Thread

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

570 points · 536 comments · chmaynard

kpcyrd · 1790876562 · focus · HN ↗
This article is full of mistakes and misleading claims:
1) It's claiming SHA1 insecurity is theoretical, while SHAttered from 2017 was specifically a pratical proof of concept. The only reason Git wasn't affected, is because they didn't bother bruteforcing a git-blob prefix.
2) It's claiming collision attacks don't matter, only second-preimage attacks do. This is incorrect, collision attacks are enough for code-smuggling problems, when two repositories are on the same git commit (verified by the full commit hash), yet contain different code in their git checkout.
3) The Linus quote "The real security is in distribution" is arguing that "git's content-addressed system should not be used to address content". It's arguing that, in case of curl|sh, you shouldn't use a sha256sum-gate to pin the content to something you've reviewed, you should instead ensure curl is fetching from an https server.
1. schacon · 1790877053 · focus · HN ↗
  1) I link to the SHAttered paper, as well as Shambles. Git projects were not affected because it is an inefficient attack vector. I say it's impractical to exploit, which I think everyone agrees with.
  2) I specifically argue that even if both attacks were practical and cheap, it's still not the problem we should be focusing on.
  3) Have you read this email (that I linked to)? It is almost the same general message (20 years ago) that this blog post is. It literally goes though a theoretical object replacement attack and how dumb this scenario is and so SHA-1 is fine.
  <a href="https://lore.kernel.org/git/Pine.LNX.4.58.0504291221250.18901@ppc970.osdl.org/" rel="nofollow">https://lore.kernel.org/git/Pine.LNX.4.58.0504291221250.1890...
  1. onion2k · 1790922602 · focus · HN ↗
    I say it's impractical to exploit, which I think everyone agrees with.
    Impractical for an individual, definitely. For a large org, maybe, but if the payoff was big enough? For a nation state level actor intent on doing something, absolutely not.
    The go-to example is Stuxnet. Some countries wanted to attack Iran's nuclear enrichment programme, so they spent 5 years developing a worm that used multiple zero day exploits to attack a specific controller in a specific model of gas centrifuge. Could Mythos write Stuxnet? Unlikely, but a knowledgable team with access to it could probably write it in a lot less than 5 years.
    'impractical' has very different values for different groups.
    qdotme · 1790955250 · focus · HN ↗
    And it’s also the question of different use cases.
    Asking to trust in an authority (while the main authority Microsoft/GitHub has is essentially figuring out enterprise sales well enough to be acquired by a company desperately needing developers after fumbling badly in the 2000s) is exactly the opposite of my stance - it is a large corporation, with heavy employee rotation, with substantial exposure to various forms of regulatory pressure and to various forms of corruption.
    Which is why the cryptography exists to prove the developer-to-consumer trust without trusting the intermediaries. Yes, I do check GPG signatures. Yes, I do include a git commit hash in the binaries I build. And surely I want to make sure that this doesn’t mutate because some unknown engineer at GitHub had a bad case of gambling debt.

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