How safe are signed git tags? Only as safe as SHA-1 or somehow safer?


How safe are signed git tags? Especially because git uses SHA-1. There is contradictory information around.

So if one verifies a git tag (_git tag -v tagname_), then _checksout_s the tag, and checks that _git status_ reports no untracked/modified files, without further manually auditing the code, how secure is this actually? Is it only as safe as SHA-1?

Let’s assume an adversary, that is capable of producing SHA-1 collisions.

Linus Torvalds said: [1]

Git uses SHA-1 not for security

And goes on.

The security parts are elsewhere

Could you please elaborate on this? Where are the security parts? Can you please briefly explain how these work? Where can I read more about this?

Wikipedia says. [2]

Nonetheless, without second preimage resistance [3] of SHA-1 signed commits and tags would no longer secure the state of the repository as they only sign the root of a Merkle tree [4].

Which contradicts what Linus Torvalds said. What does that mean for security? Which statement is true?

The source control management system Git uses SHA-1 not for security but for ensuring that the data has not changed due to accidental corruption. Linus Torvalds has said, “If you have disk corruption, if you have DRAM corruption, if you have any kind of problems at all, Git will notice them. It’s not a question of if, it’s a guarantee. You can have people who try to be malicious. They won’t succeed. […] Nobody has been able to break SHA-1, 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, OK, it’s a huge security feature. It has nothing at all to do with security, it’s just the best hash you can get. […] I guarantee you, if you put your data in Git, you can trust the fact that five years later, after it was converted from your hard disk to DVD to whatever new technology and you copied it along, five years later you can verify that the data you get back out is the exact same data you put in. […] One of the reasons I care is for the kernel, we had a break in on one of the BitKeeper sites where people tried to corrupt the kernel source code repositories. [6]

If (!) I understand Mike Gerwitz ([…] GNU […]) ‘s opinion, his opinion is, that for best security each and every commit should be signed for best possible git verification security.

See also:

– Mike Gerwitz’s “A Git Horror Story: Repository Integrity With Signed Commits” [7]

Verbose reply by Mike Gerwitz to my question. [8]

Similar question on security stackexchange. [9] Quote:

Nevertheless, If somebody managed to find a way how to find SHA1 collisions easily, then git would have much bigger problem.

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[1] https://www.youtube.com/watch?v=4XpnKHJAok8&t=56m20s
[2] https://en.wikipedia.org/wiki/SHA-1#Data_integrity
[3] https://en.wikipedia.org/wiki/Second_preimage_resistance
[4] https://en.wikipedia.org/wiki/Merkle_tree
[5] https://www.youtube.com/watch?v=4XpnKHJAok8&t=56m20s
[6] https://en.wikipedia.org/wiki/SHA-1#Data_integrity
[7] http://mikegerwitz.com/papers/git-horror-story
[8] https://www.whonix.org/forum/index.php/topic,538.msg4278.html#msg4278
[9] https://security.stackexchange.com/questions/67920/how-safe-are-signed-git-tags-only-as-safe-as-sha-1-or-somehow-safer
[10] https://www.whonix.org/forum/index.php/topic,538.0.html
[11] https://www.whonix.org/pipermail/whonix-devel/2014-November/000229.html
[12] https://groups.google.com/d/msg/qubes-devel/yWeDswfLUao/Iqc92SLsTB0J
[13] http://www.mail-archive.com/git@vger.kernel.org/msg61087.html

to look for alternative and as well compared against git check Fossil:


They said:

The practical impact of attacks like SHAttered and SHAmbles on the Git and Fossil blockchains isn’t clear, but you want to have your repositories moved over to a stronger hash algorithm before someone figures out how to make use of the weaknesses in the old one. Fossil had this covered for years now, so that the solution is now almost universally deployed.

so even with the upgrade of git it will use SHA-2 , Fossil using SHA-3.

Found comments by Matthew Green (expert cryptographer) and Daniel Micay on this.

I can’t see the post he linked (requires an account) but he posted a screenshot where Linus talks about how Git does actually use SHA-1 for security.

TLDR: Git depends on SHA-1 for security and thus is insecure.

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