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Breaking Classical Ciphers

Breaking Classical Ciphers. Cunsheng Ding Mordecai Golin HKUST, Hong Kong. Agenda for Today. More on breaking monoalphabetic substitution ciphers Breaking Vigenere ciphers. Breaking Monoalphabetic Substitution Ciphers. Monoalphabetic Substitution Ciphers.

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Breaking Classical Ciphers

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  1. Breaking Classical Ciphers Cunsheng Ding Mordecai Golin HKUST, Hong Kong EMB project

  2. Agenda for Today • More on breaking monoalphabetic substitution ciphers • Breaking Vigenere ciphers EMB project

  3. Breaking Monoalphabetic Substitution Ciphers EMB project

  4. Monoalphabetic Substitution Ciphers • Assign to each plaintext letter a different coding letter to ensure correct decryption. • a b c d e f g h i j k l m n o p q r s t u v w x y z • u v w c a b x y z q p o r s t e f g l m n d h i j k • Today we look into monoalphabetic substitution ciphers(plaintext) • MTCUJ HA OTTP ZSMT RTSTUOEYUVAMZW LNVLMZMNMZTS WZEYAGL (ciphertext) EMB project

  5. Frequency of Letters • Letters in order of frequency (highest to lowest): • E T A O N R I S H D L F C M U G Y P W B V K X J Q Z • The letters can be grouped further by their frequencies: • Very Common: E • Common: T • Next most common: A O N R I S • Less Common: H • Less Common Still: D L F C M U • Rare: V K X J Q Z EMB project

  6. Frequency of Digrams • Most common pairs of letters (digrams), in order of frequency: • TH HE AN RE ER IN ON AT ND ST ES EN OF TE ED OR TI HI AS TO AR OU IS IT LE NT RI SE HA AL DE EA NE RO OM IO WE VE TA TR CO ME NG MA CE RA IC NS UT US BE UN CH WA SI LA AD LI RT CA NC SO NC SO LL UR EL RS EM AC IM PR TT OT WI EC • The most common words in English are: • THE OF AND TO IN A IS THAT FOR IT BY ARE BE WAS AS HE WITH HIS EMB project

  7. Ciphertext from a Monoalphebetic Substitution Cipher • WNWHXHPBJIHGXHNWFOUBXHUX QIJCODILNDBOLWONOLJBHA, UIFMGVBW MHBPXJOBNT XUIFIABONODWDBYHWXXGGMOXY, "AIHXWJX NDXAIIY IFYYWTOQDXH TIMUIMFYYIXUIFIAT EMONVXUWMOX BNQWOZMH." NDIOXBHDBO LJIZXOOBIHGBADN IHUXDWPX WHWFTSXYXUIOTONXGO LMJXFTZIJ NDXBHNXFFXUNMWF UDWFFXHAX, VMNNIYWTNDXBJQIJC DWOWHMJAXHUTNIBN. NDXTJXNMJH NIW ZIJXONNDXT DWYQIJCXYBH, IHFTNIZBHY BNUDILLXYYIQH; NDXTDXWJIFYXJUIFFXWAMXOJXGBHBOUX WVIMNVBJYOFWON OXXHYXUWYXOWAI; NDXTQWFCIHLXJGWZJION NMJHXYGMODT VTAFIVWFQWJGBHA. • WOYXOUJBVXY BHNDBOOLXUBWFBOOMX, NDXQIJFYBOHIQ MHYXJ-AIBHA WJXGWJCWVFXOXN IZNJWHOBNBIHO: LILMFWNBIH AJIQNDDWO JXWUDXYWH BHZFXUNBIHLIBHN WHYBOONWJNBHA NIFXPXFIZZ, NDXYXPXFILBHA QIJFYBOVXUIGBHAYXPXFILXY, WHYXHPBJIHGXHNWF LJIVFXGONDWN MOXYNIVXFIUWF-BSXY WJXJWYBWNBHA XPXJTQDXJX. NDXBHNXJUIHHXUNXY UDWHAXOWJX, BHYXGIAJWLDXJ EIOXLDUDWGBX'OQIJYO, W "KMBXNJXPI-FMNBIH" NDWNJXWUDXO BHNIXPXJT UIJHXJIZFBZX. NDXTLIOXNDJXWNO VMNWFOIIZZXJ ILLIJNMHBNBXO. DWPBHAOXXH ZBJONDWHYQDWN BODWLLXHBHA, NDXWMNDIJO IZNDXWJNBUFXO DXJXDWPXNDJIQH NDXGOXFPXOBHNI NDXNWOCIZ DXFLBHAOIUBXNT HWPBAWNXNDX ODIWFOWDXWY. EMB project

