Re: [dmarc-ietf] ARC questions

Michael Thomas <> Mon, 23 November 2020 18:49 UTC

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To: John R Levine <>
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From: Michael Thomas <>
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Date: Mon, 23 Nov 2020 10:49:31 -0800
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Subject: Re: [dmarc-ietf] ARC questions
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On 11/22/20 11:56 AM, John R Levine wrote:
> On Sun, 22 Nov 2020, Michael Thomas wrote:
>>> The ARC signature has a sequence number so you can track the chain 
>>> of custody.  You are right that it is similar to the DKIM signature 
>>> but the extra ovehead doesn't seem excessive.
>> Did the wg consider just grafting that onto the DKIM signature itself 
>> instead of having essentially a duplicate signature? Receivers are 
>> already supposed to ignore any tags they don't understand so it 
>> shouldn't hurt backward compatibility.
> ARC is an experiment that came from the people who designed DMARC.  
> It's not a WG product.
> Having adapted the perl DKIM module to handle ARC signing and 
> verification, I can say that the extra signature is not a big deal.  
> If you look at mail coming from large mail systems, they're full of 
> other junk headers and the extra overhead of AMS along with DKIM is 
> not important.
 From what I can tell, the main thing that ARC is doing is binding an 
auth-res to a dkim signature-like thing. But as I recall -- it's been a 
long time -- there were ordering requirements ala received headers for 
where new dkim-signatures and auth-res go in the header. Assuming my 
memory is correct, that means you can reconstruct "what this looked like 
before i messed with it" already by signing the incoming auth-res as 
part of the new DKIM signature.

Is there something more going on here?