Subject: Re: [ietf-dkim] Case for ADSP "dkim=except-mlist"

From: Dave CROCKER <[email-masked]>
Date: Fri, 25 Jun 2010 17:34:41 +0200

On 10/16/2009 3:33 PM, Michael Deutschmann wrote:
> I'd like to more emphatically state the case for adding a
> "dkim=except-mlist" policy to ADSP.  It will soon become a practical
> issue for me, since my mailserver software (Exim) is going to support
> DKIM in its next version.

Simple question:  why?

THe hard part is that I'll ask you to describe the end-to-end scenario that 
requires this and has Internet scale utility and to do this using no technical 
language.

Remember that DKIM's job is to pass an ID that can be used for reputation.  It 
isn't it's job to 'certify' the validity of message content.

d/

-- 

   Dave Crocker
   Brandenburg InternetWorking
   bbiw.net
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---

From: Michael Deutschmann <[email-masked]>
Date: Fri, 25 Jun 2010 16:32:30 -0700 (PDT)

> > DKIM in its next version.
That's quite an old message you are responding to.

I do not expect any selfish benefit to publishing DKIM/ADSP, even were the
mailing list problem completely solved.  Someone mentally lazy enough to be
tricked in the absence of DKIM would probably be fooled by a sender using
the expected full name in combination with an e-mail address he controls.

But I'm not entirely selfish -- I would consider publishing merely to help
my fellow hacker with his spam control.  The point is not to protect him
from being *tricked*, but to help him filter out noise at his computer
before it reaches his brain.  For this purpose a low FP rate is essential.

Because of the confusion over what "all" and "discardable" actually mean,
when choosing an assertion I must consider the worst case for FPs -- that
the recipient thinks both mean "everything unsigned is *definitely* a
forgery and thus noise."

In turn, the only policy I can publish is "unknown", which provides no help.
In order to publish a stronger policy, I would need to abstain from mailing
lists and Usenet (because some newsgroups are gatewayed to mail).  Sorry,
but my charity doesn't extend that far.

But I could publish "except-mlist" right now.  And it happens that should
another site publish "except-mlist", that would be as helpful to my spam
defense as if they published "discardable".

I could reject incoming messages with missing/broken signatures when the
assertion is correctly except-mlist, and be guaranteed no *additional* false
positives.  The only way this test could false-trigger is if I forget to
whitelist a mailing list, and if that happened every message from the list
would already be doomed by my anti-Bcc: filter.

---- Michael Deutschmann <[email-masked]>
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