  8. Our Task Today • We shall break this cipher. • We were told that the original plaintext is in English and was encrypted with a one-to-one mapping from the English alphabet to itself. • We can see that some spaces between words and punctuation were deleted, while other spaces between words and punctuation are kept. • We shall use the tools at the URL: • http://cryptoclub.math.uic.edu/indexmain.html EMB project

  9. Step 1 Compute the frequency distributions of single letters and digrams using the online software at the URL: http://cryptoclub.math.uic.edu/indexmain.html EMB project

  10. In message In English X: 13.0 e: 12.7 I: 8.3 t: 9.1 N: 8.3 a: 8.2 W: 7.5 i: 7.0 H: 7.0 n: 6.7 O: 6.9 o: 6.3 B: 6.7 h: 6.1 J: 6.0 r: 6.0 D: 5.8 d: 4.3 F: 4.4 q: 4.3 Y: 4.0 l: 4.0 U: 3.1 c: 2.8 M: 2.7 u: 2.8 In message In English A: 2.6 m: 2.4 L: 2.4 w: 2.3 T: 2.1 f: 2.2 G: 1.8 s: 2.2 Z: 1.8 g: 2.0 Q: 1.7 y: 2.0 P: 1.5 p: 1.9 V: 1.4 b: 1.5 C: 0.6 v: 1.0 E: 0.2 k: 0.8 S: 0.2 j: 0.2 K: 0.1 x: 0.1 R: 0.0 z: 0.1 Frequency of Letters: Comparison EMB project

  11. Most Common Digraphs • In message: • ND, DX, BH, DW, JX, XJ, IH, HA, XY, XH • In English: • th, he, in, er, ed, an, nd, ar, re, en EMB project

  12. Step 2 Look at specialities in the ciphertext (Since some spaces between words and some punctuation symbols are kept, the most important thing is to look for special indications) EMB project

  13. Looking for Indications in Ciphertext • Read the ciphertext carefully, and write down any piece of valuable information you obtain from reading the ciphertext. • Let me know what you have discovered. EMB project

  14. Looking for Indications in Ciphertext • In the ciphertext we have the following: • BHYXGIAJWLDXJ EIOXLDUDWGBX'OQIJYO, W "KMBXNJXPI-FMNBIH" • What can you infer from it? EMB project

  15. Indications in Ciphertext • In the ciphertext we have the following: • BHYXGIAJWLDXJ EIOXLDUDWGBX'OQIJYO, W "KMBXNJXPI-FMNBIH" • What can you infer from it? • “O  s” or “X  s” (not consistent with the frequency distribution of single letters) • “W  a” EMB project

  16. Step 3 Looking at the frequency distributions of single letters and digrams EMB project

  17. Looking at Most Common Digraphs • In message: • ND, DX, BH, DW, JX, XJ, IH, HA, XY, XH • In English: • th, he, in, er, ed, an, nd, ar, re, en • What can you tell from this statistics? EMB project

  18. Looking at Most Common Digraphs • In message: • ND, DX, BH, DW, JX, XJ, IH, HA, XY, XH • In English: • th, he, in, er, ed, an, nd, ar, re, en • What can you tell from this statistics? • It is very likely that “Dh”, “Xe”, “Nt” • This is more or less consistent with the frequency distribution of single letters. EMB project

  19. The first Guess • Combining what we have on Page 15 and Page 18, we first guess the following: • W  a • Dh • Xe • Nt • Os EMB project

