[ietf-dkim] Issue #1265: Signing by parent domains

[Douglas Otis]

On May 25, 2006, at 10:23 PM, John Levine wrote:

> The arguments against parent signatures seem to me to boil down to  
> two: parents may be untrustworthy, and foolish people can do  
> foolish things.
>
> I concur with Jim's rebuttal to the first.  The DNS works on the  
> basis of delegation from the top down, subdomains are 100% at the  
> mercy of their parent domains whether you like it or not, and it's  
> 25 years too late to change it.  Sorry.

Any weakness or exploit related to selections of algorithms or keys  
within the DNS hierarchy related to DKIM parent signing precludes an  
effective response at affected levels.  If a TLD used DKIM with 768  
bit keys for example, that might be adequate when this domain's  
messages are seldom targeted.  On the other hand, <big- 
financial>.<tld> may need to react to a highly targeted attack  
employing greater resources, perhaps necessitating 1024 bit keys or  
greater.

Critical and targeted domains will be in an awkward position  
insisting unusual (movie plot) exploits are something that all their  
TLDs must defend against.  When allowing parent signing, <big- 
financial>.<tld> will be unable to directly respond to all threats  
affecting them.  By allowing parent signing, messages for <big- 
financial>.<tld> might be validated by a compromised key at <tld>.   
Perhaps <tld> retains keys for too long, or uses a key too small to  
resist the level of attack amassed against <big-financial>.<tld>.  Of  
course, the greater number of messages compromised by a single key,  
the greater the threat.

This concern is not directly related to how DNS works, but rather  
what message email-addresses are considered confirmed by a  
signature.  There is a significant reduction in security in this  
regard when parent signing is permitted.  When disallowing parent  
signing as directly validating some sub-domain email-address, the  
parent domain can still sign these messages, but would be properly  
considered a third-party signing-domain.  For most messages, this  
should not cause any problem.

-Doug