Securing the software distribution chain

[Justus Winter]

Hello :)

doing some packaging lately I noticed a weak link in Guix'
authentication chain.  Artifacts downloaded by Guix are authenticated
using a hashsum included in the packaging definition, and 'guix
download' will compute this hashsum over artifacts, but the step of
authenticating the artifact is a manual one, mentioned in the
'Submitting Patches' checklist:

> Before submitting a patch that adds or modifies a package definition,
> please run through this check list:
>
> 1. If the authors of the packaged software provide a cryptographic
>    signature for the release tarball, make an effort to verify the
>    authenticity of the archive. For a detached GPG signature file this
>    would be done with the gpg --verify command.

For me, this is a tooling problem.  'guix download' should authenticate
the downloaded artifact before it computes and prints the hashsum.
There are two problems to solve: It needs to locate the signature, and
it has to know the set of cryptographic identities eligible to create
the signature.

For some transports, like git, locating the signatures is not a problem,
but for http, there is just no standard on where the detatched signature
is located, or even what data it is computed over (for example,
kernel.org signs the uncompressed tarball).

So I think two things need to happen before this step can be improved:
The package metadata should include the URL of the signature and a set
of cryptographic identities eligible for signing the artifact.


Thinking a bit more about having a hashsum as part of the packaging
definition, it seems to me that this is a bit of a modelling error.
Because there is no standardized way of authenticating a source
distribution, Guix defers this step to the packager.  And if there is no
way to authenticate an artifact (because upstream doesn't provide
signatures), we at least get TOFU, i.e. the assurance that any user
gets the same artifact as the packager.

This doesn't seem terribly problematic, but it doesn't support parts of
the artifact changing, because in this model, this cannot be
distinguished from an attack.  Is there a way an artifact may change in
a valid way that Guix (and other distributions) may want to support?

We believe there is.  We propose to solve the problem of locating the
signatures by bundling them with the source distribution.  Instead of
using a detached PGP signature, we want to distribute the source as a
signed PGP message.

Now, if you compute a hashsum over such an artifact, you are in effect
notarizing the signatures in the message and the message payload.  If
the developers add a signature to the message, the hashsum changes and
your notarization breaks.

The preferred way to support this is to not verify that the hashsum over
the artifact matches, but to verify the PGP signatures over the payload
using the set of eligible signing keys in the package metadata.


Thoughts?
Justus

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