07 - Introduction Stream Ciphers Cryptography and Protocols...

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Unformatted text preview: Introduction Stream Ciphers Cryptography and Protocols Andrei Bulatov Cryptography and Protocols – Stream Ciphers 7-2 Pseudorandom Generators Let T(n), ε (n) be functions. A collection of random variables with is called (T, ε )-pseudorandom if A collection of functions is called a (T, ε )-pseudorandom generator if is (T, ε )-pseudorandom m(n) – n > 0 the stretch of a PRG A PRG should be (T, ε )-pseudorandom for some superpolynomial pair (T, ε ) must be efficiently computable RC4 and Blum-Blum-Shub } { n X n n X } 1 , { ∈ } { } { , n T n U X ε ≈ ) ( } 1 , { } 1 , { : n m n n g → )} ( { n n U g n g Cryptography and Protocols – Stream Ciphers 7-3 Stream Ciphers Let be a pseudorandom generator producing, given a seed of length n, bit strings of length m(n) Let (K,E,D) be a SES defined as follows: K – draws keys uniformly at random E – to encrypt a plaintext P of length m(n) it applies to the key k and computes D – same as E } { n g n g i i n i P k g C ⊕ = )) ( ( Cryptography and Protocols – Stream Ciphers 7-4 Security of Stream Ciphers Theorem ....
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07 - Introduction Stream Ciphers Cryptography and Protocols...

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