[ietf-dkim] Base issue: multiple linked signatures
leiba at watson.ibm.com
Tue Dec 26 08:21:09 PST 2006
In discussions with the IESG to sort through their "discuss" comments,
I had a talk with Lisa Dusseault, and she had one point that I want to
bring back to the mailing list: I don't think we considered, in our
discussions of multiple signatures, multiple *linked* signatures, which
could work TOGETHER to convey information, and the protocol doesn't
allow that sort of thing. The way dkim-base is set up, I don't think
this could easily be added as an extension, and it'd be a significant
change at this point. Here's the concept:
* Signer puts on two signatures (maybe as two header records, maybe as
one that contains two sigs).
* One of the signatures has minimal scope, maybe signing only "from:",
* The other signature covers as much of the message as possible... most
headers, all the boby.
* The two signatures work together. If one verifies and the other
doesn't, the verifier can consider what was changed in the message, and
possibly use that information to deal with mailing list modifications
One way this might be used is to have one signature that covers the
subject header and one that doesn't, to allow the verifier to detect a
subject change and decide whether it's OK. As the spec is now, the
verifier would just find the one signature (that doesn't cover the
subject) that works, and use that, not considering the other.
The WG did discuss related things, so maybe we'll decide that this was
covered and dismissed, but it's a wrinkle that I want to make sure we
look at. Let's beat this around for a week or so, and see where we are
with it, and what we do or don't want to do with it.
Barry Leiba, DKIM working group chair (leiba at watson.ibm.com)
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