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Re: [Cose] Key management for MACs (was Re: Review of draft-schaad-cose-msg-01)

Ilari Liusvaara <ilari.liusvaara@elisanet.fi> Mon, 13 July 2015 17:47 UTC

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Date: Mon, 13 Jul 2015 20:47:52 +0300
From: Ilari Liusvaara <ilari.liusvaara@elisanet.fi>
To: Derek Atkins <derek@ihtfp.com>
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Cc: Jim Schaad <ietf@augustcellars.com>, Brian Campbell <bcampbell@pingidentity.com>, Mike Jones <Michael.Jones@microsoft.com>, cose@ietf.org
Subject: Re: [Cose] Key management for MACs (was Re: Review of draft-schaad-cose-msg-01)
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On Mon, Jul 13, 2015 at 01:36:45PM -0400, Derek Atkins wrote:
> Ilari Liusvaara <ilari.liusvaara@elisanet.fi> writes:
> >
> > It occurs to me that any sort of asymmetric key management with MACs
> > is unsafe, since knowing just public key enables message forgery.
> 
> Pardon?  I don't understand that.  Just knowing your Public Key does not
> mean I can send messages that appear to come from you.  The key used for
> the MAC is the derived shared secret, so I would need to know your
> Private Key as well in order to generate the correct shared secret.

Forgot about static-static DH (which might actually be useful, given
that it can cut down number of keys asymptotically).

There's also MAC keyed by dynamic DH key (one protocol I cooked
up a while back did that). But from message perspective, it would
be static-static.

But many asymmetric crypto primitives are not useful there, given
that those allow to agree or transport a known key, rendering the
MAC pretty much meaningless.


-Ilari

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