cleartext© David Morgan 20111 2 3 4 5 6 7Additive ? ? ? ? ? ? ?Cipher 34591 64727 54286Clear ? ? ?Text ? ? ?Cipher 56297 49510 51550Clear ? ? ?Text ? ? ?Cipher 94226 33067 35468Clear ? ? ?Text ? ? ?Cipher 42106 25925 23475Clear ? ? ?Text ? ? ?Cipher 48682 00178 17422Clear ? ? ?Text ? ? ?7© David Morgan 20111 2 3 4 5 6 7Additive ? ? 84213 ? ? ? ?Cipher 34591 64727 54286Clear ? ? ?Text ? ? ?Cipher 56297 49510 51550Clear 72084 ? ?Text I ? ?Cipher 94226 33067 35468Clear ? ? ?Text ? ? ?Cipher 42106 25925 23475Clear ? ? ?Text ? ? ?Cipher 48682 00178 17422Clear ? ? ?Text ? ? ?© David Morgan 20111 2 3 4 5 6 7Additive ? ? 84213 ? ? ? ?Cipher 34591 64727 54286Clear ? 80514 ?Text ? eat ?Cipher 56297 49510 51550Clear 72084 ? ?Text I ? ?Cipher 94226 33067 35468Clear ? ? ?Text ? ? ?Cipher 42106 25925 23475Clear ? 41712 ?Text ? ? ?Cipher 48682 00178 17422Clear 64479 ? ?Text ? ? ?8© David Morgan 2011Letting air needlessly out ofLetting air needlessly out ofthe randomness balloonthe randomness balloon using a garble check (multiple of 3) replacing one book but not the other choosing certain, conveniently accessible indexes into additive book (e.g., ones that fall at the beginning of a printed page) with high frequency communicating independently known information (ship movements, weather activity) transmitting regularly (similar message from same station same time every day)© David Morgan 2011Info sources Info sources ––JNJN--2525 The Emporer's Codes, Breaking Japan's Secret Ciphers, Michael Smith, 2000, Arcade Publishing Double-Edged Secrets: U.S. Naval Intelligence Operations in the Pacific During World War II, W.J. Holmes Japanese JN-25 naval codehttp://www.vectorsite.net/ttcode_07.html#m1 “The Flaw in the JN25 Series of