Hello. CKSN - Ciphering Key Sequence Number is associated with Kc and is a bit trickier than trivial counter. According to GSM 04.08 (p. 278 in v5.3.0 in my case) it's comprised as follows: * 3 bits - actual key sequence (value 111 seems to be reserved) * 1 bit - always 0 * 3 bits - CKSN IEI Could you help me to understand how to properly generate it: - the last bit (8th) is not specified - does it mean it's always 0? - what is CKSN IEI and how do I calculate\use it? - am I right that 3-bit key seq. value is simple counter that I increment with each Kc generation? - what do I do when key seq. reach 6 - start over with 0 again? And of course I'm interested if CKSN-related code is available as part of libosmogsm or some other open source project: the code is always helpful in understanding and I don't have to reinvent the wheel :) best regards, Max.