Re: Linus Torvalds dismissed concerns about attacks on Git SHA-1 hashes
I am reasonably certain that Linus' point was that for this particular purpose, the hash needed to be fast and reasonably "random" in its output. It didn't need to be cryptographically secure.
I think some of my fellow commentards are thinking of hashing solely in terms of security (which is the use that gets most of the attention). Hashing gets use beyond that. Some hash functions (such as that used in the Linux ext3 system) are almost trivially insecure; they are for use in hash tables for indexing data. Personally, I've had cause to use a ludicrously insecure hash function in my astronomy software. I needed maximum hashing speed (the function was a performance bottleneck) and a good enough output to avoid collisions. Security was not an issue.
(I should note, though, that I'm taking Linus' word that the hash function for Git falls into this category. If so, I could easily see him saying : we've got the SHA-1 code sitting around; it works; we don't care if collisions can be engineered; let's use it.)