4 ms·

Git isn't relying on collision-resistance, it's relying on second-preimage[0] resistance, which is to say: in order to sneak a hash collision in to a git reposi

by gsu2 4y ago

Git isn't relying on collision-resistance, it's relying on second-preimage[0] resistance, which is to say: in order to sneak a hash collision in to a git repository, you have to sneak _something else_ that's already trusted (e.g. via code review) into the repository; collisions can't (yet) be generated for arbitrary hashes. I haven't heard of any second-preimage attacks against MD5, much less SHA-1, so mlindner was correct in asserting that MD5 would be fine (assuming 128 bits are enough). See also the analysis in [1]. More to the point, if you're able to sneak something into a repository in the first place (e.g. a benign file that generates a collision with a malicious file), then you're probably able to sneak in something more directly (e.g. [2]) that won't rely on both getting something in a trusted repository and then cloning from a different, untrusted source. [0]: https://en.wikipedia.org/wiki/Preimage_attack https://en.wikipedia.org/wiki/Preimage_attack [1]: this is getting a bit old, but should still be relevant? https://electriccoin.co/blog/lessons-from-the-history-of-attacks-on-secure-hash-functions/ https://electriccoin.co/blog/lessons-from-the-history-of-att... [2]: https://en.wikipedia.org/wiki/IDN_homograph_attack https://en.wikipedia.org/wiki/IDN_homograph_attack

oconnor663 4y ago
> if you're able to sneak something into a repository in the first place (e.g. a benign file that generates a collision with a malicious file), then you're probably able to sneak in something more directly Could you imagine using an implementation of TLS that "probably" authenticated your network traffic though? I think there are two separate reasons we prefer to make strong guarantees in cryptography: 1. That's often really what I need. If I'm downloading e.g. software updates over the network, I really need those to be authentic. 2. Even when I arguably don't need strong authenticity, like just reading some news articles, I want to use the same strong tools, because I don't want to have to study and understand (much less teach) the situations where some weaker tool fails. Inevitably I'll get that wrong or just forget, and I'll end up using the weak tool in some case where I should've used the strong one. In this case, if I imagine teaching how commit signing works with a weak hash function, it sounds like "Signing commits means that no one can sneak malicious content into your repository, unless they first steal your secret signing key, or else you ever committed (or allowed anyone else to commit) a non-text file that they created." Actually writing that second part out makes it feel really bad to me.
- gsu2 4y ago
  I'm not 100% sure I understand your point, but it sounds like you're concerned about signing something using a weak hash function (i.e. where the hash of something is what actually gets signed)? If that's the case, then my point is pretty simple: yes, SHA-1 is broken for signing untrusted input (due to weak collision resistance), but it is not broken (so far) for signing trusted input (due to strong preimage resistance). My point earlier was primarily that the contents of a repository are generally trusted (via mechanisms like code review), and signing trusted content still works even with SHA-1. Note that certificate signing vulnerabilities (which I assume is why TLS was mentioned?) usually rely on a malicious actor presenting one certificate and then presenting a different cert later; they can't arbitrarily fake existing certs from somebody else. The analogous scenario for git repositories would be to have a malicious actor make a commit (or blob, tree, etc.) that could be swapped out for another. But if you already have malicious actors able to make commits in your repository, then the hash function doesn't matter: they can cause damage in many, many other ways.
  - eru 4y ago
    > The analogous scenario for git repositories would be to have a malicious actor make a commit (or blob, tree, etc.) that could be swapped out for another. But if you already have malicious actors able to make commits in your repository, then the hash function doesn't matter: they can cause damage in many, many other ways. The malicious actor can pose as a good-faith contributor and submit Pull Requests to your repository. You review the code in the PR, and perhaps even prove it correct. Later on, the malicious actor can do the swapping trick. (Eg by running a mirroring service for your repository.)
    - gsu2 4y ago
