Introduction to Stream Cipher Sayed Mahdi Mohammad Hasanzadeh Spring 2004.

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Presentation transcript:

Introduction to Stream Cipher Sayed Mahdi Mohammad Hasanzadeh Spring 2004

Why we need to Cryptography ?

Data transfer algorithm 1) Source  “Plain text” ABCD… 2) Source coding  … 3) Encryption 4) Channel coding 5) Data Transfer

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

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,…

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

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

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

Generator Properties Randomness Provable security Bit rate Key length Complexity of algorithm Memory Resistant against every attack