Re: Commit SHA1 == SHA1 checksum?

[Konstantin Ryabitsev <konstantin@linuxfoundation.org>]

On Mon, Feb 07, 2022 at 02:49:16PM -0800, Junio C Hamano wrote:
> Given a signed commit or a signed tag that points at a commit, your
> enhanced "git archive" would create a .tar file with the contents of
> the tree object, and adds copies signed objects that tells what tree
> object the archive ought to have.  E.g. if you start from a signed
> tag, "git cat-file tag $tag" output would allow you to learn the
> object name of the tagged object, and to verify the PGP signature
> embedded in the tag, but it is likely that the tagged object is a
> commit, not a tree, so you'd also need to include "git cat-file
> commit $tag^{commit}". 

Correct, unless it's a snapshot of a signed commit, not of a tag (cgit, for
example, makes them available for download). In this case we only need to have
the cat-file contents of the commit.

> So you'd store the raw contents of the tag
> (so that we have a hash-protected record of commit object name), and
> the commit (so that we have a hash-protected record of tree object
> name).
> 
> You as the recipient will find these in the tarball:
> 
>  - the files that are supposed to be the contents of tree X.
> 
>  - the raw contents of the commit C that is supposed to record the
>    tree X.
> 
>  - the raw contents of the tag T that is supposed to point at the
>    commit C.
> 
> Starting from the contents of tag T, which is PGP signed, you know
> that the signer wanted to call commit C with the name of the tag T.
> Then the raw contents that alledgedly are from commit C, you can
> "git hash-object -t commit" it to verify that it indeed hashes down
> to C (hence, it what the signer wanted to give you), and find the
> name of the tree object X the commit records.  And when you added
> all the blobs contained in the tarball (and nothing else) to the
> index and ran write-tree on the resulting index, you would know what
> tree object the tarball contained, and if it hashes down to X, you
> know that the cryptographic hash chain starting from PGP signature
> on T attests that that tarball matches what the signer wanted you
> to have.

Exactly right. It would be slightly more complicated for things like openssh
signatures, since then you have to worry about where the allowed_signers file
comes from, but these are implementation minutae. Even if we start with just
support for PGP signatures, that would already be a great improvement over
where things are with snapshot downloads right now.

Best regards,
-K