New supply-chain security tool: backseat-signed

Adrian Bunk bunk at
Wed Apr 3 02:21:40 UTC 2024

On Wed, Apr 03, 2024 at 02:31:11AM +0200, kpcyrd wrote:
> I figured out a somewhat straight-forward way to check if a given `git
> archive` output is cryptographically claimed to be the source input of a
> given binary package in either Arch Linux or Debian (or both).

For Debian the proper approach would be to copy Checksums-Sha256 for the 
source package to the buildinfo file, and there is nothing where it would
matter whether the tarball was generated from git or otherwise.

> I believe this to be the "reproducible source tarball" thing some people
> have been asking about.

The lack of a reliably reproducible checksum when using "git archive" is 
the problem, and git cannot realistically provide that.

Even when called with the same parameters, "git archive" executed in 
different environments might produce different archives for the same
commit ID.

It is documented that auto-generated Github tarballs for the same tag 
and with the same commit ID downloaded at different times might have 
different checksums.

> This tool highlights the concept of "canonical sources", which is supposed
> to give guidance on what to code review.

How does it tell the git commit ID the tarball was generated from?

Doing a code review of git sources as tarball would would be stupid,
you really want the git metadata that usually shows when, why and by
whom something was changed.


"This requires some squinting since in Debian the source tarball is 
 commonly recompressed so only the inner .tar is compared"

This doesn't sound true.

> Let me know what you think. 🖤
> Happy feet,
> kpcyrd


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