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Keeping your bits private!

Keeping your bits private!. Science of cryptography = Art of keeping secrets. Tony C Smith Dept. Computer Science University of Waikato. terminology. plaintext (the message). terminology. plaintext. sender (cryptographer). receiver. terminology. plaintext. sender. receiver.

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Keeping your bits private!

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  1. Keeping your bits private! Science of cryptography = Art of keeping secrets Tony C Smith Dept. Computer Science University of Waikato

  2. terminology plaintext (the message)

  3. terminology plaintext sender (cryptographer)

  4. receiver terminology plaintext sender

  5. receiver terminology plaintext ciphertext encrypt (transmission) sender

  6. receiver terminology plaintext plaintext ciphertext encrypt decrypt sender

  7. terminology Transmission medium is insecure!

  8. receiver terminology cryptanalyst plaintext plaintext encrypt decrypt sender

  9. encryption process:transform plaintext into ciphertext goal:deciphering the ciphertext is as hard as just guessing the plaintext message.

  10. plaintext and ciphertext are in 27 letter English: 26 letters of the alphabet, plus space space is 0 A is 1 B is 2 C is 3 … Z is 26 also, letters can be treated as numbers:

  11. because we can treat letters as numbers, we can do arithmetic with them: B + C = E 2 + 3 = 5 if the sum of two numbers goes beyond 26 then we wrap around back to the beginning of the alphabet W + H = ? 23 + 8 = 31 Z + E = ? 26 + 5 = 31 … same as ... five letters beyond the end = fifth from start … so, W + H = E

  12. receiver cryptanalyst plaintext plaintext encrypt decrypt sender

  13. knows the encryption scheme plaintext plaintext encrypt decrypt key key guessing the key is as hard as guessing the message

  14. Caesar Cipher Used by Julius Caesar to communicate with his army. Thought to be the first general use of encryption, 75 B.C.. replace each letter with the k-th letter after it in the alphabet

  15. space + 3 = C A + 3 = D B + 3 = E … W + 3 = Z X + 3 = space Y + 3 = A Z + 3 = B Caesar Cipher E.g. k = 3 ATTACK AT DAWN … becomes ... DWWDFNCDWCGDZQ ciphertext:

  16. knows the encryption scheme plaintext plaintext encrypt decrypt key key guessing the key is as hard as guessing the message

  17. Caesar Cipher Easily attacked! Simply try every possible value for k … from 1 to 26

  18. Caesar Cipher Easily attacked! Simply try every possible value for k … from 1 to 26 ciphertext = DWWDFNCDWCGDZQ

  19. Caesar Cipher Easily attacked! Simply try every possible value for k … from 1 to 26 DWWDFNCDWCGDZQ k = 1 ? CVVCEMBCVBFCYP

  20. Caesar Cipher Easily attacked! Simply try every possible value for k … from 1 to 26 DWWDFNCDWCGDZQ k = 1 ? CVVCEMBCVBFCYP k = 2 ? BUUBDLABUAEBXO

  21. Caesar Cipher Easily attacked! Simply try every possible value for k … from 1 to 26 DWWDFNCDWCGDZQ k = 1 ? CVVCEMBCVBFCYP k = 2 ? BUUBDLABUAEBXO k = 3 ? ATTACK AT DAWN

  22. Keyed Substitution A more general substitution scheme.

  23. Keyed Substitution A more general substitution scheme. Replaces each letter with another.

  24. Keyed Substitution A more general substitution scheme. Replaces each letter with another. Any letter can stand in for any other letter.

