# 02 - Classical Cryptosystems Introduction Cryptography and...

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Introduction Classical Cryptosystems Cryptography and Protocols Andrei Bulatov

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Cryptography and Protocols - Classical Cryptosystems 2-2 Notation Alice Bob message Eve Protocol: (K, E, D) K – key generation algorithm E – encryption algorithm D – decryption algorithm Plaintext Ciphertext Key
Cryptography and Protocols - Classical Cryptosystems 1-3 Three Types of Cryptosystems Steganography `Security by obscurity’ Transposition cryptosystems: E permutes (transposes) the letters of plaintext D applies the converse transposition Example: Spartans Scytale

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1-4 Three Types of Cryptosystems (cntd) Substitution cryptosystems E substitutes each letter of the plaintext with another letter or symbol D applies the converse substitution Example: Caesar cipher He made messages secret by shifting each letter three letters forward. Thus we can replace letters by integers from 0 to 25. For Next, Then E adds 3 modulo 25 to every letter. To decrypt a message,
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## This note was uploaded on 03/05/2012 for the course CMPT 404 taught by Professor Andreia.bulatov during the Spring '12 term at Simon Fraser.

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02 - Classical Cryptosystems Introduction Cryptography and...

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