Now consider the opposite problem: using 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, etc. Show that this scheme is not secure by solving the following problem. Given a two-block message B1, B2, and its hash

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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RSAH(B, B) = RSA(RSA(B1) + B2)