Re: [6tisch] TSCH and CCM security proofs

[Tero Kivinen <[e-mail]>]

Michael Richardson writes:
>     >   Implementations MUST use different L2 keys when using different MIC
>     > lengths, as using same key with different MIC lengths might be unsafe
>     > (i.e., using same key for both MIC-32 and MIC-64). See IEEE 802.15.4
>     > Annex B.4.3 for more information.
> 
> This seems like it isn't a problem.
> It would apply to network-wide keying only.

It applies all keys, not only network-wide keys. 

> While I guess we could include multiple Link_Layer_Key with the same
> key_id, and a different key_usage, that wasn't the intention.  I
> guess one could use a different K1 and K2 with a different MIC length, but
> have no idea why a network would want to mix MIC-32 and MIC-64.

Most common use might be someone using MIC-32 for beacons, but using
MIC-64 for actual data or something like that.
draft-ietf-6tisch-minimal-security seems to be using different keys
when the mic length is different (k1, and k2), and when k1 and k2 are
same it will always use same mic length, so there is no problem there.

Anyways it might good idea to add the warning somewhere, just incase
someone adds new combinations without realizing this problem.
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