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Understanding Cryptography: From Caesar to RSA Nicola Chiapolini UZH Inverted CERN School of Computing, 3-4 March 2011 Understanding Cryptography Outline 1 Basic Substitution & Attacks 2 More Complex Methods 3 Something Completely

  1. Understanding Cryptography Multiple Cipher Equivalents - Attack GMAZD VACKC ARHPG IYTZW RYKHY OTDEK GWDCS EATJU HEOKA MAHAI GETGR YKSEH ADRKZ AHYCU JXAKH YAKJU HKYGV DTHYU TCGZM DCKYE MYAWA HAKAC RYHYR ZAKPG ARHDT GCKJU HKPDW UHAYB GMDTA ARWAM AWUEZ KYUCG PACUH XYCKD BBYTH CKYTG XYMDG MAZBT YXPAX CDZBE RWBTY XECZE TFGEO ATOZD ECPGO ERARB ZUDRO G ERWH PDXYT G PDGV DTOAC GCPAX CDZBA RKPAC WUHAK PGXYT DIACG ZAPDW ATGOH CPACD BBYTK C ERWH PGXYT D CUOO GCCBU ZKPDC GDBBY THCET GKPDX YTGPD IAZZS TYJEJ ZAAXS TYMGE RWDVE ZHPAC YIRXA RW ERW KPGXY TD OYX SZGHD ZAWYG CPDES SGETK YBUZB AZHPD IAZZY BPACO TGEKY T 11 iCSC2011, Nicola Chiapolini, UZH

  2. Understanding Cryptography Summary of the Basics Ideas 1 shift alphabet 2 randomise alphabet 3 use multiple cipher equivalents Attack Methods brute force plaintext frequencies (letters, pairs, trigrams) repeated segments 12 iCSC2011, Nicola Chiapolini, UZH

  3. Understanding Cryptography Summary of the Basics Ideas 1 shift alphabet 2 randomise alphabet 3 use multiple cipher equivalents Attack Methods brute force plaintext frequencies (letters, pairs, trigrams) repeated segments 12 iCSC2011, Nicola Chiapolini, UZH

  4. Understanding Cryptography Summary of the Basics Ideas 1 shift alphabet 2 randomise alphabet 3 use multiple cipher equivalents Attack Methods brute force plaintext frequencies (letters, pairs, trigrams) repeated segments 12 iCSC2011, Nicola Chiapolini, UZH

  5. Understanding Cryptography Vigenere Cipher - Algorithm Idea use more than one full cipher alphabet. plain: ABCDEFGHIJKLMNOPQRSTUVWXYZ cipher C: C DEFGHIJKLMNOPQRSTUVWXYZAB cipher E: E FGHIJKLMNOPQRSTUVWXYZABCD cipher R: R STUVWXYZABCDEFGHIJKLMNOPQ cipher N: N OPQRSTUVWXYZABCDEFGHIJKLM plaintext: WETRY THISS IMPLE MESSA GEHER E key: CERNC ERNCE RNCER NCERN CERNC E ciphertext: YIKEA XYVUW ZZRPV ZGWJN IIYRT I 13 iCSC2011, Nicola Chiapolini, UZH

  6. Understanding Cryptography Vigenere Cipher - Algorithm Idea use more than one full cipher alphabet. plain: ABCDEFGHIJKLMNOPQRSTUV W XYZ cipher C: CDEFGHIJKLMNOPQRSTUVWX Y ZAB cipher E: EFGHIJKLMNOPQRSTUVWXYZABCD cipher R: RSTUVWXYZABCDEFGHIJKLMNOPQ cipher N: NOPQRSTUVWXYZABCDEFGHIJKLM plaintext: W ETRY THISS IMPLE MESSA GEHER E key: CERNC ERNCE RNCER NCERN CERNC E ciphertext: Y IKEA XYVUW ZZRPV ZGWJN IIYRT I 13 iCSC2011, Nicola Chiapolini, UZH

