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throwaway2048

because you can create two hashes that match, but you cannot create a hash that matches an arbitrary hash you do not control.

That is a much more serious weakness called a "second pre-image attack"

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So, as I understand, that's expected to happen in foreseeable future. Otherwise, why switch from SHA1 if you can't create a collision with unaltered data?

It is not expected to happen in the foreseeable future, MD5 for instance hasn't broken in a second pre-image way, more than a decade after it was known to be weak.

This class of attacks is MUCH harder to construct against a cryptographic hash.

Then, why did BitTorrent work on such a costly change if it's not vulnerable against it?

For the purpose of operating a bait and switch on the files, the torrent creators controls the two hashes somewhat, so it's an easier to pull attack.

If you have control over the bait, I don't understand the reason to switch at all. Just make the bait evil and be done with it?

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