  25. Keyed Substitution A more general substitution scheme. Replaces each letter with another. Any letter can stand in for any other letter. Each letter in ciphertext resolves to a unique letter in plaintext. plaintext: _ABCDEFGHIJKLMNOPQRSTUVWXYZ

  26. Keyed Substitution A more general substitution scheme. Replaces each letter with another. Any letter can stand in for any other letter. Each letter in ciphertext resolves to a unique letter in plaintext. plaintext: _ABCDEFGHIJKLMNOPQRSTUVWXYZ substitution: SJKEIOAHWDPMGRNVT_QLZBFYCXU

  27. Keyed Substitution A random substitution is hard to remember or validate. Solution? Use a key phrase to set the substitution alphabet. Write down each unique letter of the key phrase in order, then append the rest of the unused letters of the alphabet. E.g. key phrase = THE QUICK BROWN FOX JUMPED plaintext: _ABCDEFGHIJKLMNOPQRSTUVWXYZ substitution: THE_QUICKBROWNFXJMPDAGLSVYZ

  28. Keyed Substitution Security? Any letter can stand for any other letter. Thus there are 27! possible different substitutions schema. 27 * 26 * 25 * … * 3 * 2 = 1029 At one permutation tested per second, it would take a million, million, million centuries to test them all. Seems pretty secure …….. but ...

  29. ciphertext: WSCZSDZHAKVSHQZAKZKE_ZDXXKZVHZ KEXZBSHCQZSDZGSLXZOXZTXXKZEXAJ ZSPJZLSVYXQZOXZXHKJXAKZWSCZCXD XHCZSPJZDJXXZGAHCZ

  30. ciphertext: WSCZSDZHAKVSHQZAKZKE_ZDXXKZVHZ KEXZBSHCQZSDZGSLXZOXZTXXKZEXAJ ZSPJZLSVYXQZOXZXHKJXAKZWSCZCXD XHCZSPJZDJXXZGAHCZ letter frequency: Z 23

  31. ciphertext: WSCZSDZHAKVSHQZAKZKE_ZDXXKZVHZ KEXZBSHCQZSDZGSLXZOXZTXXKZEXAJ ZSPJZLSVYXQZOXZXHKJXAKZWSCZCXD XHCZSPJZDJXXZGAHCZ letter frequency: probability in English: Z 23 space .20

  32. ciphertext: WSCSDHAKVSHQAKKE_DXXKVH KEXBSHCQSDGSLXOXTXXKEXAJ SPJLSVYXQOXXHKJXAKWSCCXD XHCSPJDJXXGAHC letter frequency: probability in English: Z 23 space .20

  33. ciphertext: WSC SD HAKVSHQ AK KE_ DXXK VH KEX BSHCQ SD GSLX OX TXXK EXAJ SPJ LSVYXQ OXXHKJXAK WSC CXD XHC SPJ DJXX GAHC letter frequency: probability in English: Z 23 X 16 space .20

  34. ciphertext: WSC SD HAKVSHQ AK KE_ DXXK VH KEX BSHCQ SD GSLX OX TXXK EXAJ SPJ LSVYXQ OXXHKJXAK WSC CXD XHC SPJ DJXX GAHC letter frequency: probability in English: Z 23 X 16 space .20 e .15

  35. ciphertext: WSC SD HAKVSHQ AK KE_ DeeK VH KEe BSHCQ SD GSLe Oe TeeK EeAJ SPJ LSVYeQ OeeHKJeAK WSC CeD eHC SPJ DJee GAHC letter frequency: probability in English: Z 23 X 16 space .20 e .15

  36. ciphertext: WSC SD HAKVSHQ AK KE_ DeeK VH KEe BSHCQ SD GSLe Oe TeeK EeAJ SPJ LSVYeQ Oe eHKJeAK WSC CeD eHC SPJ DJee GAHC letter frequency: probability in English: Z 23 X 16 S 10 space .20 e .15

  37. ciphertext: WSC SD HAKVSHQ AK KE_ DeeK VH KEe BSHCQ SD GSLe Oe TeeK EeAJ SPJ LSVYeQ Oe eHKJeAK WSC CeD eHC SPJ DJee GAHC letter frequency: probability in English: Z 23 X 16 S 10 space .20 e .15 o .09

