ietf-dkim
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Re: [ietf-dkim] Seriously.

2008-01-24 09:36:52
The protocol consistency must be consistent for the entire x822.From domain
list.  At best, we can recommend that the signer use the "primary" domain
it is signing for, as the first address in the list. This might require
reordering of the list when the message is first submitted for signing.

Example of reordering (optional optimization)

    From: A, C, B, D

A new message was created by a user B and he wishes to add co-authors A, C
and D to the From: header.   For some odd-ball, but still technically legal
reason, the user B address is not the first:

Nonetheless, User B submits it to his/her MSA and the MSA signing for
domain B can reorder this prior to signing:

    From: B, A, C, D
    Dkim-Signature: d=B


+1



  +-[ NOTE ]+-------------------------------------------+
  |                                                     |
  | The MSA may also want to check the SSP for domains  |
  | A, C and D to make sure they are not DKIM=STRICT.   |
  | See Note1 below on this.                            |
  |                                                     |
  +-----------------------------------------------------+

I am not so sure about this, please see my reasoning below


As the logic is currently defined, a 3PS (3rd Party Signature) is when the
DKIM d= domain is not matching the x822.From domain part.


I believe that 3rd party signing and this issue are separate.

So in this case, the signature d=B helps by checking the primary domain B
regardless of the From: list order.

Only if d=B and there has been a reorder. Without the reorder, the
responsible party (at least in the readers eyes) is A.


The Sender:, if provided, can possible add "weigh" if it was also pointing
to domain B.

    Sender: B
    From: A, C, B, D
    Dkim-Signature: d=B

and under this "all" matching case, there would be no need to reorder the
From: list.

o Missing Signature

Of course, the situation is when we don't have a signature and we want to
find what policies apply to the unsigned message.

I don't think we can trust relying on Sender: especially if the domain is
not among the From: list.  So at best, an verifier can possible choose to
use Sender: only if it matches and it helps define the order of the
preferred lookup.

I believe that we should say that in the absence of a d= the first
from address will be the default author.


In all cases, we can not violate the basic principle in SSP From: domain
requirement - the single From: domain logic must be consistent with a
multiple From: domains logic.

Note #1:

What rule is necessary for multiple SSP policies?


None. Regardless of the policies of the other From: addresses, if a d=
exists and there has been a reorder, the author is, at this point,
defaulted to the first address. Anything else and we are going to
break VERP implementations.

This question pertains to how mixed SSP policies are interpreted.

In order to be protocol consistent, IMO, any domain exposing a DKIM=STRICT
or DKIM=ALL policy can not be violated by other mixed restrictive policies.

I think we need to look at the mixed possibilities in order to get a handle
on this multiple From: domains SSP logic. Here are few examples:

Example 1:

  From:  A, B, C
  DKIM-signature: d=A

     A -->  DKIM=STRICT
     B -->  NO SSP (DKIM=UNKNOWN)
     C -->  NO SSP (DKIM=UNKNOWN)

  Issues:

     I see no Issue. Perfect SSP/DKIM protection. If the message
     was not signed, then it would be viewed as suspicious.

Agree


Example 2:

  From:  A, B, C
  DKIM-signature: d=B

     A -->  DKIM=STRICT
     B -->  NO SSP (DKIM=UNKNOWN)
     C -->  NO SSP (DKIM=UNKNOWN)

  Issues:

     Domain A is unprotected. This message should be suspicious.

Disagree-
There should be a reorder in this case where after processing:

From: B,A,C
DKIM-signature: d=B
A is no longer the author.


Example 3:

  From:  A, B, C
  DKIM-signature: d=A

     A -->  DKIM=STRICT
     B -->  DKIM=ALL
     C -->  DKIM=STRICT

  Issues:

     Can't have two DKIM=STRICT?

     One consideration might be, maybe as long as a valid
     signature was found any of the STRICT domains, in this case
     D=A matches one of the STRICT domains, this this may be an
     exemption.


This is ok. The author is correctly identified and checked.

     However, this would be a LOOPHOLE if we allow this.

Example 4:

  From:  A, B, C
  DKIM-signature: d=B

     A -->  DKIM=STRICT
     B -->  DKIM=ALL
     C -->  DKIM=STRICT

  Issues:

     Even though domain B allows anyone to sign, can it sign
     on its own behalf and still use strict co-domains?

     If we allow this, then we have a loophole.


Disagree-
If we allow reordering, it would then be allowed.


--
Sincerely

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



Regards,
Damon Sauer
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