      > You review the code in the PR, and perhaps even prove it correct. Later on, the malicious actor can do the swapping trick. (Eg by running a mirroring service for your repository.) Having a copy of code that is reviewable and then searching for a malicious collision is a preimage attack; extending two chosen prefixes (e.g. one "valid" and one "malicious") until they meet at a hash collision is how most practical (?) collision attacks work. The latter scenario produces large junk sections in the results, which should be obvious under even mild scrutiny. If the reviewer misses the kilobytes of garbage in the middle of a file they're reviewing, then an attacker can just sneak malicious code in directly without requiring a hash collision. If the project relies on an effectively unreviewable binary file that could hold kilobytes of junk (like some YAML files I've seen...), then that's already breaking the review process without requiring a hash collision. Ignoring all of that, anybody grabbing code from an untrusted source is already vulnerable to whatever attacks that untrusted source wants to employ, with "exploiting hash collision" being one of the higher-effort attacks that could be mounted. Essentially, any repository that would be vulnerable to any of the known hash collision attacks (via bad review, untrusted upstream, etc.) would be vulnerable to more mundane, easier attacks against the same weaknesses that do not depend on hash collisions.
dagenix 4y ago
I don't believe that is accurate. First - if git really didn't care about collision resistance, there wouldn't have been a need to switch to SHA1DC as the hash function. They switched because they care enough that they were willing to accept the performance penalty. Second - imagine this scenario: a user creates two commits with the same hash, one with a valid change and the second with a malicious one. The collision could be created by playing around with some data in a binary file - so, this is a collision attack not 2nd pre-image. The user then submits the change to the upstream and gets it approved. The user maintains a mirror of the upstream repo into which they place the malicious commit. Anyone that pulls from this mirror will think they have the same code as the upstream, even if they compare hashes. So don't use an untrusted mirror? I guess - but that is something that should be possible with a strong hash. And if git really didn't want you to do that, it would provide for better ways of tracking where objects were actually pulled from. Anyway, collision attacks are real and can impact git. They just aren't as bad as a 2nd pre-image attack.
- gsu2 4y ago
  > First - if git really didn't care about collision resistance, there wouldn't have been a need to switch to SHA1DC as the hash function. They switched because they care enough that they were willing to accept the performance penalty. Git didn't _need_ to switch to SHA1DC, but they did because the cost was minimal and it's still a good idea to defend against known attacks. > Second - imagine this scenario: a user creates two commits with the same hash, one with a valid change and the second with a malicious one. The collision could be created by playing around with some data in a binary file - so, this is a collision attack not 2nd pre-image. The user then submits the change to the upstream and gets it approved. This is a general problem with binary files: they're hard to properly review. Having unreviewable files in a repository (binaries, machine-generated configs, etc.) is already a security problem; hash collisions would just be one (very difficult) way of exploiting that problem. > The user maintains a mirror of the upstream repo into which they place the malicious commit. Anyone that pulls from this mirror will think they have the same code as the upstream, even if they compare hashes. Having people pull data from an attacker-controlled source is a security issue, regardless of hash collisions. > So don't use an untrusted mirror? I guess - but that is something that should be possible with a strong hash. And if git really didn't want you to do that, it would provide for better ways of tracking where objects were actually pulled from. Git was designed for collaboration between trusted parties; collaboration between untrusted parties (e.g. pulling changes from untrusted sources) is a much harder problem that git doesn't pretend to solve. > Anyway, collision attacks are real and can impact git. They just aren't as bad as a 2nd pre-image attack. Collision attacks are real, but they have yet to impact git (beyond adopting SHA1DC, I guess), despite how big of a target popular git repositories are.
eru 4y ago
> Git isn't relying on collision-resistance, it's relying on second-preimage[0] resistance, which is to say: in order to sneak a hash collision in to a git repository, you have to sneak _something else_ that's already trusted (e.g. via code review) into the repository; collisions can't (yet) be generated for arbitrary hashes. Yes, I know. I was arguing the more general point that 'The use of SHA-1 in Git is not for security purposes,'. Of course, for anything crypto related we go by the maxim 'guilty, until proven innocent'. MD5 might not have a published second-preimage attack, yet; but its broken enough, that you shouldn't rely on it for anything anymore: it's not a acceptable crypto-hash, and if you don't need a crypto-hash, you can use something simpler like a CRC instead.

