Douglas Otis:
>
> On Sep 6, 2006, at 5:39 PM, Wietse Venema wrote:
>
> >> Why? The signature must be valid and the email-address must be
> >> assured to be valid. How is the email-address susceptible?
> >
> > I can answer that. Exploitation of the mapping from recipient
> > address to DNS record name, by the application of brute force.
>
> If policy attempts to list all valid email-addresses, then it would
> be possible to use these records to discover valid email-addresses as
> you suggest.
>
> This is not how the mechanism is envisioned to be used however. As
> opposed to John's suggestion, this mechanism would automate
> annotations for "select" email-addresses within a domain. These
> email-addresses are likely already widely known, and are useful only
> in conjunction with a trusted domain. These "select" email-addresses
> offer a means to differentiate messages the trusted domain wishes to
> automatically convey as trustworthy.
With only a small number of email addresses in a domain, the existing
mechanism is plenty sufficient. Simply use an appropriate selector
field in the DKIM signature. This problem can easily be solved
without introducing complexity in the form of per-user mechanisms.
As far as I can tell, we're talking about a solution for which a
convincing problem has yet to be found.
Wietse
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