# Lecture3 - Technology ITEC443 LECTUREIII...

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LECTURE - III Eastern Mediterranean University School of Computing and  Technology ITEC 443 Cryptography and Network Security

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Substitution Cipher (Review) Replaces each character of a plaintext message with  something else. All that is needed a table of plaintext characters and  their associated substitute characters.   Or you can create a simple algorithm to do this job.  (For ex. Ceasar Cipher. ROT-3 Dr. E. Iscioglu                                                Cryptography and Network Security 2
Substitution Cipher  (Review). .. Or; Simple Substitution Cipher; You can use sentence or word; Start from first letter of that sentence; For example; if the sentence is;   security. ...... S       A E       B C       C U       D R       E I       F T       G Y        H .... Dr. E. Iscioglu                                                Cryptography and Network Security 3 Be careful no repetition. For ex; If you say S       A. You have to do all S      A.

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Playfair Cipher (Review) Ex. 3: Keyword: FAMAGUSTA Encrypt: COME HERE PLEASE Dr. E. Iscioglu                                                Cryptography and Network Security 4 F A M G U S T B C D E H I K L N O P Q R V W X Y Z
Polyalphabetic Ciphers Use more than  one substitution  alphabet  Makes cryptanalysis harder since have more alphabets to guess  and flattens frequency distribution  same plaintext letter gets replaced by several  ciphertext letter, depending on which alphabet  is used use a key to select which alphabet is used for each  letter of the message  Best example is; Vigenère Cipher. Dr. E. Iscioglu                                                Cryptography and Network Security 5

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Vigenère Cipher Proposed by GiovanBatista Belaso(1553) and reinvented by Blaisede  Vigenère (1586), called “ le chiffreindéchiffrable ( the unbreakable  cipher) ”for 300 years Basically multiple Caesar ciphers  Key is multiple letters long (k is a word)  K = k 1  k 2  ... k d   i th  letter specifies  i th  alphabet to use  use each alphabet in turn, repeating from start after d letters in message  Encryption Encryption Read one letter  from the plaintext and one letter  from the key is encrypted according to the Caesar cipher with key  k When the key word is finished, start the reading of the key from the  beginning Decryption works in reverse
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Lecture3 - Technology ITEC443 LECTUREIII...

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