Re: [ietf-smtp] Email explained from first principles

[Dave Crocker <[e-mail]>]

On 5/24/2021 7:03 AM, John Levine wrote:
> It appears that Kaspar Etter  <[e-mail]> said:
>> 2. List-Name header field: Mailing lists shouldn’t rewrite the messages of others and break DKIM signatures in the process.
> 
> Sorry, but this shows some serious misunderstandings about both DKIM and mailing lists.
> 
> DKIM is a transport signature, which in this case shows that the message was sent from the author
> to the mailing list system.  

Since DKIM is often misunderstood -- and especially with beliefs that go 
far beyond what it actually does -- it is important that we be very 
careful in its description.

Although DKIM is, indeed, applied and interpreted by operators rather 
than users, it is /not/ a transport mechanism.

DKIM does not know or care about sessions or transport (for any 
definition of transport.)  DKIM is an object-level mechanism. At an 
architectural level, it is comparable to OpenPGP and S/MIME, though of 
course its semantics and algorthms are wholly different.

In technical terms, DKIM actually /can/ be created and processed by 
end-user software.  That it isn't is a matter of operational 
preferences, rather than technical design.

Further, DKIM does not say anything at all about the author.  Nothing. 
Nada.  Some signers have stringent policies that well might permit 
making assessments relative to the author, but that is fully and 
completely outside the DKIM specification.


> List apply their own DKIM signature on the mail they send.
> Mailing lists have been editing messages for 40 years, long before anyone
> ever thought of DKIM or DMARC. 

Yup.

Any attempt to pretend that 'we' can dictate much to the list 'them' has 
proven to be a non-starter.  For ever.


> The whole point of ARC is to provide recipient systems with info to help recognize when they should
> ignore DMARC and deliver mail from lists and other legitimate senders that don't happen to match the
> assumptions that DMARC makes.

Yeah.  No.  It does not say you can ignore DMARC.

Rather it says that the list that broke DMARC is reporting what their 
DMARC evaluation was.  If you trust that list's reporting, then indeed 
you can very much pay attention to DMARC.

That is, the main motivation for ARC is to provide a plausible basis for 
paying attention to DMARC, even when its underlying authentication 
mechanism are broken by the time they show up at the receiver.

d/


-- 
Dave Crocker
Brandenburg InternetWorking
bbiw.net