  7. Understanding Cryptography Vigenere Cipher - Algorithm Idea use more than one full cipher alphabet. plain: ABCD E FGHIJKLMNOPQRSTUVWXYZ cipher C: CDEFGHIJKLMNOPQRSTUVWXYZAB cipher E: EFGH I JKLMNOPQRSTUVWXYZABCD cipher R: RSTUVWXYZABCDEFGHIJKLMNOPQ cipher N: NOPQRSTUVWXYZABCDEFGHIJKLM plaintext: W E TRY THISS IMPLE MESSA GEHER E key: CERNC ERNCE RNCER NCERN CERNC E ciphertext: Y I KEA XYVUW ZZRPV ZGWJN IIYRT I 13 iCSC2011, Nicola Chiapolini, UZH

  8. Understanding Cryptography Vigenere Cipher - Algorithm Idea use more than one full cipher alphabet. plain: ABCDEFGHIJKLMNOPQ R STUVWXYZ cipher C: CDEFGHIJKLMNOPQRSTUVWXYZAB cipher E: EFGHIJKLMNOPQRSTUVWXYZABCD cipher R: RSTUVWXYZABCDEFGHIJ K LMNOPQ cipher N: NOPQRSTUVWXYZABCDEFGHIJKLM plaintext: WE T RY THISS IMPLE MESSA GEHER E key: CERNC ERNCE RNCER NCERN CERNC E ciphertext: YI K EA XYVUW ZZRPV ZGWJN IIYRT I 13 iCSC2011, Nicola Chiapolini, UZH

  9. Understanding Cryptography Vigenere Cipher - Algorithm Idea use more than one full cipher alphabet. plain: ABCDEFGHIJKLMNOPQRS T UVWXYZ cipher C: CDEFGHIJKLMNOPQRSTUVWXYZAB cipher E: EFGHIJKLMNOPQRSTUVWXYZABCD cipher R: RSTUVWXYZABCDEFGHIJKLMNOPQ cipher N: NOPQRSTUVWXYZABCD E FGHIJKLM plaintext: WET R Y THISS IMPLE MESSA GEHER E key: CERNC ERNCE RNCER NCERN CERNC E ciphertext: YIK E A XYVUW ZZRPV ZGWJN IIYRT I 13 iCSC2011, Nicola Chiapolini, UZH

  10. Understanding Cryptography Vigenere Cipher - Algorithm Idea use more than one full cipher alphabet. plain: ABCDEFGHIJKLMNOPQRSTUVWX Y Z cipher C: CDEFGHIJKLMNOPQRSTUVWXYZ A B cipher E: EFGHIJKLMNOPQRSTUVWXYZABCD cipher R: RSTUVWXYZABCDEFGHIJKLMNOPQ cipher N: NOPQRSTUVWXYZABCDEFGHIJKLM plaintext: WETR Y THISS IMPLE MESSA GEHER E key: CERNC ERNCE RNCER NCERN CERNC E ciphertext: YIKE A XYVUW ZZRPV ZGWJN IIYRT I 13 iCSC2011, Nicola Chiapolini, UZH

  11. Understanding Cryptography Vigenere Cipher - Algorithm Idea use more than one full cipher alphabet. plain: ABCDEFGHIJKLMNOPQRSTUVWXYZ cipher C: CDEFGHIJKLMNOPQRSTUVWXYZAB cipher E: EFGHIJKLMNOPQRSTUVWXYZABCD cipher R: RSTUVWXYZABCDEFGHIJKLMNOPQ cipher N: NOPQRSTUVWXYZABCDEFGHIJKLM plaintext: WETRY THISS IMPLE MESSA GEHER E key: CERNC ERNCE RNCER NCERN CERNC E ciphertext: YIKEA XYVUW ZZRPV ZGWJN IIYRT I 13 iCSC2011, Nicola Chiapolini, UZH

  12. Understanding Cryptography Vigenere Cipher - Attack 1 determine key-length n 2 split into n columns 3 use letter frequency for each column separately Coincidence Counting ciphertext: YIKEA XYVUW ZZRPV ZGWJN IIYRT I shift 2: YIK EAXYV UWZZR PVZGW JNIIY RT I shift 4: Y IKEAX YVUWZ ZRPVZ GWJNI IYRT I key-length: 4 14 iCSC2011, Nicola Chiapolini, UZH

