A method performed by user equipment, UE, during idle mode inter-system change, while the UE is in a single registration mode, comprising: 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, if any one or more of following conditions are met: condition a) the source cellular network is an evolved universal terrestrial radio access network, e-UTRAN; and the target cellular network is a Next Generation Radio Access Network, ng-RAN of a 5G system, 5GS; and the UE does not have a valid native 5G NAS security context; and the UE has a packet data network, PDN, connection for emergency bearer services; and the UE has a current Evolved Packet System, EPS, security context including NAS security algorithms set to a null integrity protection algorithm and null ciphering algorithm; and the UE has received an indication that the source cellular network does not support interworking with the target cellular network without a signaling channel between mobility management entities of the EPS and the 5GS; condition b) the source cellular network is the ng-RAN; and the target cellular network is the e-UTRAN; and the UE has received an indication that the source cellular network does not support interworking with the target cellular network without a signaling channel between mobility management entities of the EPS and the 5GS; condition c) the source cellular network is the ng-RAN; and the target cellular network is the e-UTRAN; and the UE does not support sending an ATTACH REQUEST message containing a PDN CONNECTIVITY REQUEST message with request type set to "handover" to transfer a Protocol Data Unit, PDU, session from N1 mode to S1 mode; condition d) the source cellular network is the ng-RAN; and the target cellular network is the e-UTRAN; and the UE has received an indication that the source cellular network supports interworking with the target cellular network without a signaling channel between mobility management entities of the EPS and the 5GS; and the UE supports sending an ATTACH REQUEST message containing a PDN CONNECTIVITY REQUEST message with request type set to "handover" to transfer a PDU session from N1 mode to S1 mode; and the UE has a valid native EPS security context. condition a) the source cellular network is an evolved universal terrestrial radio access network, e-UTRAN; and the target cellular network is a Next Generation Radio Access Network, ng-RAN of a 5G system, 5GS; and the UE does not have a valid native 5G NAS security context; and the UE has a packet data network, PDN, connection for emergency bearer services; and the UE has a current Evolved Packet System, EPS, security context including NAS security algorithms set to a null integrity protection algorithm and null ciphering algorithm; and the UE has received an indication that the source cellular network does not support interworking with the target cellular network without a signaling channel between mobility management entities of the EPS and the 5GS; condition b) the source cellular network is the ng-RAN; and the target cellular network is the e-UTRAN; and the UE has received an indication that the source cellular network does not support interworking with the target cellular network without a signaling channel between mobility management entities of the EPS and the 5GS; condition c) the source cellular network is the ng-RAN; and the target cellular network is the e-UTRAN; and the UE does not support sending an ATTACH REQUEST message containing a PDN CONNECTIVITY REQUEST message with request type set to "handover" to transfer a Protocol Data Unit, PDU, session from N1 mode to S1 mode; condition d) the source cellular network is the ng-RAN; and the target cellular network is the e-UTRAN; and the UE has received an indication that the source cellular network supports interworking with the target cellular network without a signaling channel between mobility management entities of the EPS and the 5GS; and the UE supports sending an ATTACH REQUEST message containing a PDN CONNECTIVITY REQUEST message with request type set to "handover" to transfer a PDU session from N1 mode to S1 mode; and the UE has a valid native EPS security context.