alt Hacker NewsBecause you're not killling two birds; you're not killing the security bird with a better content hash.
A SHA-256 sum, though very good, only assures you with great confidence that you're looking at the same thing you looked at before, or that someone else is looking at elsewhere.
It is not a digital signature, and we don't want digital signatures to serve the role of content hashes.
Speaking of signatures, we have support for them in Git; you can use gpg to sign commits, and set it up to be done automatically.
Nobody is going to fake your commit such that the fake has the same SH-1 hash and your GPG signature.
The worry there is that the key holder (whether the legitimate one, or a malicious party who got a hold of the key) somehow does this: creates a new commit, signed with their key, which somehow has the same SH-1 as an existing signed commit. The git hash includes the GPG signature, so there is a significant layer of difficulty there which is likely harder than faking an unsigned SHA-256 commit.
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Please educate yourself what a merkle tree is. It's a well understood building block of various security systems, including certificate transparency (which explicitly uses sha256).
You refer to PGP signed Git objects, but you also argue:
> Git hashes are not supposed to be a security mechanism
Guess what the Git PGP signature is signing.
Dude... please bow out gracefully...
The attack is I pre-author `Makefile => foo: echo "hello"; bar: echo "world"` along with `Makefile => foo: echo "hello"; bar: rm -rf / ; /* $ELDRITCH_SHA1_SPIRITS_GO_HERE */` that both hash to `ff1234...`
I then prepopulate the repo with `echo "hello"`, wait 6-9 months, then submit a commit for `echo "hello" ; echo "world"` and keep (in my back pocket) the alternate implementation that also includes $ELDRITCH_SPIRITS to force a collision and MY predetermined change in functionality.
I then have free choice as to whether I serve them "hello world" or "hello && rm -rf", and THAT's the plausible problem to avoid: the ability to "cloak" content anywhere within the repo if you have enough $ELDRITCH_SPIRITS and GPU's.
You have _really_ good points, but are woefully confused. The proper answer is (would have been) to include `tree ff12354...` along with `tree-sha256 abc123456789...` for another 20 years along with a `[git.hash_strictness]: default/lazy/strict`, and some oddball `git-rerere` type packfile extension which lets you map `sha1:ff1234... => sha256:abc123456789...` "transparently" rather than the horrific situation you're laying out (correctly!) that forks the ecosystem in to "longhash" and "shorthash" when most repos don't even care in the end.