  13. Understanding Cryptography Vigenere Cipher - Attack 1 determine key-length n 2 split into n columns 3 use letter frequency for each column separately Coincidence Counting ciphertext: YIKEA XYVUW ZZRPV ZGWJN IIYRT I shift 2: YIK EAXYV UWZZR PVZGW JNIIY RT I shift 4: Y IKEAX YVUWZ ZRPVZ GWJNI IYRT I key-length: 4 14 iCSC2011, Nicola Chiapolini, UZH

  14. Understanding Cryptography Vigenere Cipher - Attack 1 determine key-length n 2 split into n columns 3 use letter frequency for each column separately Coincidence Counting ciphertext: YIKEA XYVUW ZZRPV Z GWJN IIYRT I shift 2: YIK EAXYV UWZZR PVZGW JNIIY RT I shift 4: Y IKEAX YVUWZ Z RPVZ GWJNI I YRT I key-length: 4 14 iCSC2011, Nicola Chiapolini, UZH

  15. Understanding Cryptography CSC 2010: HTML riddle Code 1 Take password and create hash 2 check hash against given value 3 if check passed, decipher secret message by XOR with password letters 15 iCSC2011, Nicola Chiapolini, UZH

  16. Understanding Cryptography CSC 2010: HTML riddle - Key Length 0x68 0x56 0x42 0x18 0x50 0x4B 0x52 0x18 0x47 0x5C 0x45 0x41 0x11 0x5A 0x42 0x4A 0x58 0x56 0x42 0x4B 0x11 0x49 0x52 0x4A 0x42 0x56 0x59 0x19 0x11 0x76 0x7C 0x16 0x11 0x70 0x17 0x54 0x58 0x4F 0x52 0x18 16 iCSC2011, Nicola Chiapolini, UZH

  17. Understanding Cryptography CSC 2010: HTML riddle - Key Length 0x68 0x56 0x42 0x18 0x50 0x4B 0x52 0x18 0x47 0x68 0x56 0x42 0x18 0x50 0x4B 0x52 0x18 0x5C 0x45 0x41 0x11 0x5A 0x42 0x4A 0x58 0x56 0x47 0x5C 0x45 0x41 0x11 0x5A 0x42 0x4A 0x58 0x42 0x4B 0x11 0x49 0x52 0x4A 0x42 0x56 0x59 0x56 0x42 0x4B 0x11 0x49 0x52 0x4A 0x42 0x56 0x19 0x11 0x76 0x7C 0x16 0x11 0x70 0x17 0x54 0x59 0x19 0x11 0x76 0x7C 0x16 0x11 0x70 0x17 0x58 0x4F 0x52 0x18 0x54 0x58 0x4F 0x52 0x18 16 iCSC2011, Nicola Chiapolini, UZH

  18. Understanding Cryptography CSC 2010: HTML riddle - Key Length 0x68 0x56 0x42 0x18 0x50 0x4B 0x52 0x18 0x47 0x68 0x56 0x42 0x18 0x50 0x4B 0x52 0x5C 0x45 0x41 0x11 0x5A 0x42 0x4A 0x58 0x56 0x18 0x47 0x5C 0x45 0x41 0x11 0x5A 0x42 0x4A 0x42 0x4B 0x11 0x49 0x52 0x4A 0x42 0x56 0x59 0x58 0x56 0x42 0x4B 0x11 0x49 0x52 0x4A 0x42 0x19 0x11 0x76 0x7C 0x16 0x11 0x70 0x17 0x54 0x56 0x59 0x19 0x11 0x76 0x7C 0x16 0x11 0x70 0x58 0x4F 0x52 0x18 0x17 0x54 0x58 0x4F 0x52 0x18 16 iCSC2011, Nicola Chiapolini, UZH

  19. Understanding Cryptography CSC 2010: HTML riddle - Key Length 0x68 0x56 0x42 0x18 0x50 0x4B 0x52 0x18 0x47 0x68 0x56 0x42 0x18 0x50 0x4B 0x5C 0x45 0x41 0x11 0x5A 0x42 0x4A 0x58 0x56 0x52 0x18 0x47 0x5C 0x45 0x41 0x11 0x5A 0x42 0x42 0x4B 0x11 0x49 0x52 0x4A 0x42 0x56 0x59 0x4A 0x58 0x56 0x42 0x4B 0x11 0x49 0x52 0x4A 0x19 0x11 0x76 0x7C 0x16 0x11 0x70 0x17 0x54 0x42 0x56 0x59 0x19 0x11 0x76 0x7C 0x16 0x11 0x58 0x4F 0x52 0x18 0x70 0x17 0x54 0x58 0x4F 0x52 0x18 16 iCSC2011, Nicola Chiapolini, UZH

