[Reproducible-builds] Preliminary review of dpkg-genbuildinfo

[Daniel Kahn Gillmor]

On Fri 2015-02-13 03:36:20 -0500, Hans-Christoph Steiner wrote:
> I think it would be much simpler to just have the single package signature
> that is embedded in the package file itself, like Android APKs and Java JARs.
>  Since the package is built reproducibly, anyone who builds it can just copy
> the canonical signature into their copy of the package they just built, and
> it'll match the sha512sum of the signed apt metadata.

It seems like you're saying everyone will be able to agree on which
signing authority is "canonical" for any given package.  I'm not
convinced that's the case.

> The big question there is determining the where the canonical
> signature happens.  It seems like it should be the official Debian
> build process, since it is the only process guaranteed to be the same

Even though i'm personally likely to treat Debian as "the canonical
source" i care about, i don't want it to be that way.  I would like
Debian to be able to be a downstream as well as an upstream (see the
work feeding back into debian from ubuntu, for example); if a .deb
package can contain an internal signature, and i'm looking at a given
.deb in isolation on my debian system, i want to see it signed *by
debian*, not by whoever happened to produce it first.  Otherwise, it's
not clear to me that the embedded signature is useful to me as an end
user at all.

> Another question is whether dpkg checks whether the signers match when
> upgrading, like the Android model (a package can only be upgraded by
> another package signed by the same key).  This would be nice, but
> seems optional and hard to do in Debian.

Maybe this is the question we need to answer to move the discussion
forward to make sure we're taking the desire for embedded signatures
into account when thinking about reproducible .debs: how exactly do we
expect an embedded signature to be used/evaluated?  by who, and in what
context?

> I think the .buildinfo file is useful, but for a separate process.  It should
> be the canonical file for running a reproducible build.

I'm not sure what this means.  I'd be very happy if *all* of my debian
packages were reproducible builds, and i could have a way of verifying
it.  I'd consider that more valuable than knowing that all my .debs were
signed by any individual authority.  So if we're really talking about a
tradeoff between signed buildinfo files and signed packages, i'd
certainly prefer signed buildinfo files.

But my proposal was an attempt to let people have both, without forcing
the entire ecosystem to agree on "who is a canonical authority for
package X, without whom a reproducible package is impossible"

          --dkg