  38. ciphertext: WoC oD HAKVoHQ AK KE_ DeeK VH KEe BoHCQ oD GoLe Oe TeeK EeAJ oPJ LoVYeQ Oe eHKJeAK WoC CeD eHC oPJ DJee GAHC letter frequency: probability in English: Z 23 X 16 S 10 space .20 e .15 o .09

  39. ciphertext: WoC oD HAKVoHQ AKKE_ DeeK VH KEe BoHCQ oD GoLe Oe TeeK EeAJ oPJ LoVYeQ Oe eHKJeAK WoC CeD eHC oPJ DJee GAHC letter frequency: probability in English: Z 23 X 16 S 10 K 8 space .20 e .15 o .09

  40. ciphertext: WoC oD HAKVoHQ AKKE_ DeeK VH KEe BoHCQ oD GoLe Oe TeeK EeAJ oPJ LoVYeQ Oe eHKJeAK WoC CeD eHC oPJ DJee GAHC letter frequency: probability in English: Z 23 X 16 S 10 K 8 space .20 e .15 o .09 t .08

  41. ciphertext: WoC oD HAtVoHQ AttE_ Deet VH tEe BoHCQ oD GoLe Oe Teet EeAJ oPJ LoVYeQ Oe eHtJeAt WoC CeD eHC oPJ DJee GAHC letter frequency: probability in English: Z 23 X 16 S 10 K 8 space .20 e .15 o .09 t .08

  42. ciphertext: WoC oD HAtVoHQ At tE_ Deet VH tEe BoHCQ oD GoLe Oe Teet EeAJ oPJ LoVYeQ Oe eHtJeAt WoC CeD eHC oPJ DJee GAHC letter frequency: probability in English: Z 23 X 16 S 10 K 8 H 7 space .20 e .15 o .09 t .08

  43. ciphertext: WoC oD nAtVonQ At tE_ Deet Vn tEe BonCQ oD GoLe Oe Teet EeAJ oPJ LoVYeQ Oe entJeAt WoC CeD enC oPJ DJee GAnC letter frequency: probability in English: Z 23 X 16 S 10 K 8 H 7 space .20 e .15 o .09 t .08 n .07

  44. ciphertext: WoC oD nAtVonQ At tE_ Deet Vn tEe BonCQ oD GoLe Oe Teet EeAJ oPJ LoVYeQ Oe entJeAt WoC CeD enC oPJ DJee GAnC letter frequency: probability in English: Z 23 X 16 S 10 K 8 H 7 C 6 space .20 e .15 o .09 t .08 n .07

  45. ciphertext: Wod oD nAtVonQ At tE_ Deet Vn tEe BondQ oD GoLe Oe Teet EeAJ oPJ LoVYeQ Oe entJeAt Wod deD end oPJ DJee GAnd letter frequency: probability in English: Z 23 X 16 S 10 K 8 H 7 C 6 space .20 e .15 o .09 t .08 n .07 d .06

  46. ciphertext: Wod oD nAtVonQ At tE_ Deet Vn tEe BondQ oD GoLe Oe Teet EeAJ oPJ LoVYeQ Oe entJeAt Wod deD end oPJ DJee GAnd

  47. ciphertext: Wod oD nAtVonQ At tE_ Deet Vn tEe BondQ oD GoLe Oe Teet EeAJ oPJ LoVYeQ Oe entJeAt Wod deDend oPJ DJee GAnd

  48. ciphertext: Wod oD nAtVonQ At tE_ Deet Vn tEe BondQ oD GoLe Oe Teet EeAJ oPJ LoVYeQ Oe entJeAt Wod deDend oPJ DJee GAnd

  49. ciphertext: Wod of nAtVonQ At tE_ feet Vn tEe BondQ of GoLe Oe Teet EeAJ oPJ LoVYeQ Oe entJeAt Wod defend oPJ fJee GAnd

  50. ciphertext: Wod of nAtVonQ At tE_ feet Vn tEe BondQ of GoLe Oe Teet EeAJ oPJ LoVYeQ Oe entJeAt Wod defend oPJ fJee GAnd

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