  20. After the First Guess • ataHeHPBJIHGeHtaFsUBeHUe QIJCshILthBsLastsLJBHA, UIFMGVBa MHBPeJsBtT eUIFIABstshahBYHaeeGGMseY, "AIHeaJe theAIIY IFYYaTsQheH TIMUIMFYYIeUIFIAT EMstVeUaMse BtQasZMH." thIseBHhBs LJIZessBIHGBAht IHUehaPe aHaFTSeYeUIsTsteGs LMJeFTZIJ theBHteFFeUtMaF UhaFFeHAe, VMttIYaTtheBJQIJC hasaHMJAeHUTtIBt. theTJetMJH tIa ZIJesttheT haYQIJCeYBH, IHFTtIZBHY BtUhILLeYYIQH; theTheaJIFYeJUIFFeaAMesJeGBHBsUe aVIMtVBJYsFast seeHYeUaYesaAI; theTQaFCIHLeJGaZJIst tMJHeYGMshT VTAFIVaFQaJGBHA. • asYesUJBVeY BHthBssLeUBaFBssMe, theQIJFYBsHIQ MHYeJ-AIBHA aJeGaJCaVFeset IZtJaHsBtBIHs: LILMFatBIH AJIQthhas JeaUheYaH BHZFeUtBIHLIBHt aHYBsstaJtBHA tIFePeFIZZ, theYePeFILBHA QIJFYBsVeUIGBHAYePeFILeY, aHYeHPBJIHGeHtaF LJIVFeGsthat MseYtIVeFIUaF-BSeY aJeJaYBatBHA ePeJTQheJe. theBHteJUIHHeUteY UhaHAesaJe, BHYeGIAJaLheJ EIseLhUhaGBe'sQIJYs, a "KMBetJePI-FMtBIH" thatJeaUhes BHtIePeJT UIJHeJIZFBZe. theTLIsethJeats VMtaFsIIZZeJ ILLIJtMHBtBes. haPBHAseeH ZBJsthaHYQhat BshaLLeHBHA, theaMthIJs IZtheaJtBUFes heJehaPethJIQH theGseFPesBHtI thetasCIZ heFLBHAsIUBetT HaPBAatethe shIaFsaheaY. EMB project

  21. What Shell We do Now • After the first guess and first replacement, we should check if there is anything wrong in the partially decrypted text. • If YES, this means that some part in our first guess must be wrong. So modification of the first guess should be done. • If the partially decrypted text looks okay, we should try to get information on the partially decrypted text. EMB project

  22. Anything Wrong in the Text: Page 20? • If yes, tell me what is wrong? • If no, look for further information in the text of Page 20. EMB project

  23. The short phrase: “tIa” on page 20 It cannot be “tea”, because “Xe” already. Hence it should be two words, and thus “to a”. Thus “Io” Then look at the short phrase: “shoaFsaheaY” The only possibility is “shoals ahead” Hence, “Fl”, “Yd” The second guess Io Fl Yd Seeking Information on Page 20 EMB project

  24. The Second Guess • The First guess • W  a • Dh • Xe • Nt • Os • The second guess • Io • Fl • Yd EMB project

  25. After the Second Guess • ataHeHPBJoHGeHtalsUBeHUe QoJCshoLthBsLastsLJBHA, UolMGVBa MHBPeJsBtT eUoloABstshahBdHaeeGGMsed, "AoHeaJe theAood olddaTsQheH ToMUoMlddoeUoloAT EMstVeUaMse BtQasZMH." thoseBHhBs LJoZessBoHGBAht oHUehaPe aHalTSedeUosTsteGs LMJelTZoJ theBHtelleUtMal UhalleHAe, VMttodaTtheBJQoJC hasaHMJAeHUTtoBt. theTJetMJH to a ZoJesttheT hadQoJCedBH, oHlTtoZBHd BtUhoLLeddoQH; theTheaJoldeJUolleaAMesJeGBHBsUe aVoMtVBJdslast seeHdeUadesaAo; theTQalCoHLeJGaZJost tMJHedGMshT VTAloValQaJGBHA. • asdesUJBVed BHthBssLeUBalBssMe, theQoJldBsHoQ MHdeJ-AoBHA aJeGaJCaVleset oZtJaHsBtBoHs: LoLMlatBoH AJoQthhas JeaUhedaH BHZleUtBoHLoBHt aHdBsstaJtBHA tolePeloZZ, thedePeloLBHA QoJldBsVeUoGBHAdePeloLed, aHdeHPBJoHGeHtal LJoVleGsthat MsedtoVeloUal-BSed aJeJadBatBHA ePeJTQheJe. theBHteJUoHHeUted UhaHAesaJe, BHdeGoAJaLheJ EoseLhUhaGBe'sQoJds, a "KMBetJePo-lMtBoH" thatJeaUhes BHtoePeJT UoJHeJoZlBZe. theTLosethJeats VMtalsooZZeJ oLLoJtMHBtBes. haPBHAseeH ZBJsthaHdQhat BshaLLeHBHA, theaMthoJs oZtheaJtBUles heJehaPethJoQH theGselPesBHto thetasCoZ helLBHAsoUBetT HaPBAatethe shoals ahead. EMB project

