Re: Is GIT_DEFAULT_HASH flawed?

On 2023-05-02 at 23:46:02, Felipe Contreras wrote:

That is practically very difficult and it means that it's hard to have confidence in the later history because SHA-1 is weak and you have to rely on it to verify the SHA-256 history later. Since attacks always get better, SHA-1 will eventually be so weak that collisions can be computed in the amount of time we now take for MD4 or MD5 collisions (i.e., seconds), and with your plan, we'd have to retain that history forever with the resulting lack of confidence in part of the history.

This also doesn't work with various structures like trees, the index, and pack and index formats, which have no indication of the algorithm used and simply rely on fixed-size, often 4-byte aligned object IDs without any metadata. In addition, the internals of the code often don't pass around enough data to make these values variable and thus this approach would substantially complicate the code in many ways.

Also, we've already decided on the current design a long time ago with the transition plan after extensive, thoughtful discussion by many people. Very few people other than me have worked on sending patches to work on the hash function transition, and that work up to now has all been done on my personal time, without compensation of any sort, out of a desire to improve the project. Lots of people have opined on how it should have been different without sending any patches.

If you would like to propose patches for the extensive amount of work to implement your solution, then we could consider them, although I will warn you that your approach will likely require at least several hundred patches. However, I refer you to the list archives to determine why your approach is not the one we chose and is not, in my view, the best path forward. I should also be clear that I have no intention of submitting patches to change our approach now or in the future, or redoing the patches I've already sent.

No, it doesn't. It means that our empty repositories until recently lacked any indication of the algorithm or other capabilities, which was a mistake in our original protocol design that has now been corrected.

If you interact with the repository later on when it has data, then if you're using the wrong hash algorithm, you'll find that you get a helpful error message that that's not yet supported. If you patched Git to ignore that check, you'd find that your repository would just be very broken in many ways with lots of random crashing and seemingly unrelated error messages instead of subtly using the wrong algorithm.