Re: experimental .deb packages using "signed-by"

[Graham Percival <gperciva@tarsnap.com>]

Apologies for being unclear here: we have an experimental
tarsnap-archive-keyring package, but no new tarsnap packages.

If tarsnap-archive-keyring 2021.09.29 installs without any errors or warnings,
then all is well and there is nothing further to test.

Cheers,
- Graham

On Wed, Oct 13, 2021 at 11:45:38AM -0700, Graham Percival wrote:
> We have experimental packages and instructions for .deb packages which support
> "signed-by" repositories.  This is relevant for people using Debian, Ubuntu,
> Mint, ElementaryOS, Pop!_OS, etc.
> 
> The old method (using apt-key) is marked as deprecated, and is planned to be
> removed from new distributions in mid-2022.  As far as I'm aware, any existing
> installation should be fine in the future -- this isn't something that they
> would change in the middle of a stable release's lifetime.  (For example, if
> you're running Ubuntu 18.04, don't worry.)
> 
> We're currently testing:
> - a new tarsnap-archive-keyring package.
> - installation instructions which depend on the user's version of apt.
> https://www.tarsnap.com/pkg-deb.html#experimental
> 
> If there's no reports of problems, we'll migrate them to the real packages in a
> week.
> 
> 
> History
> - before apt 1.1~exp9 2015-08-18, gpg keys for third-party
>   repositories had to either be added to /etc/apt/trusted.gpg directly,
>   or added to /etc/apt/trusted.gpg.d/ as a separate file.  These keys
>   are completely trusted by the system.
> 
>   Hypothetically, if a third-party repository includes a package called
>   "passwd" which has a higher version number than the normal "passwd"
>   package, the distro would happily install the third-party package.
>   I leave the security implications of this as an exercise for the reader.
> 
> - after apt 1.1~exp9 (related stable release: apt 1.1 2015-11-26),
>   repositories entries in sources.list could be marked as "signed-by".
>   For example,
>     deb http://pkg.tarsnap.com/deb/stretch ./
>   turns into:
>     deb [signed-by=/usr/share/keyrings/tarsnap-archive-keyring.gpg] http://pkg.tarsnap.com/deb/stretch ./
>   (all on one line)
> 
>   As a result, the system no longer needs to completely trust
>   third-party keys; a malicious third-party "passwd" package would not
>   be viewed as an upgrade to the base system "passwd" package.
> 
> - apt 2.1.8 2020-08-04 marked "apt-key" (which manages
>   /etc/apt/trusted.g) as deprecated; scheduled for removal in Q2/2022.
> 
> 
> What does that mean for us?
> 
> - people using old (2015 or earlier) distros still need the
>   tarsnap-archive-keyring package to write the keyring to
>   /etc/apt/trusted.gpg.d/
> - people using the most recent (2021 or later) distros should use the
>   "signed-by" method for new installations.
> - people using distros between 2016 and 2021 could use either method for
>   new installations.  (We will tell them to use the newer method.)
> - if at all possible, people with existing tarsnap installations should
>   not need to know about any changes.
>   (i.e. `apt-get update && apt-get upgrade` should just work.)
> 
> Our solution for the tarsnap-archive-keyring packate is:
> - if the system already has a
>     /etc/apt/trusted.gpg.d/tarsnap-archive-keyring.gpg
>   then copy the new archive keyring over that existing file.
> - otherwise, don't write to /etc/apt/trusted.gpg/
> 
> This requires the use of postinst / postrm scripts, which is discouraged by the
> Debian New Maintainer's Guide, but I can't see an alternate solution which
> would work for new installations and existing installations at the same time
> (without requiring existing users to manually edit files in /etc/).
> 
> 
> For more details, here's the initial discussion:
> https://github.com/Tarsnap/tarsnap/issues/475
> and here's the commit in the tarsnap-public-keys repository:
> https://github.com/Tarsnap/tarsnap-public-keys/commit/cc32d2b5fc2c3e955efe651dbeb65a6c708b8fab
> 
> Cheers,
> - Graham Percival