3.8 Now consider the opposite problem: Use an encryption algorithm to construct a oneway hash function. Consider using RSA with a known key.Then process a message consisting of a sequence of blocks as follows: Encrypt the first block, XOR the result with the second block and encrypt again, and so on. Show that this scheme is not secure by solving the following problem. Given a two-block message B1, B2, and its hash, we have RSAH(B1, B2) RSA(RSA(B1) B2) Given an arbitrary block C1, choose C2 so that RSAH(C1, C2) RSAH(B1, B2). Thus, the hash function does not satisfy weak collision resistance. | |
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