Subject: [ietf-dkim] draft-ietf-dkim-threats-00 Overlooking a practical solution while also recommending a highly unfair solution

From: Douglas Otis <[email-masked]>
Date: Tue, 24 Jan 2006 13:51:30 -0800

---[D: Overlooking a practical solution while also recommending a  
highly unfair solution.]
,---
|4.1.4.  Chosen Message Replay
|
| ... One approach to this problem is for the
| domain to only sign email for clients that have passed a vetting
| process to provide traceability to the message originator in the
| event of abuse.
'---

Unless there is an expectation that individuals obtain their own  
certificates from a trusted authority, individual reputations on a  
cost-free email-address would be completely futile and unfair as DKIM  
does not necessary verify the valid use of an email-address anyway.

Another strategy not mentioned would be establishing a practice where  
incoming signatures are overlaid with verification results.   
Recommending an overlay practice should replace recommending the  
impossible of establishing the reputation for individual email- 
addresses.   There is _no_ means that would be fair without using  
individual CA certificates.  The recipient domain can be fairly held  
accountable for ensuring that incoming signatures are protected using  
signature overlays.  The vetting process would be made when deciding  
whether it would be _safe_ to sign a message destine for a particular  
domain.

Replay abuse can not assume the email-address associated with the  
message had participated.  There is _no_ fair means for holding an  
email-address accountable!  A domain or IP address must always be  
made accountable with respect to any reputation scheme!

-Doug

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---

From: Stephen Farrell <[email-masked]>
Date: Tue, 24 Jan 2006 22:36:20 +0000

Doug,

Douglas Otis wrote:
> ---[D: Overlooking a practical solution while also recommending a highly 
> unfair solution.]
> ,---
> impossible of establishing the reputation for individual 
> email-addresses.   There is _no_ means that would be fair without using 
> email-address accountable!  A domain or IP address must always be made 
> accountable with respect to any reputation scheme!

This appears to be a fairly pure rant. What's the suggested alternate
text? (And here's a hint: I doubt very much that it'd be useful to
suggest defining a new mechanism like the above in this section where
we're basically describing threats.)

Stephen.

---

From: Douglas Otis <[email-masked]>
Date: Tue, 24 Jan 2006 14:56:14 -0800

On Jan 24, 2006, at 2:36 PM, Stephen Farrell wrote:
>> cost-free email-address would be completely futile and unfair as  
>> DKIM does not necessary verify the valid use of an email-address  
>> anyway.
>> Another strategy not mentioned would be establishing a practice  
>> where incoming signatures are overlaid with verification results.   
>> individual CA certificates.  The recipient domain can be fairly  
>> held accountable for ensuring that incoming signatures are  
>> protected using signature overlays.  The vetting process would be  
>> made when deciding whether it would be _safe_ to sign a message  
>> destine for a particular domain.
> This appears to be a fairly pure rant. What's the suggested  
> alternate text? (And here's a hint: I doubt very much that it'd be  
> useful to suggest defining a new mechanism like the above in this  
> section where we're basically describing threats.)

Sorry.  My hackles rise with suggestions to hold the email-address  
accountable.

The text should be:
: As it is impossible to know whether the email-address associated
: with the signature or just the email-address associated with the
: RCPT TO: caused the replay abuse, one approach may be to develop
: a strategy that always holds the receiving domain accountable
: for exposing the signature and allowing a recipient within the
: domain to either act alone or in conjunction with the sender to
: perpetrate message replay abuse.

-Doug

---

From: <[email-masked]>
Date: Tue, 24 Jan 2006 19:55:10 -0500

Doug,
I would like to say that the only thing that a properly resolved dkim sig suggests is that the message came from the signing domain, no more no less. It allows better resolution of responsibility without any absolute assigning of same.
thanks,
Bill

-----Original Message-----
From: [email-masked] on behalf of Douglas Otis
Sent: Tue 1/24/2006 5:56 PM
To: Stephen Farrell
Cc: IETF-DKIM
Subject: Re: [ietf-dkim] draft-ietf-dkim-threats-00 Overlooking a practicalsolution while also recommending a highly unfair solution

The text should be:

-Doug

---

From: Douglas Otis <[email-masked]>
Date: Tue, 24 Jan 2006 18:03:16 -0800

On Jan 24, 2006, at 4:55 PM, <[email-masked]>  
<[email-masked]> wrote:
>
> I would like to say that the only thing that a properly resolved  
> dkim sig suggests is that the message came from the signing domain,  
> no more no less. It allows better resolution of responsibility  
> without any absolute assigning of same.

