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

Derek Atkins <derek@ihtfp.com> Mon, 13 July 2015 17:36 UTC

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From: Derek Atkins <derek@ihtfp.com>
To: Ilari Liusvaara <ilari.liusvaara@elisanet.fi>
References: <CA+k3eCQUPxZfWM9XcKaTLN-WOx2cHEi9SAGSRFTtv71iSCUqdQ@mail.gmail.com> <559576A9.9090002@gmx.net> <BY2PR03MB442C02F758E34B29BBD0CEAF5970@BY2PR03MB442.namprd03.prod.outlook.com> <001001d0b90c$3c874af0$b595e0d0$@augustcellars.com> <00c501d0b9a5$c8f869d0$5ae93d70$@augustcellars.com> <CA+k3eCS-7UK9RDfnkKCLK0ApTdNhSamYY3LL73+e1=rBvz7vDA@mail.gmail.com> <sjmlheo6t1j.fsf@securerf.ihtfp.org> <20150710171118.GA20991@LK-Perkele-VII>
Date: Mon, 13 Jul 2015 13:36:45 -0400
In-Reply-To: <20150710171118.GA20991@LK-Perkele-VII> (Ilari Liusvaara's message of "Fri, 10 Jul 2015 20:11:18 +0300")
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Cc: Jim Schaad <ietf@augustcellars.com>, Brian Campbell <bcampbell@pingidentity.com>, Derek Atkins <derek@ihtfp.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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Ilari Liusvaara <ilari.liusvaara@elisanet.fi> writes:

> On Fri, Jul 10, 2015 at 09:28:24AM -0400, Derek Atkins wrote:
>> 
>> Performing a MAC using the output of a DH (be it ECDH, AEDH, or raw DH)
>> can still tell you that the transmission has not been modified between
>> you and the party you're talking to.  You're right that it tells you
>> nothing about the party you're talking to (i.e., without authentication
>> of the DH keys you have no idea if you're talking to a MITM or the real
>> endpoint).  However, using DH + MAC does tell you that you're not being
>> spoofed by a "passive" eavesdropper.
>> 
>> So IMHO it does have some benefit.
>
> 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.

At worst I could record and replay the session, assuming the other end
also uses a static public/private keypair.  Or I could act as an active
man-in-the-middle.

> This does not straightforwardly extend to symmetric schems (like
> key wrapping), but are those useful with MACs?
>
>
> -Ilari

-derek
-- 
       Derek Atkins                 617-623-3745
       derek@ihtfp.com             www.ihtfp.com
       Computer and Internet Security Consultant

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