5 ms·

It's not a concern for Linus, but maybe it should be? If SHA1's security properties don't matter, why was SHA1 chosen and not something faster? What about signi

It's not a concern for Linus, but maybe it should be? If SHA1's security properties don't matter, why was SHA1 chosen and not something faster? What about signing commits? This is affected by SHA1 collisions.

grzm 9y ago
Would you mind elaborating on this? What scenario are you envisioning where this is an issue? I'd caveat that with "in practice", but I'm willing to set that aside to avoid derailing the conversation into what's practical or not.
- thethirdone 9y ago
  > What scenario are you envisioning where this is an issue? Not the parent, but the problem specifically with using github repos with a hash to identify a specific commit is that if someone can make a collision, they can make the code anything they want. Linus doesn't consider this a problem, because he doesn't use untrusted repositories.
  - grzm 9y ago
    The hash is based on the content of the repo (and its history), is it not? Or am I misunderstanding? It's not that they can make a repo with arbitrary code of their choice cause a collision, correct? The chance of the collision be something meaningful in the context of the code at hand is vanishingly small, isn't it? Is it the concern that this can happen at all?
    - thethirdone 9y ago
      > The hash is based on the content of the repo (and its history), is it not? Or am I misunderstanding? That is correct. The hash of a commit is based on the parent commit's hash, the hash of the tree (file state), and commit message (and maybe more, I don't remember). >It's not that they can make a repo with arbitrary code of their choice cause a collision, correct? They can make a repo with arbitrary code, but they need to change make specific (potentially wierd) commits to get a specific hash. > The chance of the collision be something meaningful in the context of the code at hand is vanishingly small, isn't it? Is it the concern that this can happen at all? The chance by normal users is really small. But someone malicious could intentionally try to manipulate it. This could be done by varying commit messages. So if you trust such a person does not have control over the repo (Linus's position)its fine, but if a hash isn't cryptographically strong, malicious actors can make repos with commits pointing to arbitrary code with a specified hash.
      - grzm 9y ago
        Thanks for confirming. Getting back to the issue of using Github as a source of truth for deployment, that's got a host of issues besides reliance on SHA1, but I appreciate the reasoned response to the issues surrounding the hash itself as well. Appreciated!
        whyever 9y ago
        Note that Github actually checks for the SHA1 collisions discussed above: https://github.com/blog/2338-sha-1-collision-detection-on-github-com https://github.com/blog/2338-sha-1-collision-detection-on-gi...
- whyever 9y ago
  People use the SHA1 hash when signing commits. If you can create collisions you can create repositories with arbitrary code without breaking the signature. See Github's blog post on the mitigations for more details: https://github.com/blog/2338-sha-1-collision-detection-on-github-com https://github.com/blog/2338-sha-1-collision-detection-on-gi...

grzm 9y ago

Would you mind elaborating on this? What scenario are you envisioning where this is an issue? I'd caveat that with "in practice", but I'm willing to set that aside to avoid derailing the conversation into what's practical or not.

thethirdone 9y ago
> What scenario are you envisioning where this is an issue? Not the parent, but the problem specifically with using github repos with a hash to identify a specific commit is that if someone can make a collision, they can make the code anything they want. Linus doesn't consider this a problem, because he doesn't use untrusted repositories.
- grzm 9y ago
  The hash is based on the content of the repo (and its history), is it not? Or am I misunderstanding? It's not that they can make a repo with arbitrary code of their choice cause a collision, correct? The chance of the collision be something meaningful in the context of the code at hand is vanishingly small, isn't it? Is it the concern that this can happen at all?
  - thethirdone 9y ago
    > The hash is based on the content of the repo (and its history), is it not? Or am I misunderstanding? That is correct. The hash of a commit is based on the parent commit's hash, the hash of the tree (file state), and commit message (and maybe more, I don't remember). >It's not that they can make a repo with arbitrary code of their choice cause a collision, correct? They can make a repo with arbitrary code, but they need to change make specific (potentially wierd) commits to get a specific hash. > The chance of the collision be something meaningful in the context of the code at hand is vanishingly small, isn't it? Is it the concern that this can happen at all? The chance by normal users is really small. But someone malicious could intentionally try to manipulate it. This could be done by varying commit messages. So if you trust such a person does not have control over the repo (Linus's position)its fine, but if a hash isn't cryptographically strong, malicious actors can make repos with commits pointing to arbitrary code with a specified hash.
    - grzm 9y ago
      Thanks for confirming. Getting back to the issue of using Github as a source of truth for deployment, that's got a host of issues besides reliance on SHA1, but I appreciate the reasoned response to the issues surrounding the hash itself as well. Appreciated!
      - whyever 9y ago
        Note that Github actually checks for the SHA1 collisions discussed above: https://github.com/blog/2338-sha-1-collision-detection-on-github-com https://github.com/blog/2338-sha-1-collision-detection-on-gi...
whyever 9y ago
People use the SHA1 hash when signing commits. If you can create collisions you can create repositories with arbitrary code without breaking the signature. See Github's blog post on the mitigations for more details: https://github.com/blog/2338-sha-1-collision-detection-on-github-com https://github.com/blog/2338-sha-1-collision-detection-on-gi...