--On 16 September 2010 09:49:40 -0700 "Murray S. Kucherawy"
<msk(_at_)cloudmark(_dot_)com> wrote:
-----Original Message-----
From: ietf-dkim-bounces(_at_)mipassoc(_dot_)org [mailto:ietf-dkim-
bounces(_at_)mipassoc(_dot_)org] On Behalf Of Ian Eiloart
Sent: Thursday, September 16, 2010 3:20 AM
To: Hector Santos; ietf-dkim(_at_)mipassoc(_dot_)org
Subject: Re: [ietf-dkim] draft-vesely-dkim-joint-sigs
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.
I'm worried about that third sentence. If people are encouraged not to
sign Subject:, for example, which is a popular display header field, one
could spamify that field and re-send the message.
If you subscribe to the idea that a DKIM signature reflects a domain
taking some responsibility for a message, I'd have a hard time not
signing Subject: (or From:) for any reason.
I guess for this to work, the MLM admin needs to be looking for a good full
signature. Lists that don't rewrite the subject are going to work better
here, but more serious is the lack of signature for the body. Still, the
attack that you describe is fairly esoteric, and targetted specifically to
the list (remember, the sender can add a list-id header, and sign that!) It
may be too expensive to be profitable.
--
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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