On 23 Jun 2011, at 20:00, Douglas Otis wrote:
This seems like a completely bogus argument to me. You're saying that
some domains can't be trusted, therefore none can be trusted. That's
a logical fallacy.
Not at all. Acceptance policies and results for DKIM MUST align with
what is being displayed in the message. Otherwise malefactors may be
able to exploit open and large volume domain's signatures and their lack
of duplicates in the signed header list (which most don't do). The
pre-pended header fields could then be that of any high value domain.
These messages might have been accepted on the false premise of being
from a high volume domain when based upon valid DKIM signature indications.
Right, but DKIM is checked at the MTA. If I think that messages DKIM signed by,
say, my local council, are trustworthy, then I apply a spam score accordingly.
The fact that someone else might spoof a From: header in a different mail
stream says nothing about whether I can trust the stream from my local council.
So, it may be that the practical outcome is to improve the deliverability of
mail for a trusted signer, which is a different problem. But that's still
useful. With ADSP, of course, there's also a chance of spotting spoofed
messages.
And, if multiple "From:" headers become a popular spoofing mechanism, I guess
sites will stop accepting them.
I accept that DKIM doesn't solve every problem, but that doesn't mean that it
has no value.
--
Ian Eiloart
Postmaster, University of Sussex
+44 (0) 1273 87-3148
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