Subject: [ietf-dkim] user-based keys / another protocol

From: "Florian Sager" <[email-masked]>
Date: Sat, 4 Aug 2007 00:17:50 +0200

In June 2006 Eric Allman wrote:

>From eric+dkim at sendmail.org Thu Jun 1 07:36:07 2006
>Date: Thu Jun 1 07:36:57 2006
>Subject: [ietf-dkim] base-03: Key lookup parameters
>
>The point of passing i= is to allow extension in the future to 
>possible per-user keying. You wouldn't do this in DNS, but another 
>protocol should be able to handle it easily.
>eric

In the last days I was thinking about an easy way to deploy multiple selectors/public keys (e.g. for per-user keying) to different DNS servers in an environment of a mailserver with multiple virtual mail domains: a typical webhosting scenario with DNS-zones at different providers.

At the point of view of an administrator it seems to be best that public keys have to be provided directly by the authorities signing outgoing mail (reason: cost efficiency).
I outlined s.th. at http://dkim-connector.agitos.de/trac/wiki/DeploymentVersionTwo to support this idea. I'm sure this kind of deployment was already considered earlier - is there any information available about that?

Looking for "another protocol" Eric mentioned in the email above I couldn't find any more hints in the mailing list archives.
Is there something in progress?

Best regards,
Florian
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From: Douglas Otis <[email-masked]>
Date: Fri, 3 Aug 2007 16:28:39 -0700

On Aug 3, 2007, at 3:17 PM, Florian Sager wrote:

> In the last days I was thinking about an easy way to deploy  
> multiple selectors/public keys (e.g. for per-user keying) to  
> different DNS servers in an environment of a mailserver with  
> multiple virtual mail domains: a typical webhosting scenario with  
> DNS-zones at different providers.
>
> At the point of view of an administrator it seems to be best that  
> public keys have to be provided directly by the authorities signing  
> outgoing mail (reason: cost efficiency).
> I outlined s.th. at http://dkim-connector.agitos.de/trac/wiki/ 
> DeploymentVersionTwo to support this idea. I'm sure this kind of  
> deployment was already considered earlier - is there any  
> information available about that?

Per user keys would be a bad idea.

Consider how TPA-SSP can replace key distribution:

http://www1.tools.ietf.org/wg/dkim/draft-otis-dkim-tpa-ssp-01.txt

Rather than distributing keys, the mail-server is authorized in a  
simple and scalable fashion instead.  This better protects DKIM from  
a calamity when a private key becomes compromised at a highly shared  
MTA.  Using TPA-SSP, the resulting security warning would be limited  
to a single domain.

When distributing keys are employed, a single compromised server  
might then impact thousands of domains.  A compromised server is not  
that uncommon.  Reporting thousands of domains have been compromised  
when a single server has been compromised key will result in a much  
greater loss of confidence.  Of course, distributing keys imposes  
greater administrative costs as well.

-Doug