by gsu2 4y ago

oconnor663 4y ago

> if you're able to sneak something into a repository in the first place (e.g. a benign file that generates a collision with a malicious file), then you're probably able to sneak in something more directly Could you imagine using an implementation of TLS that "probably" authenticated your network traffic though? I think there are two separate reasons we prefer to make strong guarantees in cryptography: 1. That's often really what I need. If I'm downloading e.g. software updates over the network, I really need those to be authentic. 2. Even when I arguably don't need strong authenticity, like just reading some news articles, I want to use the same strong tools, because I don't want to have to study and understand (much less teach) the situations where some weaker tool fails. Inevitably I'll get that wrong or just forget, and I'll end up using the weak tool in some case where I should've used the strong one. In this case, if I imagine teaching how commit signing works with a weak hash function, it sounds like "Signing commits means that no one can sneak malicious content into your repository, unless they first steal your secret signing key, or else you ever committed (or allowed anyone else to commit) a non-text file that they created." Actually writing that second part out makes it feel really bad to me.

gsu2 4y ago
I'm not 100% sure I understand your point, but it sounds like you're concerned about signing something using a weak hash function (i.e. where the hash of something is what actually gets signed)? If that's the case, then my point is pretty simple: yes, SHA-1 is broken for signing untrusted input (due to weak collision resistance), but it is not broken (so far) for signing trusted input (due to strong preimage resistance). My point earlier was primarily that the contents of a repository are generally trusted (via mechanisms like code review), and signing trusted content still works even with SHA-1. Note that certificate signing vulnerabilities (which I assume is why TLS was mentioned?) usually rely on a malicious actor presenting one certificate and then presenting a different cert later; they can't arbitrarily fake existing certs from somebody else. The analogous scenario for git repositories would be to have a malicious actor make a commit (or blob, tree, etc.) that could be swapped out for another. But if you already have malicious actors able to make commits in your repository, then the hash function doesn't matter: they can cause damage in many, many other ways.
- eru 4y ago
  > The analogous scenario for git repositories would be to have a malicious actor make a commit (or blob, tree, etc.) that could be swapped out for another. But if you already have malicious actors able to make commits in your repository, then the hash function doesn't matter: they can cause damage in many, many other ways. The malicious actor can pose as a good-faith contributor and submit Pull Requests to your repository. You review the code in the PR, and perhaps even prove it correct. Later on, the malicious actor can do the swapping trick. (Eg by running a mirroring service for your repository.)
  - gsu2 4y ago
    > You review the code in the PR, and perhaps even prove it correct. Later on, the malicious actor can do the swapping trick. (Eg by running a mirroring service for your repository.) Having a copy of code that is reviewable and then searching for a malicious collision is a preimage attack; extending two chosen prefixes (e.g. one "valid" and one "malicious") until they meet at a hash collision is how most practical (?) collision attacks work. The latter scenario produces large junk sections in the results, which should be obvious under even mild scrutiny. If the reviewer misses the kilobytes of garbage in the middle of a file they're reviewing, then an attacker can just sneak malicious code in directly without requiring a hash collision. If the project relies on an effectively unreviewable binary file that could hold kilobytes of junk (like some YAML files I've seen...), then that's already breaking the review process without requiring a hash collision. Ignoring all of that, anybody grabbing code from an untrusted source is already vulnerable to whatever attacks that untrusted source wants to employ, with "exploiting hash collision" being one of the higher-effort attacks that could be mounted. Essentially, any repository that would be vulnerable to any of the known hash collision attacks (via bad review, untrusted upstream, etc.) would be vulnerable to more mundane, easier attacks against the same weaknesses that do not depend on hash collisions.

dagenix 4y ago

I don't believe that is accurate. First - if git really didn't care about collision resistance, there wouldn't have been a need to switch to SHA1DC as the hash function. They switched because they care enough that they were willing to accept the performance penalty. Second - imagine this scenario: a user creates two commits with the same hash, one with a valid change and the second with a malicious one. The collision could be created by playing around with some data in a binary file - so, this is a collision attack not 2nd pre-image. The user then submits the change to the upstream and gets it approved. The user maintains a mirror of the upstream repo into which they place the malicious commit. Anyone that pulls from this mirror will think they have the same code as the upstream, even if they compare hashes. So don't use an untrusted mirror? I guess - but that is something that should be possible with a strong hash. And if git really didn't want you to do that, it would provide for better ways of tracking where objects were actually pulled from. Anyway, collision attacks are real and can impact git. They just aren't as bad as a 2nd pre-image attack.

gsu2 4y ago
> First - if git really didn't care about collision resistance, there wouldn't have been a need to switch to SHA1DC as the hash function. They switched because they care enough that they were willing to accept the performance penalty. Git didn't _need_ to switch to SHA1DC, but they did because the cost was minimal and it's still a good idea to defend against known attacks. > Second - imagine this scenario: a user creates two commits with the same hash, one with a valid change and the second with a malicious one. The collision could be created by playing around with some data in a binary file - so, this is a collision attack not 2nd pre-image. The user then submits the change to the upstream and gets it approved. This is a general problem with binary files: they're hard to properly review. Having unreviewable files in a repository (binaries, machine-generated configs, etc.) is already a security problem; hash collisions would just be one (very difficult) way of exploiting that problem. > The user maintains a mirror of the upstream repo into which they place the malicious commit. Anyone that pulls from this mirror will think they have the same code as the upstream, even if they compare hashes. Having people pull data from an attacker-controlled source is a security issue, regardless of hash collisions. > So don't use an untrusted mirror? I guess - but that is something that should be possible with a strong hash. And if git really didn't want you to do that, it would provide for better ways of tracking where objects were actually pulled from. Git was designed for collaboration between trusted parties; collaboration between untrusted parties (e.g. pulling changes from untrusted sources) is a much harder problem that git doesn't pretend to solve. > Anyway, collision attacks are real and can impact git. They just aren't as bad as a 2nd pre-image attack. Collision attacks are real, but they have yet to impact git (beyond adopting SHA1DC, I guess), despite how big of a target popular git repositories are.

eru 4y ago

> Git isn't relying on collision-resistance, it's relying on second-preimage[0] resistance, which is to say: in order to sneak a hash collision in to a git repository, you have to sneak _something else_ that's already trusted (e.g. via code review) into the repository; collisions can't (yet) be generated for arbitrary hashes. Yes, I know. I was arguing the more general point that 'The use of SHA-1 in Git is not for security purposes,'. Of course, for anything crypto related we go by the maxim 'guilty, until proven innocent'. MD5 might not have a published second-preimage attack, yet; but its broken enough, that you shouldn't rely on it for anything anymore: it's not a acceptable crypto-hash, and if you don't need a crypto-hash, you can use something simpler like a CRC instead.