[LinkSec] Factoid: MIC size
Under the standard birthday attack collision assumptions, the probability
of collision becomes significant when the number of messages under a given
key approachs sqrt(2^size-of-mic).
That means that for a 64-bit MIC, you need to change keys before 2^32 messages
have been sent under a key. For a 10gbit link, under assumption of average
1000-bit messages, that's a key change every 7.15 minutes or so. That impacts
the efficiency of the key-exchange algorithm, and the decision-point for
key-change (if you algorithm for key-changing is slow, you want to start trying
to do a key-change rather early, so that there's NO possibility of exceeding
the collision limit).
Since we seem to be in the "how many bits can we realistically add to the header"
phase, I'd vote for more MIC bits than 64.
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Marcus Leech Mail:
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