Subject: [secdir] SECDIR review of draft-melnikov-smtp-priority-09

From: Chris Lonvick <[email-masked]>
Date: Sun, 11 Mar 2012 18:05:20 -0700 (PDT)

Hi,

I have reviewed this document as part of the security directorate's
ongoing effort to review all IETF documents being processed by the
IESG.  These comments were written primarily for the benefit of the
security area directors.  Document editors and WG chairs should treat
these comments just like any other last call comments.

Overall, the proposal to insert a MT-PRIORITY header appears reasonable. 
I don't think that it opens any security threats that are not noted in the 
document.

I am concerned that this feature can disrupt the DKIM protection as noted 
in section 11.1.  I see the problem here and I can't offer any suggestions 
to fix that.  I think that the authors have done well to document this.

Thanks,
Chris

---

From: Dave Crocker <[email-masked]>
Date: Mon, 12 Mar 2012 09:44:54 -0700

(copying Murray, in case he has further thoughts...)

On 3/11/2012 6:05 PM, Chris Lonvick wrote:
> I am concerned that this feature can disrupt the DKIM protection as noted in
> section 11.1. I see the problem here and I can't offer any suggestions to fix
> that. I think that the authors have done well to document this.

Predictably, the DKIM reference caught my attention.

I think the existing draft text in 11.1 is good.  I think it entirely reasonable 
to leave it alone.

That said, since the concern about broken DKIM signatures is raised it's worth 
at least considering adding more text.  However I think there's a challenge in 
what that 'more' should be.

In general, the world is moving in a direction that is likely to impose 
penalties when an origination DKIM signature is broken.  (I'm not happy about 
that; I think it represents some problematic thinking about DKIM, but it is 
nonetheless the direction things are going.)

The draft might suggest /not/ covering the MT-Priority: field with a signature. 
  There is a common view that DKIM "protects" and or "validates" the fields it 
covers.

Although DKIM obviously provides data integrity between the time of signing and 
validating, DKIM semantics do not offer anything like authentication of 
validation of the data that are signed.  It only validates the authorization of 
the associated identifier (sdid, d=), with the message it is attached to.

As such, having MT-Priority: be outside a signature probably isn't that onerous, 
unless there is a serious threat of in-transit header field modification attacks.

Arguably, it's better to have the agent assigning priority be the one to sign. 
In the case of revising the priority, that's not the originator.  This is only 
problematic for cases that demand origination signatures (usually translating 
into requiring the DKIM sdid (d=) value be the same as, for example, the 
rfc5322.From domain.

There is private pursuit of a specification that allows intermediaries that 
break DKIM signatures to report that they were valid before being broken.  For 
this mechanism to be useful, there needs to be transitive trust between the 
intermediary and the validating receiver.  But, as I say, that's work in 
progress.  Although it sounds appealing, I've no idea how useful it will 
actually prove to be.

d/

-- 

   Dave Crocker
   Brandenburg InternetWorking
   bbiw.net

---

From: "Murray S. Kucherawy" <[email-masked]>
Date: Mon, 12 Mar 2012 16:52:43 +0000

> -----Original Message-----
> From: Dave Crocker [mailto:[email-masked]]
> Sent: Monday, March 12, 2012 9:45 AM
> To: Chris Lonvick
> Cc: [email-masked]; [email-masked]; [email-masked]; Murray S. Kucherawy
> Subject: Re: SECDIR review of draft-melnikov-smtp-priority-09
> 

I agree with Dave's points, and would also add that this draft doesn't need to call out a "right" solution.  It would probably be just fine calling out the pros and cons regarding choosing whether to cover the MT-Priority header field with a DKIM signature, and leaving it to the implementation to decide.

-MSK