Subject: Re: [ietf-dkim] RFC4871bis - whether to drop -- k: Key type

From: "Dave Crocker" <[email-masked]>
Date: Tue, 2 Jun 2009 11:29:49 -0700

There are much easier ways to do a dos attack.

/d

--
  Dave Crocker
   bbiw.net

> ------- Original Message -------
> From: Eliot Lear <[email-masked]>
> To: Murray Kucherawy <[email-masked]>
> Sent: 02-Jun-09, 10:59:30 AM
> Subject: Re: [ietf-dkim] RFC4871bis - whether to drop  --  k: Key type
> 
> On 6/2/09 7:41 PM, Murray Kucherawy wrote:
> > So if a=rsa-sha256, then the key I get from issuing a query based on 
s= and d= will be an RSA key, or it will fail to verify.  What k= offers is 
somewhat redundant except for the fact that I can avoid the crypto overhead 
if I can detect early on that it won't work anyway.
> >    
> 
> That's an important point, in the case of a DOS attack.  The next 
> question is whether anybody implements it as such?
> 
_______________________________________________
NOTE WELL: This list operates according to 
http://mipassoc.org/dkim/ietf-list-rules.html

---

From: Douglas Otis <[email-masked]>
Date: Tue, 2 Jun 2009 11:43:23 -0700

> There are much easier ways to do a dos attack.

IIRC, this feature was intended to reduce the number of unsupported  
algorithms that might be otherwise accepted because the algorithm was  
not yet adopted by the receiver.

Unless the key indicates rsa-md6 for example, then accepting messages  
that do not verify because MD6 is not yet supported by the receiver  
would not be given a pass because the DKIM key did not indicate the  
domain uses MD6.

This is not about DoS avoidance, although this might be one of the  
benefits.  This is about algorithm agility.

-Doug