  20. Understanding Cryptography CSC 2010: HTML riddle - Key Length 0x68 0x56 0x42 0x18 0x50 0x4B 0x52 0x18 0x47 0x68 0x56 0x42 0x18 0x50 0x5C 0x45 0x41 0x11 0x5A 0x42 0x4A 0x58 0x56 0x4B 0x52 0x18 0x47 0x5C 0x45 0x41 0x11 0x5A 0x42 0x4B 0x11 0x49 0x52 0x4A 0x42 0x56 0x59 0x42 0x4A 0x58 0x56 0x42 0x4B 0x11 0x49 0x52 0x19 0x11 0x76 0x7C 0x16 0x11 0x70 0x17 0x54 0x4A 0x42 0x56 0x59 0x19 0x11 0x76 0x7C 0x16 0x58 0x4F 0x52 0x18 0x11 0x70 0x17 0x54 0x58 0x4F 0x52 0x18 16 iCSC2011, Nicola Chiapolini, UZH

  21. Understanding Cryptography CSC 2010: HTML riddle - Columns 0x68 0x56 0x42 0x18 0x50 0x4B 0x52 0x18 0x47 0x5C 0x45 0x41 0x11 0x5A 0x42 0x4A 0x58 0x56 0x42 0x4B 0x11 0x49 0x52 0x4A 0x42 0x56 0x59 0x19 0x11 0x76 0x7C 0x16 0x11 0x70 0x17 0x54 0x58 0x4F 0x52 0x18 0x58 0x57 0x17 0x5B 0x58 0x4D 0x4E 0x18 0x7A 0x78 0x7A 0x7D 0x7F 0x6A 0x7C 0x15 0x64 0x6B 0x76 0x74 0x62 0x72 0x7E 0x61 0x1D 0x19 0x62 0x4A 0x50 0x55 0x17 0x4A 0x54 0x5E 0x5E 0x57 0x5F 0x17 0x17 0x71 0x11 0x58 0x5A 0x18 0x03 0x0C 0x17 0x41 0x54 0x58 0x45 0x4B 0x11 0x56 0x5B 0x5C 0x1F 0x19 0x7A 0x41 0x11 0x5F 0x56 0x4E 0x5E 0x4B 0x5E 0x4C 0x54 0x19 0x43 0x50 17 iCSC2011, Nicola Chiapolini, UZH

  22. Understanding Cryptography CSC 2010: HTML riddle - Columns 0x68 0x56 0x42 0x18 0x50 0x4B 0x52 0x18 0x47 0x5C 0x45 0x41 0x11 0x5A 0x42 0x4A 0x58 0x56 0x42 0x4B 0x11 0x49 0x52 0x4A 0x42 0x56 0x59 0x19 0x11 0x76 0x7C 0x16 0x11 0x70 0x17 0x54 0x58 0x4F 0x52 0x18 0x58 0x57 0x17 0x5B 0x58 0x4D 0x4E 0x18 0x7A 0x78 0x7A 0x7D 0x7F 0x6A 0x7C 0x15 0x64 0x6B 0x76 0x74 0x62 0x72 0x7E 0x61 0x1D 0x19 0x62 0x4A 0x50 0x55 0x17 0x4A 0x54 0x5E 0x5E 0x57 0x5F 0x17 0x17 0x71 0x11 0x58 0x5A 0x18 0x03 0x0C 0x17 0x41 0x54 0x58 0x45 0x4B 0x11 0x56 0x5B 0x5C 0x1F 0x19 0x7A 0x41 0x11 0x5F 0x56 0x4E 0x5E 0x4B 0x5E 0x4C 0x54 0x19 0x43 0x50 17 iCSC2011, Nicola Chiapolini, UZH

  23. Understanding Cryptography CSC 2010: HTML riddle - Key Letter 0x11 = 0001 0001 0x20 = 0010 0000 0011 0001 = 0x31 = ‘1’ http://www.physik.uzh.ch/~nchiapol/icsc 18 iCSC2011, Nicola Chiapolini, UZH

