One way to improve the strength of a signature is to sign with salt, i.e. sign random number + document instead of document alone (you can put the random number into the hash key accumulator as starting point). This basically removes the possibility to create a pair of documents that will result with the same hash in advance, because the random number of the signer is still unknown (unless of course, the hash has a vulnerability, where a known sequence of bytes removes the history in the accumulator). This is a remedy that can be implemented right now, even with SHA-1. Several of the SHA-3 proposals recommend something in that direction, though e.g. Bruce Schneier recommends to start with your public key as salt - this is less useful, since the public key is known to the attacker. Though a document with several signers makes it a lot more difficult for him.
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