calcopiritus comments on Git 3.0's upcoming SHA-256 default will be a costly mistake

[–] calcopiritus@lemmy.world 57 points 1 month ago (8 children)

I tried to implement my own git from scratch. And when I got to choose a hashing algorithm, I reached the exact same conclusion as the author pretty fast.

The hashing algorithm doesn't really matter as long as it's good enough to prevent accidental collisions. And if there is a collision, it is extremely easy to check.

If you are about to create a new object and an object with that hash already exists, you compare the 2 objects. If they are the same, nothing happened. If they are different, you just found a collision. Throw some obscure error that will only be witnessed once in the lifetime of the universe and be done with it. Tell the user to change a single bit of the input and be done with it.

The purpose of object hashes in git was never to provide security. Security is achieved through other means.

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[–] chris@l.roofo.cc 7 points 1 month ago (6 children)

Even if it's not the purpose it can fulfill it. Hashes are used for all kinds of things like traceability or for pinning versions. They are used in a context where people depend on that a hash always has the same code behind it and SHA1 can not guarantee that anymore. A better hashing algo is needed to keep the way how people use git secure.

It's not enough to say "you shouldn't use it that way". We have to accept the reality of how git is used and adapt.

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[–] calcopiritus@lemmy.world 6 points 1 month ago (5 children)

If someone has access to write to a repo you are downloading code from, and you don't trust that someone, you shouldn't be running the code in that repo.

The only "security" vulnerability is: I have audited and verified that the code in commit 05682ab36ca. And I will use only that version.

What you do then is: download that commit and store it in a local repo, and package it so you can distribute it to your clients.

Auditing external software is tremendous effort, even if it is open source. Using your local repo instead of downloading that commit from GitHub every time is very little extra effort in comparison.

And if you really need it. You can always hash with sha256 yourself and verify that the commit with that sha1 still produces the same sh256. So you can keep downloading it each time, just need to store the sha256.

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[–] chris@l.roofo.cc 2 points 1 month ago (4 children)

You are exactly doing what I described. You are saying "your using it wrong" instead of acknowledging that's how people use it and and then improving that. You might not need this feature because you are "using it correctly" but that doesn't matter. For the lived reality of many people sha256 is the correct move.

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[–] calcopiritus@lemmy.world 4 points 1 month ago (3 children)

I've never heard of anyone relying on git's sha1 hashed for security

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[–] chris@l.roofo.cc 1 point 1 month ago (2 children)

I have. In some languages you can use git repos as dependencies. It's good practice to use hashes instead if tags because tags are not immutable. SHA1 hashes are better than tags but better hases would be even better.

Also traceability. You can build a chain from your commit over ci to the artifact if you do it right. If you can change the files to a hash you can destroy that trust chain. Sha256 makes that impossible.

People use the hashes in ways you might not. It wasn't meant as a trust anchor but it became one. And that is now being addressed. I also think that the git maintainers have put a lot of thought into backward compatability and making the change as painless as possible.

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[–] calcopiritus@lemmy.world 1 point 1 month ago (1 child)

Good practice != Something that ensures security. As I said, it's an even better practice to just host your own fork if what you want is security. The reason to use git hashes is mostly so you don't have to trust the author following semver in there version number. It's a matter of ensuring that your code will always compile, not a security feature.

Have you read the article? Your arguments derive from an assumption of "sha1 is mutable, sha256 is immutable". First of all, every hashing algorithm is going to have collisions, that's an unavoidable "feature" of hashes. And sha1 is in no way "mutable", it takes a great amount of effort (and money) to generate a collision. Furthermore, those collisions are not arbitrary. You have to calculate them beforehand. As the article says: what is more likely? Paying 40k€ in compute time to generate a single collision? Or just paying an open source maintainer 40k€ to let you do 1 new commit that most people are going to download anyway?

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[–] chris@l.roofo.cc 1 point 1 month ago

Yes I read the article. I don't agree with all the conclusions.

Now is the best time to change to a new algo. SHA1 is not completely broken yet. It most probably will be at some point.

Also AFAIK they will have compatibility tools for SHA1 repos in git. I don't see the big deal with the path they chose.

We will probably nor agree on that one. I think the hashes are important and that they are mutable right new with effort and later without. It's okay that we don't agree though. The git maintainers have made their decision anyways.

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[–] Kajika@lemmy.ml 6 points 1 month ago* (last edited 1 month ago)

Security is achieved through other means.

Let's explicit this: you can, and should, sign your commits if you want security (no commit tampering). You need to:

generated a gpg key: gpg --full-generate-key
set git to look for the public key to use: git config --global user.signingkey [public key ID]
set git to sign your commits: git config --global commit.gpgsign true

Note that I used --global here but you can do without to sign on a per-project basis.

Also gpg UX is terrible, to find the [public key ID] use the command gpg --list-keys, it should looks like:

pub   ed25519 2026-01-01 [SC] <- type algorithm date and capability (this key is only to Sign and Certify)
      662E3CDD6FE329002D0CA5BB40339DD82B12EF16 <- Public key ID
uid           [ultimate] my full name (Master Key) <my_email@domain.com> <- owner information (should be your name and email address)
sub   rsa4096 2026-01-01 [E] <- sub key used to encrypt
sub   ed25519 2026-01-01 [S] [expires: 2027-01-01] <- subkey used to sign

Last, if you want to save/backup you keys the default location of gpg files is ~/gnupg or you can export keys with gpg --export [key ID] > path/to/file.key for the public part and gpg --export-secret-keys [private key ID] for the private part. Both export can have a -a or --armor argument to output as base64 text instead of raw binary. [private key ID] can be found with gpg --list-secret-keys.

EDIT: after writing this I checked https://git-scm.com/book/en/v2/Git-Tools-Signing-Your-Work and TIL you can sign tags too.

EDIT 2: you can also use your ssh key to sign -> git config --global gpg.format ssh (I am less inclined to do this as you won't have subkeys and expiration date but this would be simplier indeed)

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