  24. Understanding Cryptography CSC 2010: HTML riddle - Key Letter 0x11 = 0001 0001 0x20 = 0010 0000 0011 0001 = 0x31 = ‘1’ http://www.physik.uzh.ch/~nchiapol/icsc 18 iCSC2011, Nicola Chiapolini, UZH

  25. Understanding Cryptography CSC 2010: HTML riddle - Key Letter 0x11 = 0001 0001 0x20 = 0010 0000 0011 0001 = 0x31 = ‘1’ http://www.physik.uzh.ch/~nchiapol/icsc 18 iCSC2011, Nicola Chiapolini, UZH

  26. Understanding Cryptography CSC 2010: HTML riddle - Key Letter 0x11 = 0001 0001 0x20 = 0010 0000 0011 0001 = 0x31 = ‘1’ http://www.physik.uzh.ch/~nchiapol/icsc 18 iCSC2011, Nicola Chiapolini, UZH

  27. Understanding Cryptography CSC 2010: HTML riddle - Key Letter 0x11 = 0001 0001 0x20 = 0010 0000 0011 0001 = 0x31 = ‘1’ http://www.physik.uzh.ch/~nchiapol/icsc 18 iCSC2011, Nicola Chiapolini, UZH

  28. Understanding Cryptography Extending Vigenere: Wheel Cipher 36 disks key: disk ordering assemble plaintext in one line read off ciphertext in any other line 19 iCSC2011, Nicola Chiapolini, UZH

  29. Understanding Cryptography Extending Vigenere: Plaintext Auto-Key plaintext: WETRY THISS IMPLE MESSA GEHER E key: CERN W ETRYT HISSI MPLEM ESSAG E ciphertext: YIKEU XAZQL PUHDM YTDWM KWZEX I Weakness The key has the properties of plaintext. Strings in key and plaintext at fixed offset. 20 iCSC2011, Nicola Chiapolini, UZH

  30. Understanding Cryptography Extending Vigenere: Plaintext Auto-Key plaintext: WETRY THISS IMPLE MESSA GEHER E key: CERN W ETRY T HISSI MPLEM ESSAG E ciphertext: YIKEU XAZQL PUHDM YTDWM KWZEX I Weakness The key has the properties of plaintext. Strings in key and plaintext at fixed offset. 20 iCSC2011, Nicola Chiapolini, UZH

  31. Understanding Cryptography Extending Vigenere: Plaintext Auto-Key plaintext: WETRY THISS IMPLE MESSA GEHER E key: CERNW ETRYT HISSI MPLEM ESSAG E ciphertext: YIKEU XAZQL PUHDM YTDWM KWZEX I Weakness The key has the properties of plaintext. Strings in key and plaintext at fixed offset. 20 iCSC2011, Nicola Chiapolini, UZH

  32. Understanding Cryptography Extending Vigenere: Plaintext Auto-Key plaintext: WETRY THISS IMPLE MESSA GEHER E key: CERNW ETRYT HISSI MPLEM ESSAG E ciphertext: YIKEU XAZQL PUHDM YTDWM KWZEX I Weakness The key has the properties of plaintext. Strings in key and plaintext at fixed offset. 20 iCSC2011, Nicola Chiapolini, UZH

  33. Understanding Cryptography Extending Vigenere: Key Auto-Key simple rule (sum of last 3 key letters) hard to do manually simple rules are probably weak mechanical devices pseudo random number generator plaintext: WETRY THISS IMPLE MESSA GEHER E key: 2473 4 41944 75689 30257 46770 4 ciphertext: YIAUC XIRWW PRVTN PEUXH KKOLR I 21 iCSC2011, Nicola Chiapolini, UZH

  34. Understanding Cryptography Extending Vigenere: Key Auto-Key simple rule (sum of last 3 key letters) hard to do manually simple rules are probably weak mechanical devices pseudo random number generator plaintext: WETRY THISS IMPLE MESSA GEHER E key: 24734 41944 75689 30257 46770 4 ciphertext: YIAUC XIRWW PRVTN PEUXH KKOLR I 21 iCSC2011, Nicola Chiapolini, UZH