Without a strategy to prevent replay abuse, DKIM is indeed limited to  
identifying the initial domain of a message.  Within the typical  
domain, a torrent of abuse may be derived from an extremely difficult  
to detect series of messages sent slowly from perhaps thousands of  
compromised systems.  However, these messages may be replayed a  
billion fold when redirected.  Identifying the initial domain does  
not indicate whether the domain is also capable of preventing even  
small amounts of abuse bearing their signature.  Most can not.  The  
use of the signature as a means for basing acceptance, either by way  
of white-listing, or by reputation, will be eviscerated by replay  
abuse.  The signature will not remain suitable as a basis for  
acceptance.

The signature may still permit differentiation between a phish and a  
financial enterprise using registered recognitions at the MUA or  
within filters.  Phishers will be able to sign messages, and that  
also greatly weakens a sending policy conformance strategy.

Holding the recipient domain accountable for replay abuse would be a  
viable means to control this exploit.  The recipient domain needs to  
ensure all signatures are overlaid with the verification results  
prior to delivery to the mailbox, preferably at the edge of the  
AdmD.  The sending domain needs to ensure the recipient domain is  
secure and has adopted the overlay practice.

-Doug

---

From: Eliot Lear <[email-masked]>
Date: Wed, 25 Jan 2006 08:55:42 +0100

Doug,

I'm having a problem entering this as an issue.  I'm creating a
placeholder called "Does Section 4.1.4 properly address traceability and
accountability?"  Please see below.

> does not necessary verify the valid use of an email-address anyway.
I don't see why you went here.  Use of certificates is certainly one
valid approach but it is not the only one.  Within an enterprise it
should be possible to impose policies in conjunction with simpler
mechanisms such as DKIM to get traceability. Once again, one size does
NOT fit all.  Especially if the message is protected.  Remember,
deciding whether something is spam generally does not involve a single
input but instead involves many.

Eliot

---

From: Douglas Otis <[email-masked]>
Date: Wed, 25 Jan 2006 02:24:37 -0800

On Wed, 2006-01-25 at 08:55 +0100, Eliot Lear wrote:
> mechanisms such as DKIM to get traceability.

Even assuming that the holder of an email-address can be identified in
some manner, such as the use of individual CA certificates, there is
_no_ way to know which email-address to hold accountable in the case of
a replay attack.  Was it exclusively due to the recipient, or due to a
cooperation between the sender and the recipient.  Indeed, the email-
address to initially suspect would be the recipient.  Looking at the
content _may_ provide a clue whether the sender may have been partially
culpable.  Who knows, perhaps the sender was duped into responding to a
message sent to them.  Perhaps it was an innocent newsletter.

Tracing the problem demands there to be something that can be traced.
When most domains leak enough abuse to create a major problem with
respect to the value of signatures, signatures will not mean much with
respect to reputation.  In the case of replay abuse, without a practice
of overlaying the signature, attempts to hold an email-address culpable
is sure to be futile.  Email-addresses are a dime a billion.  

> Once again, one size does NOT fit all.  Especially if the message is
> protected.  Remember, deciding whether something is spam generally
> does not involve a single input but instead involves many.

A signed message is not, in itself, protected from replay abuse,
regardless of the size of the domain.  The size of the domain does not
matter.  The best moderator will not be able to spot spam that contains
a link to topic related material.  Even by hand, filtering can not be
done reliably enough.  Most domains send _some_ abuse for many reasons.
If that were not true, we would not be talking about DKIM.  

The definition of spam largely depends upon _where_ the message is sent
and much less about what is in the message.  When the message appears to
largely benefit the sender, this would be a clue when there is a
question about whether an individual email was spam.  When there are
millions of such messages sent to those that never desired receipt, even
this message, this would be spam without question.

Unless there is a practice that overlays the signature with results,
there is no one that can be held accountable.  It is not practical to
revoke keys fast enough to make a significant difference in the level of
the abuse.  Any domain would be at risk, some more so than others of
course.  While some domains may remain above the fray, the diminished
value obtained by a signature with respect to acceptance for most
domains, will also likely mean that reputation systems will not offer
much value either.  Even SSP would make little sense for the minimal
benefit.

DKIM has value without being used to base acceptance (sort out spam).
Perhaps adding signatures and not caring about whether they provide any
acceptance or conformance value would be okay.  DKIM could improve upon
filtering of phish attacks, and perhaps that is good enough.

-Doug

---

From: Stephen Farrell <[email-masked]>
Date: Wed, 25 Jan 2006 10:49:18 +0000

Doug, Eliot,

 > : perpetrate message replay abuse.

So I think we could just see if that (or some variant) is
supported or not without going into all the background.

(I assume the proposal here is for this to be additional text
and not to replace the existing stuff.)

Personally, it strikes me as very odd to be trying to hold
the receiving domain accountable for anything.

But maybe what you're arguing for is really more like
traitor-tracing assuming the receiving domain's inbound MTA
is not colluding in the replays.

Whether or not that can work (in this context) is difficult to
say absent a (clear;-) proposal.

The taint of DRM that traitor-tracing sort-of brings with
it may also be problematic.

Stephen.