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I'm confused, doesn't "collision attack" mean being able to create two (or more) documents with an identical hash? (As opposed to preimage attacks which involve

I'm confused, doesn't "collision attack" mean being able to create two (or more) documents with an identical hash? (As opposed to preimage attacks which involve creating documents that have a given hash). So why would that prevent the attack billpg described?

niyazpk 17y ago
You are correct. That is exactly why this attack cannot be used to forge documents (at least in a direct way)! To forge a document, you need to find another document having the same hash. So it is a preimage attack. Using a collision attack you can find collisions in the hash space, but that is not useful directly.