CS284StreamCiphers

# CS284StreamCiphers - Stream Ciphers CSCI284-162 Spring 2009...

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CSCI284-162 Spring 2009 GWU Stream Ciphers

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05/15/09 CS284-162/Spring09/GWU/Vora/ Strea 2 Example: one-time pad P = C = Z 2 n d K =e K (x 1 , x 2 , …x n ) = (x 1 +K 1 , x 2 +K 2 , …x n +K n ) mod 2 Problems?
05/15/09 CS284-162/Spring09/GWU/Vora/ Strea 3 One-time pad is best But key too long to be practical Can we use a pseudo-random key then, which would be generated from a short truly random string?

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05/15/09 CS284-162/Spring09/GWU/Vora/ Strea 4 Stream Ciphers Message does not consist of words of symbols, but an (unending) stream of symbols. A key stream (not a single key) encrypts the stream of symbols y 1 y 2 … = e z 1 (x 1 )e z 2 (x 2 )… Synchronous : when keystream generated by a key, independent of plaintext z 1 ,z 2 ,… = g(K)
05/15/09 CS284-162/Spring09/GWU/Vora/ Strea 5 Definition: Synchronous Stream Cipher P : plaintext C : ciphertext K : keyspace (so far exactly that in block ciphers) L : keystream alphabet g: keystream generator z 1 ,z 2 ,… = g(K) where K K , and z i L e z i :  P C d z i : C P d z i (e z i (x)) = x

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CS284StreamCiphers - Stream Ciphers CSCI284-162 Spring 2009...

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