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Published byHoward Stanley Modified over 9 years ago
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Introduction to Stream Cipher Sayed Mahdi Mohammad Hasanzadeh Hasanzadeh@Raymandcrypto.ir Spring 2004
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Why we need to Cryptography ?
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Data transfer algorithm 1) Source “Plain text” ABCD… 2) Source coding 100101001010111… 3) Encryption 4) Channel coding 5) Data Transfer
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Why we need to Cryptography We have redundancy in every language Pr(A) Pr(B) Pr(C) … Pr(Z) Plain text redundancy leakage to Source Coding => Pr(1) Pr(0) ½ Attacker guesses some of the bites from sequence Then attacker guesses the plain text
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Goals in cryptography 1) Cryptography remove the redundancy from sequence Pr(1)=Pr(0)=1/2 2) Cryptography is a mapping in the sequence space 3)Cryptography provide security in some cases: data integrity, Authentication and identification,…
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Tools in Cryptography Symmetric encryption Hash functions Message authentication codes Random number generators Public-key encryption Digital signatures Authentication and identification Key establishment, management, and certification
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Symmetric Encryption There are basically two ways to make a symmetric cipher: Stream cipher: The encryption rule depends on the plaintext symbol’s position in the stream of plaintext symbols. e.g.: Vigenere, RC4, A5 Block cipher: Encrypt several plaintext symbols at once in a block. e.g.: DES, AES, Twofish, RC6
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Stream Cipher Pr(Ci=0) =Pr(Ci=1) =1/2 Pr(Pi=0) Pr(Pi=1) 1/2 Pr(ai=0) =Pr(ai=1) =1/2
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Generator Properties Randomness Provable security Bit rate Key length Complexity of algorithm Memory Resistant against every attack
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