Thread

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

570 points · 536 comments · chmaynard

gandreani · 1790877958 · focus · HN ↗
One of my favorite fun facts about Fossil SCM (another source control by the devs of sqlite) is that they patched their use of SHA1 6 days after the shattered attack was published:
"Both Fossil and Git started out using only SHA1 hashes. But when the SHAttered attack against SHA1 was published on 2017-02-23, the need to migrate to a stronger hash algorithm was recognized. Fossil added the ability to use SHA3-256 as an alternative on 2017-03-01 (six days after the SHAttered attack was first published). SHA3-256 is now the default for all new repositories and check-ins in Fossil, though older check-ins that occurred prior to SHAttered can still use their original SHA1 hash. Hence, no repositories had to be rebuilt and no hyperlinks were broken."
<a href="https://fossil-scm.org/home/doc/trunk/www/hundredandone.md" rel="nofollow">https://fossil-scm.org/home/doc/trunk/www/hundredandone.md
To me it's so interesting watching in realtime Git is still battling with this decision and for Fossil it was just another week of development.
That whole page is fun to read. Another fun fact somewhere else in the docs is that Fossil uses a grow-only set to store commits. They came up with this scheme some years before it was formalized by CRDTs!
1. schacon · 1790878734 · focus · HN ↗
  I mean, there are two things here. One is how difficult it is to have a different hashing mechanism. Brian and other heroes in the Git core group have done amazing work to make this _technically_ possible on a repo level. To test some of my theories, I trivially implemented MD5 and an insanely dumb and easily breakable hash backend. It's not _hard_ to change the mechanism now. It's about the community.
  Fossil isn't difficult to change not because it's technically harder for Git but because Git has a community and ecosystem that Fossil does not. The cost is not in the individual project for Git, the cost is because there is _so much_ in Git and this bifurcates everything.
  1. schacon · 1790879228 · focus · HN ↗
    Also, interestingly, Git today does _not_ use a straight SHA1 because of these attacks. It uses `sha1dc`, a slower collision detecting variant that specifically checks for this vector of attacks. So currently, Git's SHA-1 variant is not susceptible to the SHAttered/Shambles attacks.
    jmyeet · 1790954397 · focus · HN ↗
    Sure but that's just kicking the can down the street. It will eventually be susceptible to a collision attack. As will SHA256 btw. What then? Are we doing to go through all this again?
    Online video has handled this. There are various codex, container formats and transport protocols. The TLS handshake does this. The ability to deprecate and replacing the hashing algorithm should've been built in from day 1.

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