Method and apparatus for deriving a cryptographic protection to a new initial non-access stratum, NAS, message for a target network from an existing security context in an idle mode inter-system change when one or more of conditions a) to d) is met, and for deriving a mapped 5G NAS security context from a source cellular network that is an EPS security context maintained by a source MME of the EPS, in an idle mode inter-system change, when one or more of conditions 1) to 4) is met, optionally after receiving a REGISTRATION REQUEST message without integrity protection and encryption.