  35. Understanding Cryptography Extending Vigenere: Key Auto-Key simple rule (sum of last 3 key letters) hard to do manually simple rules are probably weak mechanical devices pseudo random number generator plaintext: WETRY THISS IMPLE MESSA GEHER E key: 24734 41944 75689 30257 46770 4 ciphertext: YIAUC XIRWW PRVTN PEUXH KKOLR I 21 iCSC2011, Nicola Chiapolini, UZH

  36. Understanding Cryptography Extending Vigenere: Key Auto-Key simple rule (sum of last 3 key letters) hard to do manually simple rules are probably weak mechanical devices pseudo random number generator plaintext: WETRY THISS IMPLE MESSA GEHER E key: 24734 41944 75689 30257 46770 4 ciphertext: YIAUC XIRWW PRVTN PEUXH KKOLR I 21 iCSC2011, Nicola Chiapolini, UZH

  37. Understanding Cryptography Extending Vigenere: The Enigma The Enigma randomised alphabets generates different alphabet for each letter sequence defined by rotors configuration huge periods: 26 nRotors 22 iCSC2011, Nicola Chiapolini, UZH

  38. Understanding Cryptography A provably secure cipher The One-Time Pad generate truly random key with same length as message use key only once 23 iCSC2011, Nicola Chiapolini, UZH

  39. Understanding Cryptography Block Ciphers ciphers worked on one letter at a time could use groups instead 24 iCSC2011, Nicola Chiapolini, UZH

  40. Understanding Cryptography Playfair Playfair C E R N A encrypt pairs of letters at a time B D F G H pair in row: next to right I K L M O pair in column: next below P Q S T U V W X Y Z else: take diagonally opposed Example plaintext: WETRY THISS IMPLE MESSA GEHER E ciphertext: EDSNN YBOXX KOSIN KRQUR DNDAN R 25 iCSC2011, Nicola Chiapolini, UZH

  41. Understanding Cryptography Playfair Playfair C E R N A encrypt pairs of letters at a time B D F G H pair in row: next to right I K L M O pair in column: next below P Q S T U V W X Y Z else: take diagonally opposed Example plaintext: WETRY THISS IMPLE MESSA GEHER E ciphertext: EDSNN YBOXX KOSIN KRQUR DNDAN R 25 iCSC2011, Nicola Chiapolini, UZH

  42. Understanding Cryptography Playfair Playfair C E R N A encrypt pairs of letters at a time B D F G H pair in row: next to right I K L M O pair in column: next below P Q S T U V W X Y Z else: take diagonally opposed Example plaintext: WE TRY THISS IMPLE MESSA GEHER E ciphertext: EDSNN YBOXX KOSIN KRQUR DNDAN R 25 iCSC2011, Nicola Chiapolini, UZH

  43. Understanding Cryptography Playfair Playfair C E R N A encrypt pairs of letters at a time B D F G H pair in row: next to right I K L M O pair in column: next below P Q S T U V W X Y Z else: take diagonally opposed Example plaintext: WE TR Y THISS IMPLE MESSA GEHER E ciphertext: EDSNN YBOXX KOSIN KRQUR DNDAN R 25 iCSC2011, Nicola Chiapolini, UZH

  44. Understanding Cryptography Playfair Playfair C E R N A encrypt pairs of letters at a time B D F G H pair in row: next to right I K L M O pair in column: next below P Q S T U V W X Y Z else: take diagonally opposed Example plaintext: WETR Y T HISS IMPLE MESSA GEHER E ciphertext: EDSNN YBOXX KOSIN KRQUR DNDAN R 25 iCSC2011, Nicola Chiapolini, UZH

  45. Understanding Cryptography Playfair Playfair C E R N A encrypt pairs of letters at a time B D F G H pair in row: next to right I K L M O pair in column: next below P Q S T U V W X Y Z else: take diagonally opposed Example plaintext: WETRY THISS IMPLE MESSA GEHER E ciphertext: EDSNN YBOXX KOSIN KRQUR DNDAN R 25 iCSC2011, Nicola Chiapolini, UZH