  26. Anything Wrong in the Text: Page 25? • If yes, tell me what is wrong? • If no, look for further information in the text of Page 25. EMB project

  27. Any Indication on Page 25 • Look at the phrase “thetasCoZ” • It is most likely “the task of” • Hence, “Ck”, “Zf” • This is consistent with the frequency distribution of single letters. EMB project

  28. The Third Guess • The First guess • W  a • Dh • Xe • Nt • Os • The second guess • Io • Fl • Yd • The third guess • Ck • Zf EMB project

  29. After the Third Guess • ataHeHPBJoHGeHtalsUBeHUe QoJkshoLthBsLastsLJBHA, UolMGVBa MHBPeJsBtT eUoloABstshahBdHaeeGGMsed, "AoHeaJe theAood olddaTsQheH ToMUoMlddoeUoloAT EMstVeUaMse BtQasfMH." thoseBHhBs LJofessBoHGBAht oHUehaPe aHalTSedeUosTsteGs LMJelTfoJ theBHtelleUtMal UhalleHAe, VMttodaTtheBJQoJk hasaHMJAeHUTtoBt. theTJetMJH to a foJesttheT hadQoJkedBH, oHlTtofBHd BtUhoLLeddoQH; theTheaJoldeJUolleaAMesJeGBHBsUe aVoMtVBJdslast seeHdeUadesaAo; theTQalkoHLeJGafJost tMJHedGMshT VTAloValQaJGBHA. • asdesUJBVed BHthBssLeUBalBssMe, theQoJldBsHoQ MHdeJ-AoBHA aJeGaJkaVleset oftJaHsBtBoHs: LoLMlatBoH AJoQthhas JeaUhedaH BHfleUtBoHLoBHt aHdBsstaJtBHA tolePeloff, thedePeloLBHA QoJldBsVeUoGBHAdePeloLed, aHdeHPBJoHGeHtal LJoVleGsthat MsedtoVeloUal-BSed aJeJadBatBHA ePeJTQheJe. theBHteJUoHHeUted UhaHAesaJe, BHdeGoAJaLheJ EoseLhUhaGBe'sQoJds, a "KMBetJePo-lMtBoH" thatJeaUhes BHtoePeJT UoJHeJoflBfe. theTLosethJeats VMtalsooffeJ oLLoJtMHBtBes. haPBHAseeH fBJsthaHdQhat BshaLLeHBHA, theaMthoJs oftheaJtBUles heJehaPethJoQH theGselPesBHto thetaskof helLBHAsoUBetT HaPBAatethe shoals ahead. EMB project

  30. Anything Wrong in the Text: Page 29? • If yes, tell me what is wrong? • If no, look for further information in the text of Page 29. EMB project

  31. Any Indication on Page 29 • Look at the phrases “foJesttheT” and “VMttodaTtheBJQoJk” • It is very likely that “Jr”, “Ty” • This is consistent with the frequency distribution of single letters. EMB project

  32. The Fourth Guess • The First guess • W  a • Dh • Xe • Nt • Os • The second guess • Io • Fl • Yd • The third guess • Ck • Zf • The fourth guess • Jr • Ty EMB project

