NAS UL COUNT is hardcoded to 0 in derive_kgnb
in nr_nas_msg_sim.c:
derive_kgnb(kamf,0,kgnb);
If I understand correctly, this is the part of the spec that needs to be implemented:
https://www.etsi.org/deliver/etsi_ts/133500_133599/133501/16.03.00_60/ts_133501v160300p.pdf
