Subject: [ietf-dkim] SSP-01 Intro's definition of forgery a bit imprecise

From: Douglas Otis <[email-masked]>
Date: Fri, 9 Nov 2007 16:43:25 -0800

Introduction:

,--
| ... However, some domains may choose to sign all of their
| outgoing mail, for example, to protect their brand name.  It is
| highly desirable for such domains to be able to advertise that fact
| to verifiers, and that messages claiming to be from them that do not
| have a valid signature are likely to be forgeries.  This is the topic
| for sender signing practices.
'--

This statement overlooks messages forwarded by mailing-lists and the  
like where a signature might become invalid.

Perhaps change "claiming to be from them" to "claiming to be directly  
from them".

-Doug
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From: Jim Fenton <[email-masked]>
Date: Wed, 14 Nov 2007 15:50:17 -0800

> Perhaps change "claiming to be from them" to "claiming to be directly

DKIM tries to be as path-agnostic as possible, so the word "directly" is
problematic.  If it goes through a transparent (non-modifying)
forwarder, is it "directly from them"?  Probably not, so this wording
understates DKIM's value.

This case is covered under the wording "likely" in "likely to be
forgeries".  Should it say "more likely to be forgeries" instead?

-Jim

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From: Arvel Hathcock <[email-masked]>
Date: Wed, 14 Nov 2007 18:11:32 -0600

> DKIM tries to be as path-agnostic as possible, so the 
> word "directly" is problematic.

IIRC, "directly" was meant to point out that no trusted third party need be involved in the DKIM key fetching process.  An important point IMO.

Arvel

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From: Douglas Otis <[email-masked]>
Date: Wed, 14 Nov 2007 17:32:46 -0800

> understates DKIM's value.

I take your point.

> forgeries".  Should it say "more likely to be forgeries" instead?

Other factors may affect whether a message is likely to be a forgery.   
A trustworthy mailing list resigning messages (with third-party  
signatures) should not be assumed to produce likely forgeries.  Just  
the opposite would be true.  While a third-party signature might be  
valid, it would not be valid from the narrow perspective of the FROM/ 
i= identity.  I hope to finish a clean-up of the TPA-SSP draft to  
better fit with SSP.  This statement tends to preclude these other  
possible considerations.

Perhaps "claiming to be from them without a valid signature of a  
parent, trusted, or authorized domain are then likely to be  
forgeries."  This would provide some flexibility for possible future  
extensions.

-Doug

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From: "Charles Lindsey" <[email-masked]>
Date: Thu, 15 Nov 2007 11:44:20 -0000

> understates DKIM's value.

My interpretation of "directly" in the above text is that it implies

"if this message arrives without evidence of intermediate  
forwarding/mail-list-expansion/whatever, and its signature is bad/absent,  
then that is a cause for immediate and grave suspicion. But if there is  
evidence of such forwarding, then further investigation of whether such  
forwarding might removed/broken our original signature could be taken into  
account".

So if the forwarder has resigned (or even better certified that the  
original sugnature was good when seen by him) then a site that is prepared  
to trust the forwarder might choose to be less suspicious.

If that is the intention of "directly", then it is probably fine to  
include it (or mayube something more explicit, since "directly" seems toi  
have been misunderstood.

OTOH, my interpretation of "strict" means "please be suspicious if the  
signature is absent/bad even if there is plausible evidence of mangling by  
a forwarder".

-- 
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From: Jim Fenton <[email-masked]>
Date: Fri, 16 Nov 2007 16:03:28 -0800

This whole thing may become a moot point with the new wording of the
second paragraph of the introduction as suggested by Arvel.  It says:

However, the legacy of the Internet is such that not all messages will
be signed.  Therefore, the absence of a signature is not an a priori
indication of forgery. In fact, during early phases of DKIM deployment
it must be expected that most messages will remain unsigned. 
Nevertheless, some domains may find it highly desirable to advertise
that they sign all of their outgoing mail making the absence of a valid
signature a potential indication of forgery.  Without a mechanism to do
so, the benefits of DKIM are limited to cases in which a valid signature
exists and cannot be extended to cases in which signatures are missing
or are invalid.  Defining such a mechanism is the purpose of Sender
Signing Practices.

With the above wording, I don't think we need to put too sharp a point
on the definition of "forgery".

-Jim

Charles Lindsey wrote:
> forwarding/mail-list-expansion/whatever, and its signature is
> bad/absent, then that is a cause for immediate and grave suspicion.
> But if there is evidence of such forwarding, then further
> investigation of whether such forwarding might removed/broken our
> original signature could be taken into account".
>
> original sugnature was good when seen by him) then a site that is
> prepared to trust the forwarder might choose to be less suspicious.
>
> include it (or mayube something more explicit, since "directly" seems
> toi have been misunderstood.
>
> signature is absent/bad even if there is plausible evidence of
> mangling by a forwarder".
>

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From: Hector Santos <[email-masked]>
Date: Fri, 16 Nov 2007 20:41:27 -0500

Hi Jim,

+1, I think the paragraphs are well enough stated.

I think "forgery" is broad enough. If anything maybe change it to:

    a priori indication of a problematic message.

Again, I think its cool, so +1, but if I wrote it, I would probably 
change a few things:

   However, the legacy of the Internet is such that not all
   messages will be signed.  Hence, historically, the
   absence of a signature is not an a prior indication of
   forgery or problematic message.  In fact, it must be
   expected that most messages will remain unsigned.

Note the priori typo fix.  In the last sentence, I think this should be 
expected even during at any adoption level, even full deployment. The 
early deployment clause could erroneously suggest that its a short time 
issue or that DKIM itself is a short term solution during early 
development, which is not the case. In fact, I would suggest that 
maximum benefit and payoff is when you at full (super majority) 
deployment, which will optimize the idea in the next paragraph.

   This legacy consideration opens the door for DKIM to help
   protect domains.  Domain may find it highly desirable
   to expose or advertise the idea that they sign all ......

I took the word "Some" out because I think it is implied.  By having it, 
I think that is only because many are still leery of the "exclusive, I 
sign all" concept. So to me, it sounds like yet another item (double 
talk) attempt to minimize the high potential DKIM/SSP can offer.  If you 
were to follow up with a:

    However, please note, this "I SIGN ALL" idea may not apply to all
    domains[. | because ...]

Then it makes sense to start with Some.

But again, what you have is fine to me, +1 :-)

-- 
Sincerely

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

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From: "Charles Lindsey" <[email-masked]>
Date: Mon, 19 Nov 2007 09:47:17 -0000

> I think "forgery" is broad enough. If anything maybe change it to:

How about s/forgery/malpractice/?