Re: [core] OSCOAP -02: sequence number reuse?

[Christian Amsüss <[e-mail]>]

On Wed, Mar 22, 2017 at 10:17:02AM +0000, Göran Selander wrote:
> Judging by your proposal you got the point despite these errors. I’ve
> updated the draft, please check if I got it right:
>
> As an exercise, verify that this construction works even if client and
> server change roles. And also with Observe in that case.

As I read it now, it should work. (Without observe and role reversal, it
works in my implementation, the rest is pending but requires some more
streamlining in the API).

Where I do see trouble coming up is when the multicast proposals get
warmed up again. For then, I can envision at least two solutions[1], so
I suppose we're good.

Best regards
Christian


[1]: (Diverting this into a footnote so it's on record, even though it's
not relevant for this thread itself, but might spin off a new one for
multicast:)

* Approach A: Unless only one node that is allowed to do requests, all
  responses must bear a sequence number of their own instead of
  mirroring.

  This adds back the sequence numbers to the message length. (As the
  multicast responses probably need the KID as well, this just means
  that the compressed responses take the [partIV, kid, ciphertext] form.

* Approach B: Allocate not 1 but n "mirror spaces". This would take
  ceil(log2(n_participants)) bits off the sequence number range, but the
  responders would then only need to send their KID (sender ID) and the
  ciphertext.

  (Basically they'd encode the peer number in reverse bit sequence in
  the partial IV, where 0 is used for sequence numbers the device has
  dealt itself, 1 is for sequence numbers out of the list of the first
  partner (or the unicast partner), and 2 etc is for sequence numbers of
  other people's pools.

-- 
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