Lec02_Classic_Ciphers

Lec02_Classic_Ciphers - Classical Cryptosystems Example of...

Info iconThis preview shows pages 1–7. Sign up to view the full content.

View Full Document Right Arrow Icon
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 …
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
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
Background image of page 2
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 ( y ) =y-k mod 26. ( ( ) )= x mod 26. If k=3, the cipher is called Caesar cipher .
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
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
Background image of page 4
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
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
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???!!! However, it can be broken using
Background image of page 6
Image of page 7
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 10/27/2009 for the course ECE 816 taught by Professor Ren during the Spring '09 term at Michigan State University.

Page1 / 17

Lec02_Classic_Ciphers - Classical Cryptosystems Example of...

This preview shows document pages 1 - 7. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online