Re: [dmarc-ietf] A policy for direct mail flows only, was ARC questions

Alessandro Vesely <vesely@tana.it> Thu, 26 November 2020 08:59 UTC

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From: Alessandro Vesely <vesely@tana.it>
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Date: Thu, 26 Nov 2020 09:59:00 +0100
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Subject: Re: [dmarc-ietf] A policy for direct mail flows only, was ARC questions
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On 25/11/2020 20:16, Michael Thomas wrote:
> On 11/25/20 11:11 AM, Alessandro Vesely wrote:
>> On 25/11/2020 19:24, Jesse Thompson wrote:
>>> On 11/25/20 11:30 AM, Alessandro Vesely wrote:
>>>> Without resorting to ARC, it is still possible to validate author domain's 
>>>> signatures directly if the MLM just adds a subject tag and a footer, like, 
>>>> for example, this list does.   While ARC solves "deep" forwarding problems, 
>>>> which may arise in the context of email address portability, MLM 
>>>> transformation reversion solves the simpler mailing list problem, including 
>>>> reverting munged From:'s.
>>>
>>> I agree that ARC isn't really needed to do this (trust the last hop from the 
>>> MLM and determine the original authenticity from the MLM's perspective)
>>
>> I didn't mean to trust the MLM.  I meant remove the subject tag and the 
>> footer, then the original DKIM signature verifies.  See:
>> https://datatracker.ietf.org/doc/draft-vesely-dmarc-mlm-transform/
> 
> When I was at Cisco, with l= and some subject line heuristics I could get 
> probably like 90+% verification rate across the entire company, a company that 
> uses external mailing lists a lot. Definitely not 100% though.


DKIM itself is not 100%.  You always have lines beginning with "From " or 
occasional autoconversions.

l= doesn't cover multipart/alternative nor Content-Transfer-Encoding: base64. 
In addition, the DKIM spec discourages its usage and suggests that "Assessors 
might wish to ignore signatures that use the tag."


Best
Ale
--