Subject: Re: [ietf-dkim] Mailing lists and s/mime & dkim signatures - mua considerations

From: Wietse Venema <[email-masked]>
Date: Tue, 24 Aug 2010 09:45:20 -0400 (EDT)

Hector Santos:
> IMO, it is these statements that continues to raise confusion and 
> raise the barrier of industry wide adoption that includes the general 
> population of MTA developers and operators from tiny to small to even 
> large.

As a part-time MTA developer I am not confused. The DKIM signature
provides a simple piece of trace information ("I handled this mail")
that is cryptographically bound to some header and body content.

The receiver can use this trace information for any purpose that 
she deems suitable. What seems to confuse some people is that the
"using" part is entirely decoupled from the "signing" part, and 
that the signer has no control over what "use" is made.

In my view, this decoupling is beneficial (DKIM as an enabler). For
others, this open-ended design is a shortcoming (batteries required).
 
        Wietse
_______________________________________________
NOTE WELL: This list operates according to 
http://mipassoc.org/dkim/ietf-list-rules.html

---

From: "Mark Delany" <[email-masked]>
Date: 24 Aug 2010 10:23:58 -0700

> that is cryptographically bound to some header and body content.

Yes. And that the obverse is possible: "I didn't handle this mail".

As Jon Callas is fond of saying, you know a protocol is a success when
it's abused in ways you never thought possible. The bi-laterals that
others have discussed are a small example of this.

Jon got it right: we don't need to know all of what is possible with
so general a component as DKIM.

My personal motivation, going back some seven years now, was about
tools for putting credibility (back) into the email system. Clearly
this is far from the only motivation across the population of DKIM
developers. Varying motives don't necessarily mean varying tools.

Mark.

---

From: Steve Atkins <[email-masked]>
Date: Tue, 24 Aug 2010 12:08:26 -0700

> Yes. And that the obverse is possible: "I didn't handle this mail".

I don't see how DKIM can provide the obverse - the obvious way
is for a sender to assert that all their mail has a DKIM signature,
but that fails when the DKIM signature breaks in transit. Is there
a clever trick I'm missing?

> developers. Varying motives don't necessarily mean varying tools.

DKIM allows you to attach a token to an email. That's such a generally
useful thing it's no surprise people are finding a range of uses
for it.

Cheers,
  Steve

---

From: "Mark Delany" <[email-masked]>
Date: 24 Aug 2010 13:30:08 -0700

> a clever trick I'm missing?

So you're saying it can provide the obverse; you just don't like the
failure modes. Perhaps surprisingly, the failure modes are exactly
what attracts some folk to DKIM.

We also have to be patient. When DK was first discussed, folk said
that it was impossible because MTAs routinely made arbitrary changes
to the payload, but that's no longer true. Folks also said that the
mainstream players would never get on board, but that's no longer
true.

A fork-lift change was never going to occur overnight so we just have
to keep chipping away.

Mark.

---

From: Steve Atkins <[email-masked]>
Date: Tue, 24 Aug 2010 14:07:16 -0700

> what attracts some folk to DKIM.

The failure modes of DKIM are all false negatives, which is fine.
(Concrete example: I've not seen any suggestion that there
has been mail that was signed by paypal.com that wasn't
from paypal, and I suspect someone would have mentioned it).

The failure mode of anything that's based on the inverse of DKIM
are all false positives. While you can assert "I didn't handle this
mail", that's not an assertion that can be trusted, in general.
(Concrete example: I've seen email that paypal.com
asserted was not sent by them, based on lack of DKIM signature,
which actually was sent by them). I'd be surprised if that failure
mode attracted people to DKIM, and I'd be interested to hear
more about why it did.

"I'm pretty sure I didn't handle this mail" is a an assertion that
you can make in this way, though, and the level of certainty is going
to be high enough to be of value in some cases. But it's not
quite the same thing as being able to say "I didn't handle this
mail".

Cheers,
  Steve

---

From: Hector Santos <[email-masked]>
Date: Tue, 24 Aug 2010 19:35:41 -0400

Mark Delany wrote:

>> Steve Atkins:

+1

-- 
Hector Santos, CTO
http://www.santronics.com
http://santronics.blogspot.com