Section: 4.7.1.2a.4 Establishment of integrity protection of layer 3 signalling messages
Content: Establishment of integrity protection of layer 3 signalling messages and optionally user plane data is done during the attach procedure. The MS shall indicate support for integrity protection in the MS network capability IE included in the ATTACH REQUEST message and the ROUTING AREA UPDATE REQUEST message to the network. When an MS sends an ATTACH REQUEST message or a ROUTING AREA UPDATE REQUEST message, then if the MS has no UMTS security context, it shall send the ATTACH REQUEST or the ROUTING AREA UPDATE REQUEST message without integrity protection to the network. If the network supports integrity protection, then the network activates integrity protection by initiating an authentication and ciphering procedure. The network selects an integrity algorithm among the supported integrity algorithms indicated by the MS in the MS network capability IE. After successful completion of the authentication and ciphering procedure, all layer 3 signalling messages sent between the MS and network are integrity protected in the LLC layer using the new UMTS security context. When an MS sends an ATTACH REQUEST message or a ROUTING AREA UPDATE REQUEST message, then if the MS has a UMTS security context, it shall send the ATTACH REQUEST or the ROUTING AREA UPDATE REQUEST message integrity protected with the current UMTS security context to the network. The MS shall include the CKSN indicating the current UMTS security context value in the initial ATTACH REQUEST message or a ROUTING AREA UPDATE REQUEST message. The MS shall use the integrity algorithm identified by the stored integrity algorithm identifier. The network shall check whether the CKSN included in the initial ATTACH REQUEST message or the ROUTING AREA UPDATE REQUEST message belongs to an UMTS security context available in the network, and if yes, then the network re-establishes integrity protection of layer 3 signalling messages in the LLC layer: - by replying with a GMM message (ATTACH ACCEPT or ROUTING AREA UPDATE ACCEPT) that is integrity protected in LLC layer by using the current UMTS security context. From this time onward, all layer 3 signalling messages exchanged between the MS and the network are sent integrity protected, except for the messages specified in subclause 4.7.1.2a; or - by initiating an authentication and ciphering procedure. This procedure can be used by the network to select a new integrity algorithm different from the one currently used by the MS. NOTE: Even if the message authentication code protecting the ATTACH REQUEST or the ROUTING AREA UPDATE message in the LLC layer cannot be verified in the network due to the integrity key and the integrity algorithm in the LLC layer at the network not having been configured yet for this MS, the network progresses the attach procedure and the routing atra updating procedure at the GMM layer anyway without having to run a new authentication and ciphering procedure if the network decides to re-use the same encryption- and integrity keys and the same ciphering and integrity algorithms and not run a re-authentication.