Here's the scenario: We want to craft two different messages with the same MD5 hash, and a specific CRC32 checksum, simultaneously.
In other words, we want an MD5 collision attack and a CRC32 preimage attack.
This might seem like a contrived scenario, but it's exactly the one I faced while producing my PNG hashquine (Yes OK maybe that's also a contrived scenario, cut me some slack).
On its own, a CRC32 preimage attack is trivial. You can craft a 4-byte suffix that gives any message a specific checksum, calculated using a closed-form expression (which I am too lazy to derive, not even with assistance from Z3). It's not an attack per-se, since CRC32 was never meant to be cryptograpically secure in the first place.
