Subject: Re: how to (integrity) verify a whole git repo

From: Jonathan Nieder <[email-masked]>
Date: Mon, 20 Apr 2020 23:53:01 -0700

Hi Christoph,

Christoph Anton Mitterer wrote:

> How to cryptographically verify the integrity of a whole git repo (i.e.
> all it's commits/blobs/etc. in the history?

This happens automatically as part of fetch.  When you fetch, the
objects' content is transfered over the wire but not their names.  The
name of each object is a hash of its content.  Thus, whenever you
address an object by its name, you are using its verified identity.

> Assume e.g. I have the kernel sources and want to do some bisection.
> One has also retrieved Linus' and GregKH's key via some trusted path
> and assumes that SHA1 is more or less still safe enough ;-)
>
> 1) Of course there is git verify-tag and verify-commit which are signed
> with the GPPG, but these alone check, AFAIU, only the respective
> tag/commit.

Tag and commit object content include the object ids for the objects
they reference, so (assuming we are using a strong hash) their name
is enough to verify all content reachable from them.

In other words, it's a Merkle tree.

> How to check everything else? Is it enough to git fsck --full?

fsck is helpful for checking that objects are valid --- that they
don't reference any objects you don't have, that their format is
correct, and so on.  So it's good to run (or you can use the
transfer.fsckObjects setting to run fsck as part of the clone or fetch
operation).

Thanks and hope that helps,
Jonathan