alt Hacker NewsI don't understand why Git is not making the SHA-1 and SHA-256 modes far more compatible with each other.
SHA1-hashed objects should be able to refer to SHA-256-hashed objects, although this seems somewhat pointless.
But SHA-256-hashed objects should also be able to refer to SHA1-hashed objects, with a major caveat: if those objects themselves are part of a collision pair, then there is a genuine problem. But this is avoidable! Suppose that Linux decided to migrate to SHA-256. The upstream project could choose a pair of dates, say January 1 2027 and March 1 2027. Up to the first date, maintainers would be welcome to submit hashes of objects that are not yet in the repo but that they think they might submit later on, and, on that date, the upstream tree would finalize the list of these objects and reference it in the repo (with a new mechanism for this purpose). Effective the second date, the repo would start publishing SHA-256 commits and would never again accept a SHA1-hashed object that was not in the repo at the cutoff date or referenced as part of the Jan 1 block.
And now it would be impossible to get a new SHA1 collision in to the repo.
The only new git features needed would be:
a) actual compatibility so that a SHA-256-hashed object could reference a SHA1-hashed object
b) a new object type that's a list of allowed SHA1 hashes (or probably a tree of them) that is itself hashed with SHA-256 and a mechanism to link to one of these from a commit
c) a policy mechanism to set a repo to only allow SHA1-hashed-objects that a reachable from a preconfigured SHA-256-hashed commit
Replies
Hm but the date is stored inside of the commit. The only way we can know that a commit's date is authentic... is through its hash. If I can forge commits with any SHA1 hash at will, I can make a repository whose head commit has the same SHA1 as the one in torvalds: /linux but where any commit was replaced by a malicious commit with the same SHA1 and a fake date. You have no way to detect that my repo is inauthentic other than through a deep history comparison. The whole idea behind a merkle tree is that just checking the hash of the top is sufficient to know the identity of the whole tree.
I don't know what the solution is, but I'm inclined to believe that any repo with a single SHA1 commit is as weak as a repo with all SHA1 commits.
Emily's talk does a pretty good job of summarizing the issues with intermixing the hashes: https://youtu.be/eJJp0RE7cd4