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Let E k : {0, 1} 128 {0, 1} 128 be an ideal block cipher with message space {0, 1} 128 , ciphertext space {0,
Let Ek : {0, 1} 128 {0, 1}128 be an ideal block cipher with message space {0, 1}128, ciphertext space {0, 1}128, and key space {0, 1}128. Let x and y denote bitstrings of length 128. Which of the following hash functions Hi : {0, 1}256 {0, 1}128, if any, are preimage resistant?
Explain.
(a) H1(x, y) = Ey(y) x.
(b) H2(x, y) = Ey(x).
(c) H3(x, y) = Ey(x) y.
(d) H4(x, y) = Ex(y) y.
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