Lec02_Classic_Ciphers - Classical Cryptosystems Example of...

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1 Classical Cryptosystems Example of some bad ciphers Jian Ren Prof. Ren 2 At the late 16 th century, Mary Queen trusted her encryption was completely secure. Therefore, she relies on this encryption system to communicate with her trusted people and planed her conspiracy. But the truth is that her encryption was not secure, so her plan was revealed …
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2 Prof. Ren 3 Classical Cryptosystems Monoalphabetic Ciphers: each English character is mapped to a unique English character. Shift Cipher (if k=3, it is called Caesar Cipher) Substitute Cipher Affine Polyalphabetic Ciphers: the English characters could be mapped to more than one English characters. Playfair Cipher Hill Cipher Vigenére Cipher Monoalphabetic Ciphers
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3 Prof. Ren 5 Shift Cipher Shift cipher is used to encrypt ordinary English text. Each alphabetic characters is mapped to an integer from 0 to 25 as follows: 25 24 23 22 3 2 1 0 Z Y X W D C B A Prof. Ren 6 Shift Cipher The encryption of shift cipher is based on integer modular arithmetic Encryption rule: e k ( x ) =x+k mod 26, 0 k 25. Decryption rule: d k ( y ) =y-k mod 26. d k ( e k ( x ) )= x mod 26. If k=3, the cipher is called Caesar cipher .
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4 Prof. Ren 7 Shift Cipher - Cryptanalysis Shift Cipher can be broken using ciphertext only attack since it has only 26 possible keys. In other words, the key can be recovered by trying at most 26 different keys. Prof. Ren 8 Substitution Cipher Substitute cipher is also used to encrypt ordinary English text. The encryption and decryption rules are all permutations of alphabetic characters. Shift Cipher is a special case of the Substitute Cipher
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5 Prof. Ren 9 Substitution Cipher - Example Example: If the encryption rule e k is: T B W Q Z G O P H A Y N X m l k j i h g f e d c b a I D J K E U M V C R L F S z y x w v u t s r q p o n Prof. Ren 10 Substitution Cipher Then the decryption rule d k is: t p w x z e h o v y r l d M L K J I H G F E D C B A i c a k s u m n q j f g b Z Y X W V U T S R Q P O N
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6 Prof. Ren 11 Substitution Cipher For this encryption rule: Plaintext: have a nice weekend Ciphertext: GXEHXSZYHKHWHSZ Prof. Ren 12 Substitution Cipher Size of key space: 26!= 403291461126605635584000000 ~ 4 x 10 26 large enough???!!!
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