Re: [core] OSCORE: Questions about Section 5.2

Christian Amsüss <christian@amsuess.com> Thu, 11 October 2018 11:09 UTC

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Date: Thu, 11 Oct 2018 13:09:44 +0200
From: Christian =?iso-8859-1?Q?Ams=FCss?= <christian@amsuess.com>
To: Jaro Fietz <jaro.fietz@aisec.fraunhofer.de>
Cc: core@ietf.org, martin.striegel@aisec.fraunhofer.de, stefan.hristozov@aisec.fraunhofer.de, jaro.fietz@gmx.de
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Subject: Re: [core] OSCORE: Questions about Section 5.2
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Hello Jaro,

I can answer just a small part of this:

On Thu, Oct 11, 2018 at 11:14:16AM +0200, Jaro Fietz wrote:
> Given [2] and my interpretation, the Sender ID of all relevant nodes
> in a network would have the same length, only different contents.

The expectation is that the shortest (zero-length) ID would be used in
cases wherever that's beneficial, eg. when a constrained device
primarily utilizes one context in which it is addressed as a server. For
the communication partner (which might be an unconstrained device in
such scenarios, and/or rarely addressed as a server), one would pick an
ID that's long enough to tell all that device's incoming messages apart.

> While the (size, id)-pairs (1, [1]) and (2, [0,1]) would
> technically be different sender IDs, I don't know if implementations would
> actually honour that behaviour

They certainly should. Left-trimming zeros off an ID will lead to
surprising results in every 256th situation.

> while aiocoap uses the zero-stripped length of its number
> representation [6].

You briefly had me worried I got it wrong myself -- but the
left-trimming that's happenign is on the sequence numbers, not on the
sender IDs. Sequence numbers need to be represented by the shortest big
endian (but non-zero-length) representation; representing them any
different would break the AAD agreement.

> I'm currently implementing parts of OSCORE in C for a minimal OSCORE server.

Slightly off topic: Would that happen to be a freely licensed
implementation? If so, I know of an embedded operating system project
that would love to hear about this.

Best regards
Christian

-- 
A beginning is the time for taking the most delicate care that the
balances are correct.
  -- Princess Irulan, Manual of Muad'Dib