Re: [dmarc-ietf] Proposal to adopt ARC documents into the WG (toward phase 2 milestone)

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

On Wed, May 11, 2016 at 11:40 AM, Alessandro Vesely <[e-mail]> wrote:

> On Wed 11/May/2016 19:09:45 +0200 Kurt Andersen (b) wrote:
> >
> > What would an AS[0] assertion provide that would not be already asserted
> by
> > the originator's DKIM-Signature?
>
> Nothing, except that the originator's DKIM-Signature is broken after MLM
> processing.  In that respect, ARC-Seal is similar to weak signatures.
>
> > If AS[1] is untrustworthy (using the term advisedly), but AS[0] still
> > verifies, then presumably the original DKIM-Signature would also still
> > verify and ARC-based information is not needed to have a pass for the
> DMARC
> > evaluation.
>
> If the body was altered the original DKIM-Signature is broken.  If AS(0) is
> good --which is possible since it didn't sign the body-- and rfc5322.from
> matches the AS(0) signer, can we then bypass DMARC validation?  To address
> Brandon's concern, high value targets should never produce an AS(0) in the
> first place.
>

AS[0] will not be "good" in the way you propose because nearly all of the
transformations that will break DKIM will also break the AMS
(ARC-Message-Signature) and, per
https://tools.ietf.org/html/draft-andersen-arc-04#section-5.1.1.5 bullet 3,
AMS must pass for the overall ARC set to be considered valid.

I'd like to respectfully suggest that "bypassing DMARC validation" is
pretty far out of scope for what we've intended with ARC.

--Kurt