  46. Understanding Cryptography Playfair - Attack needs a lot of cipher text C E R N A B D F G H use statistics of letter pairs I K L M O hope for plaintext segments P Q S T U algorithm has typical properties V W X Y Z e.g. ER → RN 26 iCSC2011, Nicola Chiapolini, UZH

  47. Understanding Cryptography Summary Complex Methods Ideas 1 use multiple alphabets 2 generate the key during encryption 3 encrypt groups of letters Attack Methods coincidence counting reuse of key knowing the rules 27 iCSC2011, Nicola Chiapolini, UZH

  48. Understanding Cryptography Summary Complex Methods Ideas 1 use multiple alphabets 2 generate the key during encryption 3 encrypt groups of letters Attack Methods coincidence counting reuse of key knowing the rules 27 iCSC2011, Nicola Chiapolini, UZH

  49. Understanding Cryptography Summary Complex Methods Ideas 1 use multiple alphabets 2 generate the key during encryption 3 encrypt groups of letters Attack Methods coincidence counting reuse of key knowing the rules 27 iCSC2011, Nicola Chiapolini, UZH

  50. Understanding Cryptography Summary Complex Methods Ideas 1 use multiple alphabets 2 generate the key during encryption 3 encrypt groups of letters Attack Methods coincidence counting reuse of key knowing the rules 27 iCSC2011, Nicola Chiapolini, UZH

  51. Understanding Cryptography Codes Instead of letters or groups of letters we could replace whole words. limited possibilities need large code books useful for compression 28 iCSC2011, Nicola Chiapolini, UZH

  52. Understanding Cryptography Steganography mark letters with pinhole invisible ink microdots selected bits in image file 29 iCSC2011, Nicola Chiapolini, UZH

  53. Understanding Cryptography Transposition Methods Example plaintext: WE TRY THIS SIMPLE MESSAGE HERE W E T R Y T H I S S I M P L E M E S S A G E H E R E T A O N ciphertext: WT I MG EEH M E ETTI P SHARS L SEOY SE ARN 30 iCSC2011, Nicola Chiapolini, UZH

  54. Understanding Cryptography Transposition Methods Example plaintext: WE TRY THIS SIMPLE MESSAGE HERE W E T R Y T H I S S I M P L E M E S S A G E H E R E T A O N ciphertext: WT I MG EEH M E ETTI P SHARS L SEOY SE ARN 30 iCSC2011, Nicola Chiapolini, UZH

  55. Understanding Cryptography Data Encryption Standard Block Cipher Blocks of 64 Bits Key: 64 Bits (56 used) Generating Key Stream use plaintext & key XOR Permutations Substitution 31 iCSC2011, Nicola Chiapolini, UZH

  56. Understanding Cryptography Data Encryption Standard right half 32 P+ 48 1 split block in halves subkey ⊕ 48 2 prepare subkey 3 combine subkey 48 6 and right half S1 · · · S8 4 xor result and left half 5 switch halves 4 32 6 restart at 2 (16x) P out 32 iCSC2011, Nicola Chiapolini, UZH

  57. Understanding Cryptography Data Encryption Standard right half 32 P+ 48 1 split block in halves subkey ⊕ 48 2 prepare subkey 3 combine subkey 48 6 and right half S1 · · · S8 4 xor result and left half 5 switch halves 4 32 6 restart at 2 (16x) P out 32 iCSC2011, Nicola Chiapolini, UZH

  58. Understanding Cryptography Data Encryption Standard right half 32 P+ 48 1 split block in halves subkey ⊕ 48 2 prepare subkey 3 combine subkey 48 6 and right half S1 · · · S8 4 xor result and left half 5 switch halves 4 32 6 restart at 2 (16x) P out 32 iCSC2011, Nicola Chiapolini, UZH

  59. Understanding Cryptography Data Encryption Standard Result Each bit in each block depends on every other bit of the block and all the key bits. no statistical properties remain prevents differential cryptanalysis (injecting plaintexts with minimal differences) 33 iCSC2011, Nicola Chiapolini, UZH

  60. Understanding Cryptography Public Key The Basic Idea use different keys for encryption and decryption need a problem that is hard to solve but easy with additional information Example Factorise: 7031 = 34 iCSC2011, Nicola Chiapolini, UZH

