Re: [Ace] About securing last exchange CoAP-EAP

Christian Amsüss <christian@amsuess.com> Mon, 11 October 2021 10:16 UTC

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Date: Mon, 11 Oct 2021 12:16:04 +0200
From: Christian =?iso-8859-1?Q?Ams=FCss?= <christian@amsuess.com>
To: Rafa Marin-Lopez <rafa@um.es>
Cc: Dan Garcia Carrillo <garciadan@uniovi.es>, EMU WG <emu@ietf.org>, "ace@ietf.org" <ace@ietf.org>
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Subject: Re: [Ace] About securing last exchange CoAP-EAP
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Hi,

sorry for spreading this out over the sub-threads[1], just to get the
pointers right and everything addressed:

On Fri, Sep 03, 2021 at 08:32:59PM +0200, Rafa Marin-Lopez wrote:
> 2) When the CoAP message contains the OSCORE ID that hits the OSCORE
>   context without any key material, we would have to assume this is
>   CoAP-EAP: the OSCORE implementation should not discard or give a
>   fail for this coap message but "pass the control" to CoAP-EAP so
>   that we send a altAccept to the EAP state machine so we get the MSK.

It's not because the context is without key material -- it's because
that context was created by EAP and that software component, rather than
giving a key, gave a "callback" (however it's precisely implemented)
that tells the OSCORE context to rather ask for a key with metadata from
the last message.

(OSCORE appendix B.2 needs something similar to implement, so this
shouldn't be new to OSCORE implementations).

> 3) From the MSK, we derive the OSCORE key material for the OSCORE
>   context with the corresponding ID and update the OSCORE context with
>   this key material 

The key IDs need to be preconfigured for this to work, see [2] -- but
that's best practice anyway.

BR
c

[1]: https://mailarchive.ietf.org/arch/msg/emu/nb8zGGDJ3d4fUaCW8QMkf6rkhVs/
[2]: https://mailarchive.ietf.org/arch/msg/core/AK8Wxy64tXofocdRHm5HNew8dpE/

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