Subject: Re: [core] [Ace] COSE requirements from CORE/ACE

From: Hannes Tschofenig <[email-masked]>
Date: Tue, 1 Sep 2015 17:41:11 +0200

Hi Goeran,

> we really need MAC-only?

I would have said that we do not need MAC only.
Ideally, we want to have encryption as often as we can.
It does not really cost more in terms of payload size and performance.

Ciao
Hannes

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From: Carsten Bormann <[email-masked]>
Date: Tue, 01 Sep 2015 18:31:58 +0200

> Ideally, we want to have encryption as often as we can.

That is the mantra when there is personally identifiable information
involved.

In REST, we also have an important property called "visibility".
Where visibility and privacy are in conflict, the latter usually wins.
They aren't always.

Grüße, Carsten

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From: Göran Selander <[email-masked]>
Date: Fri, 4 Sep 2015 12:00:39 +0000

Hi Carsten

>Where visibility and privacy are in conflict, the latter usually wins.

Does that mean that we need MAC-only or not? If you are basing significant
actions on “visible” data, wouldn’t you also want to verify the data?

Regards,
Göran

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From: Carsten Bormann <[email-masked]>
Date: Fri, 04 Sep 2015 14:16:35 +0200

Göran Selander wrote:
> Does that mean that we need MAC-only or not? 

Yes.

> If you are basing significant

You might want to.  However, the party benefiting from the visibility is
not necessary a party to the security associations that allow integrity
checks on the data.  So that may limit what you can do.

Grüße, Carsten

---

From: Göran Selander <[email-masked]>
Date: Fri, 4 Sep 2015 13:17:26 +0000

>checks on the data.  So that may limit what you can do.

Could you please provide an example?

Thanks
Göran

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From: Carsten Bormann <[email-masked]>
Date: Sat, 05 Sep 2015 15:14:44 +0200

> Could you please provide an example?

Sure.

Imaging a scenario where we have a server that is an electricity meter,
and a client that is some billing system controlled by the electricity
vendor.  The electricity meter provides regular updates of the energy
consumed (or provided) to the billing system.  These two nodes have a
need for end-to-end integrity and therefore use a MAC.

In this scenario, the meter does not talk directly to the billing system
via IP, but uses another system in the home as a CoAP proxy.  The
confidentiality requirements are covered by using DTLS between the meter
and the proxy as well as another DTLS connection between the proxy and
the vendor.  The proxy has a requirement for visibility here: It needs a
way to ensure that only the data the home owner has agreed to provide
actually makes it to the vendor.  (It also may want to make use of these
data for an independent in-home display; the integrity desired is also
provided by the DTLS connection to the meter.)

As usual in the multi-hop scenarios that benefit from DTLS, the proxy
has a legitimate role in the conversation, with its own security
requirements.  Still, we need end-to-end security between the systems
supplied by the electricity vendor, and this is where COSE would come in.

Grüße, Carsten

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From: Hannes Tschofenig <[email-masked]>
Date: Fri, 4 Sep 2015 13:49:38 +0100

Carsten,

I don't think there is a requirement in REST that says eavesdroppers have to be able to read the data.
That's hard to believe.

Ciao
Hannes

-----Original Message-----
From: Carsten Bormann [mailto:[email-masked]]
Sent: 04 September 2015 19:17
To: Göran Selander
Cc: [email-masked]; Cuellar, Jorge; Hannes Tschofenig
Subject: Re: [core] [Ace] COSE requirements from CORE/ACE

Yes.

> actions on “visible” data, wouldn’t you also want to verify the data?

You might want to.  However, the party benefiting from the visibility is not necessary a party to the security associations that allow integrity checks on the data.  So that may limit what you can do.

Grüße, Carsten

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---

From: Göran Selander <[email-masked]>
Date: Tue, 8 Sep 2015 10:41:05 +0000

>supplied by the electricity vendor, and this is where COSE would come in.

Yes, I think that is a good example. There are keys established and data
can be verified by the intermediate, but using a different protocol.

So, unless there are further comments, I take it the answer to “2.
MAC-only?” is “yes”.

Any opinions on the other two questions?

(“1. Message size” and “3. Symmetric encryption + asymmetric signature”,
see:
http://mailarchive.ietf.org/arch/msg/core/bs4dJCBkV5gGdyJwt9RGP3vgPG8
)

Regards
Göran