  33. After the Fourth Guess • ataHeHPBroHGeHtalsUBeHUe QorkshoLthBsLastsLrBHA, UolMGVBa MHBPersBty eUoloABstshahBdHaeeGGMsed, "AoHeare theAood olddaysQheH yoMUoMlddoeUoloAy EMstVeUaMse BtQasfMH." thoseBHhBs LrofessBoHGBAht oHUehaPe aHalySedeUosysteGs LMrelyfor theBHtelleUtMal UhalleHAe, VMttodaytheBrQork hasaHMrAeHUytoBt. theyretMrH to a forestthey hadQorkedBH, oHlytofBHd BtUhoLLeddoQH; theyhearolderUolleaAMesreGBHBsUe aVoMtVBrdslast seeHdeUadesaAo; theyQalkoHLerGafrost tMrHedGMshy VyAloValQarGBHA. • asdesUrBVed BHthBssLeUBalBssMe, theQorldBsHoQ MHder-AoBHA areGarkaVleset oftraHsBtBoHs: LoLMlatBoH AroQthhas reaUhedaH BHfleUtBoHLoBHt aHdBsstartBHA tolePeloff, thedePeloLBHA QorldBsVeUoGBHAdePeloLed, aHdeHPBroHGeHtal LroVleGsthat MsedtoVeloUal-BSed areradBatBHA ePeryQhere. theBHterUoHHeUted UhaHAesare, BHdeGoAraLher EoseLhUhaGBe'sQords, a "KMBetrePo-lMtBoH" thatreaUhes BHtoePery UorHeroflBfe. theyLosethreats VMtalsooffer oLLortMHBtBes. haPBHAseeH fBrsthaHdQhat BshaLLeHBHA, theaMthors oftheartBUles herehaPethroQH theGselPesBHto the task of helLBHAsoUBety HaPBAatethe shoals ahead. EMB project

  34. Anything Wrong in the Text: Page 33? • If yes, tell me what is wrong? • If no, look for further information in the text of Page 33. EMB project

  35. Any Indication from Page 33 • Look at the phrase “"AoHeare theAood olddaysQheH ” • It is very likely that “Ag” • This is consistent with the frequency distribution of single letters. • Look at the phrase “oftheartBUles” • It is very likely that “Bi” and “Uc” EMB project

  36. The Fifth Guess • The First guess • W  a • Dh • Xe • Nt • Os • The second guess • Io • Fl • Yd • The third guess • Ck • Zf • The fourth guess • Jr • Ty • The fifth guess • A g • Bi • Uc EMB project

  37. After the Fifth Guess • ataHeHPiroHGeHtalscieHce QorkshoLthisLastsLriHg, colMGVia MHiPersity ecologistshahidHaeeGGMsed, "goHeare thegood olddaysQheH yoMcoMlddoecology EMstVecaMse itQasfMH." thoseiHhis LrofessioHGight oHcehaPe aHalySedecosysteGs LMrelyfor theiHtellectMal challeHge, VMttodaytheirQork hasaHMrgeHcytoit. theyretMrH to a forestthey hadQorkediH, oHlytofiHd itchoLLeddoQH; theyhearoldercolleagMesreGiHisce aVoMtVirdslast seeHdecadesago; theyQalkoHLerGafrost tMrHedGMshy VygloValQarGiHg. • asdescriVed iHthissLecialissMe, theQorldisHoQ MHder-goiHg areGarkaVleset oftraHsitioHs: LoLMlatioH groQthhas reachedaH iHflectioHLoiHt aHdisstartiHg tolePeloff, thedePeloLiHg QorldisVecoGiHgdePeloLed, aHdeHPiroHGeHtal LroVleGsthat MsedtoVelocal-iSed areradiatiHg ePeryQhere. theiHtercoHHected chaHgesare, iHdeGograLher EoseLhchaGie'sQords, a "KMietrePo-lMtioH" thatreaches iHtoePery corHeroflife. theyLosethreats VMtalsooffer oLLortMHities. haPiHgseeH firsthaHdQhat ishaLLeHiHg, theaMthors ofthearticles herehaPethroQH theGselPesiHto the task of helLiHgsociety HaPigatethe shoals ahead. EMB project

  38. Anything Wrong in the Text: Page 37? • If yes, tell me what is wrong? • If no, look for further information in the text of Page 37. EMB project

  39. Any Indication from Page 37 • Look at the phrase “asdescriVed ” • “Vb” • Look at the phrase “challeHge, ” • “Hn” • Look at the phrase “iHthissLecialissMe” • “Lp” • “Mu” EMB project

