The Background section of this [0] document explains it.

In short, git migrated to a hardened SHA-1 hash after the SHAttered [1] attack became known.

However, in the future more attacks against SHA-1 may be found, so git is trying to future-proof itself.

However, even in the case a hash function is no longer cryptographically secure, git doesn't lose out completely, just losing signing and shorthand fetches over the git protocol.

This project focuses on git with SHA256 and SHA-1 interchangeability, but doesn't address the git protocol side of things yet, which can come later.

[0] https://github.com/git/git/blob/master/Documentation/technic...

[1] https://shattered.io/

"Hardened SHA-1 hash" is a confusing way to characterize git's current hash behavior. There is no change in the hash--it continues to be SHA-1. The change is in git's business logic: it will detect hash inputs that look like a SHA-1 collision attack, and will refuse to proceed.

The SHA1DC algorithm implements a different hash function, since it doesn't return the same hash as SHA-1 for all inputs, those inputs just happen to be really rare.

This is SHA-1:

    hash = SHA1(input)

This is SHA-1DC in "only detect collision mode":

    collided, hash = SHA1DC(input)

Where "hash" for SHA1DC(input) will be the same value as SHA1(input), then there's the mode to work around such collisions:

    hash = SHA1DC_safe(input)

In this case "hash" will be the same as SHA1(input) in all cases, except those where the input is detected to be malicious (as in the SHAttered attack). Then SHA1DC_safe(input) will return a different ("safe") hash than SHA1(input) would.

So depending on the mode you use it in it's a different hash function than SHA-1. The Git project only uses it in the "detect a collision and die" mode: https://github.com/git/git/blob/master/sha1dc_git.c#L17-L23

Here's the part of the code where you can see it's implementing a different hash function: https://github.com/git/git/blob/v2.19.0-rc0/sha1dc/sha1.c#L1...

I.e. if detect_coll and safe_hash are set, it will return different hashes than SHA1() for the same input.

Wow, great source. Thanks!

I believe this was in response to the SHAttered[0] attack which was a demonstrated SHA-1 collision

[0] https://shattered.io/

Git uses a hardened SHA-1, not vulnerable to SHAttered, since v2.13.0 [0] But the fear of more attacks helped transition the move.

[0] https://blog.github.com/2017-05-10-git-2-13-has-been-release...