11.4 a. Consider the Davies and Price hash code scheme described in Section 11.4 and assume that DES is used as the encryption algorithm: Hi = Hi1+ E(Mi, Hi1) and recall the complementarity property of DES (Problem 3.14): If Y = E(K, X), then Y" = E(K", X"). Use this property to show how a message consisting of blocks M1, M2,..., MN can be altered without altering its hash code. b. Show that a similar attack will succeed against the scheme proposed inM [ EYE88]: Hi = Mi+ E(Hi1, Mi) | |
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