[jodrellblank]

> How hard would it be to craft a malware that has the same signature as an important system file?

Very, otherwise digital signatures wouldn’t be much use. There are no publicly known ways to make an input which hashes to the same value as another known input through the SHA256 hash algorithm any quicker than brute-force trial and error of every possibility.

This is the difficulty that BitCoin mining is based on - the work that all the GPUs were doing, the reason for the massive global energy use people complain about is basically a global brute-force through the SHA256 input space.

See the “find a custom SHA256” challenge on HN last month discussions: https://news.ycombinator.com/item?id=40683564

[weinzierl]

I was talking about malware signatures, which do necessarily use cryptographic hashes. They are probably more optimized for speed because the engine needs to check a huge number of files as fast as possible.

[jodrellblank]

Cryptographic hashes are not the fastest possible hash, but they are not slow; CPUs have hardware SHA acceleration: https://www.intel.com/content/www/us/en/developer/articles/t... - compared to the likes of a password hash where you want to do a lot of rounds and make checking slow, as a defense against bruteforcing.

That sounds even harder; Windows Authenticode uses SHA1 or SHA256 on partial file bytes, the AV will use its own hash likely on the full file bytes, and you need a malware which matches both - so the AV will think it's legit and Windows will think it's legit.