Re: [dmarc-ietf] Alternative draft text for draft-ietf-dmarc-arc-protocol

[Gene Shuman <[e-mail]>]

I've taken a look at the proposed draft and have a few notes as well.

4.  The currently specified limits on i= are not included MUST >10, SHOULD
> 50, etc

5.1 - In the current draft, it's mandated that AMS must use relaxed header
canonicalization, but that's missing from the proposed draft

5.2 - I'm a bit confused by the comment noting the importance of i=2.  What
is it that you're intending there?

5.3.1 - typo:  one of three possible values: -> one of *four* possible
values

7.2 - It may be worth elaborating more on the possible ways in which
cv=invalid can arise, if not here, maybe somewhere else

7.4 - In general I prefer this to the psuedo code in the current draft, but
I think it could still use a bit of work.  In particular, sections C-H are
exactly describing how to validate a DKIM signature and seems somewhat
unnecessary. Is there any particular reason you decided to include this, as
opposed to just relying on the DKIM spec for this?

7.5 - typo: no -> all

In general though, I agree with Brandon, the proposed draft definitely
makes some things clearer, which I think is a step in the right direction.

=Gene


On Tue, May 9, 2017 at 2:04 PM, Brandon Long <[e-mail]> wrote:

> In 5.1 defining the AMS, you say that it should cover DKIM-Signature and
> AuthRes headers.  In particular, AuthRes headers are expected to be removed
> by ADMDs, especially if the message transits the same ADMD multiple times.
> Also, the information in the AuthRes header is superseded by the ArcAuthRes
> header.  Including it means an arbitrary AMS breakage for something pretty
> minor, so I would recommend to not include it.
>
> Our implementation explicitly blacklists that header.
>
> I know some mailing lists also strip the DKIM-Signature header, but since
> they are likely to break the AMS anyways, that's less important.  I'm not
> sure what the benefit is to including it, but it seems harmless.  In
> particular, if the DKIM-Signature still passes, then the ARC isn't adding
> that much, and removing the DKIM-Signature header doesn't mean all that
> much either since it's validity was already assessed and that assessment
> included in the AAR.  We don't blacklist the DKIM-Signature method in our
> implementation, but I don't understand the advisement.
>
> You also talk about "responsibility".  I'm not sure that's how I would
> describe it.  An ARC hop is documenting that a message passed through it,
> and that it evaluated the authentication of the message.  The only
> responsibility of a hop is to correctly validate the SPF/DKIM/ARC
> information, there is no ownership implied over the message itself.
>
> With AMS, you can answer the question: which ADMD is the last ADMD to have
> modified the message.  I guess in that sense, the last modifier is
> "responsible" for the current state of the message... but that kind of
> means that the AMS of previous hops allows them to disown responsibility
> for the current state of the message...
>
> 5.2 - should we point out that there should be only one of these per hop?
> The openspf/dkim/dmarc implementations tend to add separate AuthRes headers
> for each evaluation, but ARC requires those to be a single instance.
>
> 5.3.1 - none as defined as "arrives at an MTA from an MSA", perhaps my
> understanding of those terms is slightly odd, but I would think that an MSA
> usually uses an MTA to actually send the message, and it isn't that
> "sending" MTA that's the first hop, it should be the first "receiving"
> MTA.  I mean, that's usually the point at which the DKIM signature is
> applied, and the SPF would be "from" there, not based on the location of
> the MUA.
>
> There are some missing pieces here, corresponding to the current draft
> sections 5.4 (alternate signing algorithms), 6.4.3 (arc email
> authentication method for AuthRes), 6.4.5 for dmarc xml.  I see that the
> arc is included in your IANA section, not sure if the call out outside of
> the definition is necessary or not.
>
> Overall, I think your draft makes some things clearer, and some things in
> the original are clearer.  It's worth looking into either combining or
> choosing.
>
>
> Brandon
>
> On Thu, May 4, 2017 at 12:56 AM, Murray S. Kucherawy <[e-mail]>
> wrote:
>
>> Colleagues,
>>
>> As I progress (slowly, alas) toward completing my sample implementation
>> of OpenARC, I've found myself taking a lot of notes about the current
>> draft.  This has helped me make progress; in some cases it became things I
>> posted to the list, and in others it was just to help or confirm my
>> understanding of the protocol.
>>
>> I have developed this enough to become a fairly comprehensive alternative
>> text to the current draft.  I find the layout of this version to flow
>> better for my own purposes, and in a few places I've tried to clarify some
>> of the material by rewriting chunks of it.  None of this is meant to assert
>> that the current draft is deficient; I've just found it to be a helpful
>> exercise for me.
>>
>> I offer it here to the WG as a contribution; the WG of course is free to
>> use some, all, or none of it as it wishes.
>>
>> http://blackops.org/~msk/draft-kucherawy-dmarc-arc-base.txt
>>
>> If it would be more helpful to post this as an I-D, please let me know.
>>
>> -MSK
>>
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