Gory details of USIM authentication sequence numbers

I always though I understood UMTS AKA (authentication and key
agreement), including the re-synchronization procedure. It's been years
since I wrote tools like (outside of
OsmoNITB!), that I seem to understand all the nasty little details.

I always thought for re-synchronization it is sufficient to simply
increment the SQN (sequence number). It turns out, it isn't as there is
a MSB-portion called SEQ and a lower-bit portion called IND, used for
some fancy array indexing scheme of buckets of highest-used-SEQ within
that IND bucket.

If you're interested in all the dirty details and associated spec
references (the always hide the important parts in some Annex) see the
discussion between Neels and me in Osmocom redmine issue 1965.