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0 pointssimias7y ago0 comments

I don't understand what you mean exactly. What attack vector are you considering?

If I sign a commit and push it on github anybody else can pull it from github and if they have my key than can validate that it is indeed me who made it. The only thing GH can do is modify it and strip the signature, modify it and replace the signature with a "fake" one that may fool people who don't have my public key or simply drop my commit altogether if for instance it contains a security fix (but then they also have to drop all future commits referencing this one since the hashes won't match).

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9 comments · 3 top-level

ElBarto7y ago· 4 in thread

@danieldk: "No, you do not need to hold a copy of the repository. You only need a known-good hash. Commit signing is a way to vouch that a hash is good."

The overhead is such that you are better off holding the repo yourself a la linux kernel.

Note also that SHA1 collisions are a practical attack these days: https://shattered.io/ . So if you rely on SHA1 hashes you have no guarantee...

microtonal7y ago

HackerNews pro-tip: you can react to a comment during the cool-down period by clicking on the timestamp of the comment ;). Of course, don't abuse it, there is a reason why the cool-down is there.

simiasOP7y ago

shattered is not (yet) a practical attack on git because of the way the data is represented before being hashed. Managing to get a SHA1 collision while simultaneously getting a valid git object and a meaningful commit (with, say, a backdoor inserted instead of pseudorandom garbage that will fail to compile) is still vastly too difficult to be practical, at least with publicly available knowledge.

Regarding the overhead I don't know what you mean and you're really grasping at straws at this point, how is validating a GPG signature harder than validating that, say, a website is actually hosting the official repo of a project instead of a malicious fork? What if my self-hosted website gets hacked mitm'd or DNS highjacked? GPG can still be used to validate that my commits are valid in these situations. Actually I'm not even tied to github or anything else in this situation, any mirror can be used without fear as long as my keys are secured.

tialaramex7y ago

Collision attacks allow somebody (say Greg) to produce two documents A and A' that hash the same. So hash(A) = hash(A'), with the result that signatures over A can be attached to A' and they work.

But this attack does NOT allow an adversary who sees some document B to produce a forgery B' such that hash(B) = hash(B')

If you believe Greg is a bad guy, don't let Greg make signed commits to your system. Trusting Alice and Bob to make signed commits doesn't allow Greg to attack you with a collision, only trusting Greg would do that, so don't trust Greg.

ElBarto7y ago

"If you believe Greg is a bad guy, don't let Greg make signed commits to your system."

That's exactly my point.

1 more reply

ElBarto7y ago· 1 in thread

@danieldk: "If we compare the hashes of the latest commit two git repositories"

Which implies that you hold a copy of the repository, which is therefore the master copy, as opposed to trusting a third party to do that.

microtonal7y ago

Which implies that you hold a copy of the repository, which is therefore the master copy, as opposed to trusting a third party to do that.

No, you do not need to hold a copy of the repository. You only need a known-good hash. Commit signing is a way to vouch that a hash is good.

Similarly, when downloading a distribution ISO, you do not have to trust the mirror, you only need someone trusted (e.g. the distribution maintainer) to vouch the correct hash of an ISO.

In both cases, you can hash the data that you retrieved and compare to a hash that is known to be good.

ElBarto7y ago· 1 in thread

You cannot guarantee the integrity of a repository that is controlled by a third party. You have to trust them.

microtonal7y ago

You can. Git's DAG is a Merkle tree. If we compare the hashes of the latest commit two git repositories and they are identical, then they have exactly the same history up to and including that commit [1].

If GitHub modified a commit, the hashes of all commits starting at that commit would change. This is why commit signing is important, it makes it impossible to change commits up to the signed commit, since it would change the hash of the signed commit and signature verification will fail.

[1] Or you found a way to modify the repository that leads to a hash collision. Which is unlikely.

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9 comments · 3 top-level

ElBarto7y ago· 4 in thread

@danieldk: "No, you do not need to hold a copy of the repository. You only need a known-good hash. Commit signing is a way to vouch that a hash is good."

The overhead is such that you are better off holding the repo yourself a la linux kernel.

Note also that SHA1 collisions are a practical attack these days: https://shattered.io/ . So if you rely on SHA1 hashes you have no guarantee...

microtonal7y ago

HackerNews pro-tip: you can react to a comment during the cool-down period by clicking on the timestamp of the comment ;). Of course, don't abuse it, there is a reason why the cool-down is there.

simiasOP7y ago

tialaramex7y ago

Collision attacks allow somebody (say Greg) to produce two documents A and A' that hash the same. So hash(A) = hash(A'), with the result that signatures over A can be attached to A' and they work.

But this attack does NOT allow an adversary who sees some document B to produce a forgery B' such that hash(B) = hash(B')

ElBarto7y ago

"If you believe Greg is a bad guy, don't let Greg make signed commits to your system."

That's exactly my point.

1 more reply

ElBarto7y ago· 1 in thread

@danieldk: "If we compare the hashes of the latest commit two git repositories"

Which implies that you hold a copy of the repository, which is therefore the master copy, as opposed to trusting a third party to do that.

microtonal7y ago

Which implies that you hold a copy of the repository, which is therefore the master copy, as opposed to trusting a third party to do that.

No, you do not need to hold a copy of the repository. You only need a known-good hash. Commit signing is a way to vouch that a hash is good.

Similarly, when downloading a distribution ISO, you do not have to trust the mirror, you only need someone trusted (e.g. the distribution maintainer) to vouch the correct hash of an ISO.

In both cases, you can hash the data that you retrieved and compare to a hash that is known to be good.

ElBarto7y ago· 1 in thread

You cannot guarantee the integrity of a repository that is controlled by a third party. You have to trust them.

microtonal7y ago

[1] Or you found a way to modify the repository that leads to a hash collision. Which is unlikely.

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