--On 16 September 2010 03:37:53 -0400 Hector Santos
<hsantos(_at_)isdg(_dot_)net>
wrote:
Alessandro Vesely wrote:
I see these generic possibilities:
First party Example:
From: user(_at_)example(_dot_)com
DKIM-Required: example.com
DKIM-Signature: d=example.com h="From:DKIM-Required"
Yes, that's possible, but seems useless. What I've been thinking about
is
From: user(_at_)example(_dot_)com
DKIM-Required: list.example
DKIM-Signature: d=example.com h="From:DKIM-Required"
If it were agreed that a signature may be not valid unless
countersigned by the recipient, the risk of replay attacks would be
greatly diminished.
So in the above, the originating message author is telling the world
that the domain list.example is expected to resign? so it ends up
like this?
From: user(_at_)example(_dot_)com
DKIM-Required: list.example
X-DKIM-Signature: d=example.com h="From:DKIM-Required"
DKIM-Signature: d=list.example.com h="From:DKIM-Required"
X-DKIM-Signature means that it was stripped and/or nullified in the
in distribution. Illustrated above to show there was a change.
Am I reading your proposal correct here?
I don't think so. The original signature should only sign the DKIM-required
and From headers, and perhaps enough other headers to reduce utility of
replay attacks. Importantly, they should only sign parts that are likely to
be unbroken by the MLM, thus satisfying ADSP requirements. However, the
recipient knows that a valid signature from the MLM is required, too. Thus,
the original DKIM signature is only valid for messages going through the
list - off list replay isn't possible. On-list replay can be limited by
ALSO including a full DKIM signature, for the list to check before
redistributing.
--
Ian Eiloart
IT Services, University of Sussex
01273-873148 x3148
For new support requests, see http://www.sussex.ac.uk/its/help/
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