  40. After the Sixth Replacement on Page 39 • atanenPironGentalscience Qorkshopthispastspring, coluGbia uniPersity ecologistshahidnaeeGGused, "goneare thegood olddaysQhen youcoulddoecology Eustbecause itQasfun." thoseinhis professionGight oncehaPe analySedecosysteGs purelyfor theintellectual challenge, buttodaytheirQork hasanurgencytoit. theyreturn to a forestthey hadQorkedin, onlytofind itchoppeddoQn; theyhearoldercolleaguesreGinisce aboutbirdslast seendecadesago; theyQalkonperGafrost turnedGushy byglobalQarGing. • asdescribed inthisspecialissue, theQorldisnoQ under-going areGarkableset oftransitions: population groQthhas reachedan inflectionpoint andisstarting tolePeloff, thedePeloping QorldisbecoGingdePeloped, andenPironGental probleGsthat usedtobelocal-iSed areradiating ePeryQhere. theinterconnected changesare, indeGographer EosephchaGie'sQords, a "KuietrePo-lution" thatreaches intoePery corneroflife. theyposethreats butalsooffer opportunities. haPingseen firsthandQhat ishappening, theauthors ofthearticles herehaPethroQn theGselPesinto the task of helpingsociety naPigatethe shoals ahead. EMB project

  41. Anything Wrong in the Text: Page 40? • If yes, tell me what is wrong? • If no, look for further information in the text of Page 40. EMB project

  42. Any Indication from Page 40 • Look at the phrase “Qorkshopthispastspring” • “Qw” • Look at the phrase “thedePeloping” • “Pv” • Look at the phrase “coluGbia university” • “Gm” • Finally • “Sz” • “EJ • “Kq” EMB project

  43. Then We have • atanenvironmentalscience workshopthispastspring, columbia university ecologistshahidnaeemmused, "goneare thegood olddayswhen youcoulddoecology justbecause itwasfun." thoseinhis professionmight oncehave analyzedecosystems purelyfor theintellectual challenge, buttodaytheirwork hasanurgencytoit. theyreturn to a forestthey hadworkedin, onlytofind itchoppeddown; theyhearoldercolleaguesreminisce aboutbirdslast seendecadesago; theywalkonpermafrost turnedmushy byglobalwarming. • asdescribed inthisspecialissue, theworldisnow under-going aremarkableset oftransitions: population growthhas reachedan inflectionpoint andisstarting toleveloff, thedeveloping worldisbecomingdeveloped, andenvironmental problemsthat usedtobelocal-ized areradiating everywhere. theinterconnected changesare, indemographer josephchamie'swords, a "quietrevo-lution" thatreaches intoevery corneroflife. theyposethreats butalsooffer opportunities. havingseen firsthandwhat ishappening, theauthors ofthearticles herehavethrown themselvesinto the task of helpingsociety navigatethe shoals ahead. EMB project

  44. The Decrypted Text • At an environmental science workshop this past spring, Columbia University ecologist Shahid Naeem mused, "Gone are the good old days when you could do ecology just because it was fun." Those in his profession might once have analyzed ecosystems purely for the intellectual challenge, but today their work has an urgency to it. They return to a forest they had worked in, only to find it chopped down; they hear older colleagues reminisce about birds last seen decades ago; they walk on permafrost turned mushy by global warming. As described in this special issue, the world is now under-going a remarkable set of transitions: population growth has reached an inflection point and is starting to level off, the developing world is becoming developed, and environmental problems that used to be local-ized are radiating everywhere. The interconnected changes are, in demographer Joseph Chamie's words, a "quiet revo-lution" that reaches into every corner of life. They pose threats but also offer opportunities. Having seen firsthand what is happening, the authors of the articles here have thrown themselves into the task of helping society navigate the shoals ahead. EMB project

  45. Summary of Part I • In case of no spaces between words and no punctuation in the ciphertext, it may be hard to break a substitution cipher. • It is very hard at the beginning. Special signs and short words should be investigated first. • Statistics of single letter frequency and that of digrams should then be used. • A guess could be wrong. Avoid wrong guesses. • Start and end of sentences are highest priority. • In the project assignment, we will keep some spaces between words, and some punctuation symbols, in order to reduce the difficulty. EMB project