  61. Understanding Cryptography Public Key The Basic Idea use different keys for encryption and decryption need a problem that is hard to solve but easy with additional information Example Factorise: 7031 = 34 iCSC2011, Nicola Chiapolini, UZH

  62. Understanding Cryptography Public Key The Basic Idea use different keys for encryption and decryption need a problem that is hard to solve but easy with additional information Example Factorise: 7031 = 79 · 34 iCSC2011, Nicola Chiapolini, UZH

  63. Understanding Cryptography Public Key The Basic Idea use different keys for encryption and decryption need a problem that is hard to solve but easy with additional information Example Factorise: 7031 = 79 · 89 34 iCSC2011, Nicola Chiapolini, UZH

  64. Understanding Cryptography Public Key - RSA Step by Step: Generate Key p = 11 , q = 7 n = 77 1 choose two prime numbers p , q f = 60 2 calculate n = p · q e = 23 3 calculate f = ( p − 1 ) · ( q − 1 ) 4 find two numbers e , d such that d = x · f e + 1 e · d = 1 mod f x ∈ N e 5 publish e and n , keep d secret ⇒ d = 47 ( x = 18 ) 35 iCSC2011, Nicola Chiapolini, UZH

  65. Understanding Cryptography Public Key - RSA Step by Step: Generate Key p = 11 , q = 7 n = 77 1 choose two prime numbers p , q f = 60 2 calculate n = p · q e = 23 3 calculate f = ( p − 1 ) · ( q − 1 ) 4 find two numbers e , d such that d = x · f e + 1 e · d = 1 mod f x ∈ N e 5 publish e and n , keep d secret ⇒ d = 47 ( x = 18 ) 35 iCSC2011, Nicola Chiapolini, UZH

  66. Understanding Cryptography Public Key - RSA Step by Step: Generate Key p = 11 , q = 7 n = 77 1 choose two prime numbers p , q f = 60 2 calculate n = p · q e = 23 3 calculate f = ( p − 1 ) · ( q − 1 ) 4 find two numbers e , d such that d = x · f e + 1 e · d = 1 mod f x ∈ N e 5 publish e and n , keep d secret ⇒ d = 47 ( x = 18 ) 35 iCSC2011, Nicola Chiapolini, UZH

  67. Understanding Cryptography Public Key - RSA Step by Step: Generate Key p = 11 , q = 7 n = 77 1 choose two prime numbers p , q f = 60 2 calculate n = p · q e = 23 3 calculate f = ( p − 1 ) · ( q − 1 ) 4 find two numbers e , d such that d = x · f e + 1 e · d = 1 mod f x ∈ N e 5 publish e and n , keep d secret ⇒ d = 47 ( x = 18 ) 35 iCSC2011, Nicola Chiapolini, UZH

  68. Understanding Cryptography Public Key - RSA Step by Step: Generate Key p = 11 , q = 7 n = 77 1 choose two prime numbers p , q f = 60 2 calculate n = p · q e = 23 3 calculate f = ( p − 1 ) · ( q − 1 ) 4 find two numbers e , d such that d = x · f e + 1 e · d = 1 mod f x ∈ N e 5 publish e and n , keep d secret ⇒ d = 47 ( x = 18 ) 35 iCSC2011, Nicola Chiapolini, UZH

  69. Understanding Cryptography Public Key - RSA Step by Step: Generate Key p = 11 , q = 7 n = 77 1 choose two prime numbers p , q f = 60 2 calculate n = p · q e = 23 3 calculate f = ( p − 1 ) · ( q − 1 ) 4 find two numbers e , d such that d = x · f e + 1 e · d = 1 mod f x ∈ N e 5 publish e and n , keep d secret ⇒ d = 47 ( x = 18 ) 35 iCSC2011, Nicola Chiapolini, UZH

  70. Understanding Cryptography Public Key - RSA Step by Step: Generate Key p = 11 , q = 7 n = 77 1 choose two prime numbers p , q f = 60 2 calculate n = p · q e = 23 3 calculate f = ( p − 1 ) · ( q − 1 ) 4 find two numbers e , d such that d = x · f e + 1 e · d = 1 mod f x ∈ N e 5 publish e and n , keep d secret ⇒ d = 47 ( x = 18 ) 35 iCSC2011, Nicola Chiapolini, UZH

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