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@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 o

@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/ https://shattered.io/ . So if you rely on SHA1 hashes you have no guarantee...

danieldk 8y 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.
simias 8y 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.
tialaramex 8y 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.
- ElBarto 8y ago
  "If you believe Greg is a bad guy, don't let Greg make signed commits to your system." That's exactly my point.
  - tialaramex 8y ago
    Is "let untrustworthy people change our code" a common approach in GitHub? Every GitHub project I've ever worked on expects all but their trusted insiders to submit PRs If you can't trust Greg not to make collisions, you definitely shouldn't let him write code!

danieldk 8y 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.

simias 8y 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.

tialaramex 8y 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.

ElBarto 8y ago
"If you believe Greg is a bad guy, don't let Greg make signed commits to your system." That's exactly my point.
- tialaramex 8y ago
  Is "let untrustworthy people change our code" a common approach in GitHub? Every GitHub project I've ever worked on expects all but their trusted insiders to submit PRs If you can't trust Greg not to make collisions, you definitely shouldn't let him write code!