  46. Breaking Vigenere Ciphers EMB project

  47. a b c d e f g h i j k l m n o p q r s t u v w x y z --------------------------------------------------- A B C D E F G H I J K L M N O P Q R S T U V W X Y Z B C D E F G H I J K L M N O P Q R S T U V W X Y Z A C D E F G H I J K L M N O P Q R S T U V W X Y Z A B D E F G H I J K L M N O P Q R S T U V W X Y Z A B C E F G H I J K L M N O P Q R S T U V W X Y Z A B C D F G H I J K L M N O P Q R S T U V W X Y Z A B C D E G H I J K L M N O P Q R S T U V W X Y Z A B C D E F H I J K L M N O P Q R S T U V W X Y Z A B C D E F G I J K L M N O P Q R S T U V W X Y Z A B C D E F G H J K L M N O P Q R S T U V W X Y Z A B C D E F G H I K L M N O P Q R S T U V W X Y Z A B C D E F G H I J L M N O P Q R S T U V W X Y Z A B C D E F G H I J K M N O P Q R S T U V W X Y Z A B C D E F G H I J K L N O P Q R S T U V W X Y Z A B C D E F G H I J K L M O P Q R S T U V W X Y Z A B C D E F G H I J K L M N P Q R S T U V W X Y Z A B C D E F G H I J K L M N O Q R S T U V W X Y Z A B C D E F G H I J K L M N O P R S T U V W X Y Z A B C D E F G H I J K L M N O P Q S T U V W X Y Z A B C D E F G H I J K L M N O P Q R T U V W X Y Z A B C D E F G H I J K L M N O P Q R S U V W X Y Z A B C D E F G H I J K L M N O P Q R S T V W X Y Z A B C D E F G H I J K L M N O P Q R S T U W X Y Z A B C D E F G H I J K L M N O P Q R S T U V X Y Z A B C D E F G H I J K L M N O P Q R S T U V W Y Z A B C D E F G H I J K L M N O P Q R S T U V W X Z A B C D E F G H I J K L M N O P Q R S T U V W X Y Vigenere Table EMB project

  48. Example of Encryption • Key: h o l d h o l d h o l d h o l d h o • Plaintext: t h i s i s t h e p l a i n t e x t • Ciphertext: a v t v p g e k l d w d p b e h d h • Demo for encryption using the tools in the URL • keyword: hold • keyword length: 4 • A Veginere cipher is a combination of a number of Casesar ciphers • The main task of breaking a Veginere cipher is to determine the keyword length. EMB project

  49. How to Find the Keyword Length • YBRWY JFM N QCGYFR GIL SUZJX WJMFJ. NUJ VVL VBDM VS NUJ HRNAUGIEMIBI WBSMGFHGQS GJUFJX UNG. FTGRYCZJM GMYL TZSJLRI BVR U PMIVHY OJNJJYA F HVHERQ UAI U QNGR. OYFXY NQQNDM GTIX YBR SCPPYY-FZGJL NQF, VY QNX VVLARW. NUJ VVL VBDM YFOTMYQ FHQ QUHLBRI. IAJ XND USYYE OYFXY TWUOGYQ YBR SCPPYY, MCF KUGMYE YIBP BVR UFNXR FHQ XUVI, "DRXMR, YBBXY OTSF FLR RUXNHTKOA TZ LTO. GMYL YBVSE LTO QTH'G PHBB NUJ XVRY VX QBWNU RIEJ NUFH GMY ANWXJF." WJMFJ AENHAJX NSX FFCQ, "IIA'Y QBWLL IUQ. N EATQ JMCPM CF BIEYB ZTLR. GOG NZ V YIBP NUJ XVRY, GMYL BIHQX FYIC IIVSA VY. MB KUE N'PR HIYQYPYYQ $10 IIYQUEX." EMB project

  50. How to Find the Keyword Length • If you can find strings of letters that repeat in a message, you can often figure out the key length. • The distance between repeated strings is usually a multiple of the key length. • A good guess for key length is a common factor of the distances between strings. EMB project

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