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- Hacker News
- So they’ve been talking about this for many years, planning, and finally announce when they’re going to switch the default.
So this is the right time to post that everything they’re doing is wrong? Did you engage in all the discussions about it and how best to handle it? Whether SHA-256 was the best solution?
I don’t see anywhere that it talks about alternate proposals or why they might have been better. Why the particular suggestions here were rejected.
This seems like a bunch of Monday morning quarterbacking.
by MBCook - I thought this would be a snark but it's an extremely well put together argument against the "Hashmageddon".
If you're replacing the weakness of SHA-1 just by going to another algorithm, you better be prepared to go to the next one when sha256 collisions happen, and it doesn't sound like git's design would be easy to modify for this type of crypto agility.
I do like their proposal for using signatures to establish trust and allow swapping sha256 for whatever comes next.
by 6thbit - From what I'd read, SHA256 in git is showing every sign of being another IPv6. In particular:
- It's implemented in a non-backwards-compatible way
- The benefits over the older model are a bit nebulous
- There's a large amount of tooling that needs to catch up, and little sign that there is movement there
by nicoburns - This means, if you migrate your repo, every single commit message that contains text like: "please see commit <sha1>" will now be broken.
This will be a train wreck. I hope they don't release before adding compatibility modes to keep the existing sha1's around in the database.
by purpleidea - I don't understand why Git is not making the SHA-1 and SHA-256 modes far more compatible with each other.
SHA1-hashed objects should be able to refer to SHA-256-hashed objects, although this seems somewhat pointless.
But SHA-256-hashed objects should also be able to refer to SHA1-hashed objects, with a major caveat: if those objects themselves are part of a collision pair, then there is a genuine problem. But this is avoidable! Suppose that Linux decided to migrate to SHA-256. The upstream project could choose a pair of dates, say January 1 2027 and March 1 2027. Up to the first date, maintainers would be welcome to submit hashes of objects that are not yet in the repo but that they think they might submit later on, and, on that date, the upstream tree would finalize the list of these objects and reference it in the repo (with a new mechanism for this purpose). Effective the second date, the repo would start publishing SHA-256 commits and would never again accept a SHA1-hashed object that was not in the repo at the cutoff date or referenced as part of the Jan 1 block.
And now it would be impossible to get a new SHA1 collision in to the repo.
The only new git features needed would be:
a) actual compatibility so that a SHA-256-hashed object could reference a SHA1-hashed object
b) a new object type that's a list of allowed SHA1 hashes (or probably a tree of them) that is itself hashed with SHA-256 and a mechanism to link to one of these from a commit
c) a policy mechanism to set a repo to only allow SHA1-hashed-objects that a reachable from a preconfigured SHA-256-hashed commit
by amluto - Linus Torvalds in 2007:
> but the point is the SHA-1, as far as Git is concerned, isn't even a security feature. It's purely a consistency check. The security parts are elsewhere, so a lot of people assume that since Git uses SHA-1 and SHA-1 is used for cryptographically secure stuff, they think that, Okay, it's a huge security feature. It has nothing at all to do with security, it's just the best hash you can get. ... [1]
[1] https://www.youtube.com/watch?v=4XpnKHJAok8&t=56m20s
So Torvalds used SHA-1 purely because he needed a hash function with no other property than identifying content.
by meinersbur - One of my favorite fun facts about Fossil SCM (another source control by the devs of sqlite) is that they patched their use of SHA1 6 days after the shattered attack was published:
"Both Fossil and Git started out using only SHA1 hashes. But when the SHAttered attack against SHA1 was published on 2017-02-23, the need to migrate to a stronger hash algorithm was recognized. Fossil added the ability to use SHA3-256 as an alternative on 2017-03-01 (six days after the SHAttered attack was first published). SHA3-256 is now the default for all new repositories and check-ins in Fossil, though older check-ins that occurred prior to SHAttered can still use their original SHA1 hash. Hence, no repositories had to be rebuilt and no hyperlinks were broken."
https://fossil-scm.org/home/doc/trunk/www/hundredandone.md
To me it's so interesting watching in realtime Git is still battling with this decision and for Fossil it was just another week of development.
That whole page is fun to read. Another fun fact somewhere else in the docs is that Fossil uses a grow-only set to store commits. They came up with this scheme some years before it was formalized by CRDTs!
by gandreani - This article is full of mistakes and misleading claims:
1) It's claiming SHA1 insecurity is theoretical, while SHAttered from 2017 was specifically a pratical proof of concept. The only reason Git wasn't affected, is because they didn't bother bruteforcing a git-blob prefix.
2) It's claiming collision attacks don't matter, only second-preimage attacks do. This is incorrect, collision attacks are enough for code-smuggling problems, when two repositories are on the same git commit (verified by the full commit hash), yet contain different code in their git checkout.
3) The Linus quote "The real security is in distribution" is arguing that "git's content-addressed system should not be used to address content". It's arguing that, in case of curl|sh, you shouldn't use a sha256sum-gate to pin the content to something you've reviewed, you should instead ensure curl is fetching from an https server.
by kpcyrd