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Unformatted text preview: U.C. Berkeley — CS276: Cryptography Lecture Notes: 01/29/2002 Professors Luca Trevisan and David Wagner Scribe: Deepak Rajan Lecture Notes: 01/29/2002 1 Message Indistinguishability ⇔ Semantic Security Last class, we showed that Semantic Security ( SS ) = ⇒ Message Indistinguishability ( MI ). Next, we try to prove that Message Indistinguishability ( MI ) = ⇒ Semantic Security ( SS ). In fact, we’ll prove that ( t, ) MI = ⇒ ( t , 2 ) SS , where t = t − poly ( n ). Before we do so, we need an equivalent definition for ( t, ) MI . Definition 1 For every m , m 1 ∈ { , 1 } n , for every algorithm A that runs in time ≤ t ( n ) − O ( n ) , for every a ∈ { , 1 } ∗ ,  a  ≤ n Pr ( p k ,s k ) ∈ G ( n ) [ A ( E ( m 1 , p k ) , p k ) = a ] − Pr ( p k ,s k ) ∈ G ( n ) [ A ( E ( m , p k ) , p k ) = a ] ≤ 2 ( n ) ( ∗ ) (the distribution of outputs of A () is roughly the same given the encryption of m or m 1 .) Proposition 1 ( t, ) MI = ⇒ ( ∗ ) . Proof: Assume that ∃ A, a such that Pr ( p k ,s k ) ∈ G ( n ) [ A ( E ( m 1 , p k ) , p k ) = a ] − Pr ( p k ,s k ) ∈ G ( n ) [ A ( E ( m , p k ) , p k ) = a ] > 2 ( n ) (i.e. ( ∗ ) does not hold) Define A ( c, p ) as follows A ( c, p ) = 1 if A ( c, p ) = a 0 otherwise Now, Pr ( p k ,s k ) ∈ G ( n ) [ A ( E ( m i , p k ) , p k ) = i ] = 1 2 Pr ( p k ,s k ) ∈ G ( n ) [ A ( E ( m 1 , p k ) , p k ) = 1] + 1 2 Pr ( p k ,s k ) ∈ G ( n ) [ A ( E ( m , p k ) , p k ) = 0] = 1 2 Pr ( p k ,s k ) ∈ G ( n ) [ A ( E ( m 1 , p k ) , p k ) = a ] + 1 2 1 − Pr ( p k ,s k ) ∈ G ( n ) [ A ( E ( m , p k ) , p k ) = a ] = 1 2 + 1 2 Pr ( p k ,s k ) ∈ G ( n ) [ A ( E ( m 1 , p k ) , p k ) = a ] − Pr ( p k ,s k ) ∈ G ( n ) [ A...
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This note was uploaded on 02/04/2008 for the course CS 276 taught by Professor Trevisan during the Spring '02 term at University of California, Berkeley.
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