Subject: [ietf-dkim] draft-ietf-dkim-threats-00

From: Douglas Otis <[email-masked]>
Date: Tue, 24 Jan 2006 12:48:51 -0800

---[A: Misstatement of the DKIM mechanism.]
,---
| 1.  Introduction
|
| DomainKeys Identified Mail (DKIM) [I-D.allman-dkim-base] defines a
| mechanism by which email messages can be cryptographically signed,
| permitting a signing domain to claim responsibility for the use of a
| given email address.
'---

A verified signature does not indicate that the signing domain is  
claiming responsibly for some email-address found within the  
message.  It is not a reasonable practice for a general access  
provider to inject a Sender header just to meet this expectation.   
This has already caused problems and should be avoided.

This should read:

: DomainKeys Identified Mail (DKIM) [I-D.allman-dkim-base] defines a
: mechanism by which email messages can be cryptographically signed,
: permitting a signing domain to be held accountable for the message.
: When an email-address contained within the message is also within
: the signing domain, and noted within the 'i' parameter, the signing
: domain may also be held accountable for verifying the use of the
: email-address conforms to their policies.

---[B: Limited information in a critical section.]
3.2.  Use of Specific Identities

As per a prior email, this section needs additional detail.  While  
there are separate sections expanding upon this issue, references  
should be made by this section. I will be happy to try again using  
references to other sections.

---[C: Unreasonable estimate of impact from a highly probable exploit.]
,---
| 4.1.  Attacks Against Message Signatures
|  ...
| Signed message replay                       |   Low  |    High    |
'---

This should read:

: Signed message replay                  |  Very High  |    High    |

How was this problem rated?  Any large domain has a continuous  
background of abuse being sent.  In some cases, this abuse may  
represent tens of thousands of compromised systems.  Any list-server  
is also prone, as there is no practical means to screen participants  
or expect effective outbound filters when the number of messages do  
not reflect the overall traffic until used in the replay.  Out of the  
millions of valid users within these domains, rate limiting has  
ensured these abusive systems represent a smaller percentage of the  
overall outbound email in most cases.  When used in conjunction with  
a replay strategy, rate limits will not remain effective, and yet the  
signature still remains valid.

Once the DKIM signature has any acceptance value, expect this problem  
to become paramount.

---[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: Eliot Lear <[email-masked]>
Date: Tue, 24 Jan 2006 22:07:46 +0100

Doug,

If it's not editorial related, would you mind breaking these up so we
can keep the issues separate?

Thanks,

Eliot

Douglas Otis wrote:
> claiming responsibly for some email-address found within the message. 
> It is not a reasonable practice for a general access provider to
> inject a Sender header just to meet this expectation.  This has
> already caused problems and should be avoided.
>
> This should read:
>
> overall outbound email in most cases.  When used in conjunction with a
> replay strategy, rate limits will not remain effective, and yet 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.
>
> email-address accountable!  A domain or IP address must always be made
> accountable with respect to any reputation scheme!
>
> -Doug
>
>
>
>
>
>
>
>