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

Mike Jones <Michael.Jones@microsoft.com> Fri, 10 July 2015 23:11 UTC

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From: Mike Jones <Michael.Jones@microsoft.com>
To: Jim Schaad <ietf@augustcellars.com>, 'Brian Campbell' <bcampbell@pingidentity.com>
Thread-Topic: [Cose] Key management for MACs (was Re: Review of draft-schaad-cose-msg-01)
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Date: Fri, 10 Jul 2015 23:11:39 +0000
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Cc: "cose@ietf.org" <cose@ietf.org>
Subject: Re: [Cose] Key management for MACs (was Re: Review of draft-schaad-cose-msg-01)
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Per my issue 2, I believe that the default key management mode should be “no key management” – what we called “direct” in JOSE.  If developers want to also add it in, more power to them.

Yes, key rotation will have to occasionally be done to make this safe.  That’s always been a given.  (And yes, that’s part of the meaning of “key management” that is out of scope for this working group, but is clearly necessary for any secure application.)

The overhead you’re not talking about below Jim is the computation time and power overhead of generating the ephemeral key and encrypting it.  If this isn’t absolutely necessary, it’s better to not do it.  JOSE seems to be working fine without this even being an option…

                                                                -- Mike

From: Cose [mailto:cose-bounces@ietf.org] On Behalf Of Jim Schaad
Sent: Friday, July 10, 2015 4:08 PM
To: 'Brian Campbell'
Cc: cose@ietf.org
Subject: Re: [Cose] Key management for MACs (was Re: Review of draft-schaad-cose-msg-01)

The overhead on key management is going to be based a lot on what you are doing.

If you are doing direct shared secret –then you have a kid
If you are doing ECDH-SS – then you need two kids and a nonce (which can be very short)
If you are doing ECDSA – then you need a kid

This means that to be the same size you have 24 bytes to express a second kid (which could be implicit) and a nonce (which can be a short as 2 or 3 bytes).  I don’t think you need that much space – so yes, it would be smaller.

The overhead from the basic messages (MAC vs Sign) is going to be about the same in both cases.

From: Brian Campbell [mailto:bcampbell@pingidentity.com]
Sent: Friday, July 10, 2015 3:34 PM
To: Jim Schaad
Cc: cose@ietf.org<mailto:cose@ietf.org>
Subject: Re: [Cose] Key management for MACs (was Re: Review of draft-schaad-cose-msg-01)

Really, there's no overhead from the key management part?


I've admittedly not done the comparison but the message size overhead seems like it'd be similar with something like ECDSA (admittedly not RSA).

ECDSA = 2 * SHA-256 or 32 bytes
Truncated AES-CMAC is 8 bytes (64 bits)

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