bug#19479: Package manager vulnerable

[Noam Postavsky <npostavs@gmail.com>]

Stefan Kangas <stefan@marxist.se> writes:
>
>> One more feature: include in each version of archive-contents a hash
[...]
> Does anyone understand how this would improve security in our case?
> AFAIU, it can help with APT since they support distributing package
> metadata in several files.  ELPA uses only one file, so I'm not sure it
> would make much of a difference?

Not entirely, but there's a bit more detail on the emacs-devel thread
linked from the OP:

    One final feature that isn't necessary for preventing any of the 
    vulnerabilities above, but still is helpful to make the historical record even 
    more clear, is to include in each version of archive-contents a hash (and 
    length) of the previous version of that file. This further constrains an 
    attacker who has compromised the elpa key; he can still launch attacks, but 
    it's harder to keep the attacks secret for very long, since he's forced to 
    cause a fork in what's supposed to be a linear hash chain.

I think the idea is that if the attacker has the signing key and sends
out a bad version of archive-contents, it will be revealed as soon as
the victim gets a "good" version, since its previous-version hash won't
match.  Except that only works if the user can expect to get all
versions of archive-contents, so maybe I've missed something.