Re: [dmarc-ietf] ARC-Seal is meaningless security theatre

["Kurt Andersen (b)" <[e-mail]>]

On Fri, Aug 11, 2017 at 4:54 PM, Bron Gondwana <[e-mail]>
wrote:

> . . . it's a bad idea to sign if you're not modifying, because then
> everybody has to trust you or their chain breaks, even though you didn't do
> anything which required signing.
>

<elided>

>
> In state #1, you don't need a chain of ARC Seal.  You just need each site
> to sign their own AAR and each AAR to include "arc=pass" for the previous
> AMS.  You trust the sites, so you trust them to verify the ARC status on
> ingress.
>

In the current layout, "signing [the] AAR" is done via the AS. We wanted to
keep the AAR as close to the A-R header as we could to maximize leverage of
previous definitions rather than inventing an entirely new one. Initially,
we had intended the AMS to sign over the AAR, but people objected to
signing the AAR within both the AMS and AS scopes.

<elided>

>
> And this is the crux of our disagreement.  Seth thinks it's necessary to
> do more than signing a statement that you believed the message was
> authenticated when you got it, in a way that the next hop can verify your
> signature over your own Authentication Results plus the content of the
> message.  I disagree.
>

 One could replace the AMS with an "egress DKIM" signature, but then you
would still have to decide what to do about alignment on this new DKIM
signature. That's why we built the AMS - to avoid the question of alignment
and have the ARCset as a self-contained "package".

I see the point that you are driving at regarding the claim of "forgery",
but I don't consider that any more or less of a forgery than what the IETF
mailman will do to this message en route to the recipients. Mailman changes
the headers (Subject) and body. Seems like that's about what you've done in
the sample message...but at least you took responsibility for doing so with
ARCset[7] (or someone with the private key for brong.net ;-) ).

--Kurt