Section: 4.7.7.1 Authentication and ciphering initiation by the network
Content: The network initiates the authentication and ciphering procedure by transferring an AUTHENTICATION_AND_CIPHERING REQUEST message across the radio interface and starts timer T3360. The AUTHENTICATION_AND_CIPHERING REQUEST message shall contain all parameters necessary to calculate the response parameters when authentication is performed (see 3GPP TS 43.020[ Security related network functions ]  [13] and 3GPP TS 33.102[ 3G security; Security architecture ]  [5a]). If authentication is requested, then the AUTHENTICATION_AND_CIPHERING REQUEST message shall contain either: - In a GSM authentication challenge, the GPRS ciphering key sequence number and the RAND, or - In a UMTS authentication challenge, the GPRS ciphering key sequence number, the RAND and the AUTN. In A/Gb mode, if authentication is not requested, then the AUTHENTICATION_AND_CIPHERING REQUEST message shall not contain neither the GPRS ciphering key sequence number, the RAND nor the AUTN. In A/Gb mode, if ciphering is requested, in a GSM authentication challenge or in a UMTS authentication challenge, then the AUTHENTICATION_AND_CIPHERING REQUEST message shall indicate the GPRS GSM ciphering algorithm. In A/Gb mode, an MS supporting integrity protection shall ignore a GSM authentication challenge from the network. In A/Gb mode, in a UMTS authentication challenge, if the MS has indicated support for integrity protection in MS network capability IE included in the ATTACH REQUEST or ROUTING AREA UPDATE REQUEST message to the network, then if the network supports integrity protection then the network shall activate integrity protection in the MS (see subclause 4.7.3.1.3 and subclause 4.7.5.1.3). If the network does not activate integrity protection in the MS, then the MS shall detach from the network. In A/Gb mode, if no UMTS security context is available in the network or if no common UMTS security context is available in the MS and the network, and if the MS has indicated support for integrity protection in MS network capability IE to the network, then an authentication and activation of integrity protection and setting of ciphering mode (enabled or disabled), shall take place in the same authentication and ciphering procedure. The network shall replay the MS network capability IE and the MS radio access capability IE received from the MS in the latest ATTACH REQUEST message or the latest ROUTING AREA UPDATE REQUEST message, by including the MS network capability IE and the MS radio access capability IE in the AUTHENTICATION AND CIPHERING REQUEST message to the MS. The network shall select one of the GPRS GSM integrity algorithms indicated in the MS network capability IE received from the MS in the latest ATTACH REQUEST message or ROUTING AREA UPDATE REQUEST message. The selected GPRS GSM integrity algorithm shall be included in the AUTHENTICATION AND CIPHERING REQUEST message. The network shall enable or disable ciphering and include the selected GPRS GSM ciphering algorithm if ciphering is enabled. If ciphering is enabled, then the network shall select one of the GPRS GSM ciphering algorithm indicated in the MS network capability IE received from the MS in the latest ATTACH REQUEST message or ROUTING AREA UPDATE REQUEST message.The network shall calculate a message authentication code for the AUTHENTICATION AND CIPHERING REQUEST message at the GMM layer with the new integrity key, GPRS GSM Kint key, indicated by the GPRS ciphering key sequence number included in the AUTHENTICATION AND CIPHERING REQUEST message. The new GPRS GSM Kint shall be calculated from the new UMTS security context established in the same procedure as described in the subclause 4.7.7.3b. The AUTHENTICATION AND CIPHERING REQUEST message shall include the calculated message authentication code for integrity protection. In A/Gb mode, if an established UMTS security context is available in the network and if the MS has indicated support for integrity protection in the MS network capability IE, when authentication takes place in the authentication and ciphering procedure (regardless whether a change of the integrity algorithm, ciphering algorithm or a change of ciphering mode takes place in the same procedure), then the network shall replay the MS network capability IE and the MS Radio Access Capability IE received from the MS in the latest ATTACH REQUEST message or the latest ROUTING AREA UPDATE REQUEST message, by including the MS network capability IE and the MS Radio Access Capability IE into the AUTHENTICATION AND CIPHERING REQUEST message to the MS. If a change of the GPRS GSM integrity algorithm takes place in the same procedure, then the network shall select one of the GPRS GSMintegrity algorithms indicated in the MS network capability IE received from the MS in the latest ATTACH REQUEST message or ROUTING AREA UPDATE REQUEST message. The selected GPRS GSM integrity algorithm shall be included into the AUTHENTICATION AND CIPHERING REQUEST message. If the network decides to change the ciphering mode or the ciphering algorithm or both, then if ciphering is enabled the network shall select one of the GPRS GSM ciphering algorithms indicated in the MS network capability IE received from the MS in the latest ATTACH REQUEST message or ROUTING AREA UPDATE REQUEST message. The network shall also include the ciphering mode or the selected GPRS GSM ciphering algorithm or both, if ciphering is enabled (Ciphering algorithm IE) in the AUTHENTICATION AND CIPHERING REQUEST message. The network calculates a message authentication code over the parameters included in the AUTHENTICATION AND CIPHERING REQUEST message at the GMM layer with the new integrity key GPRS GSM Kint indicated by the GPRS ciphering key sequence number included by the AUTHENTICATION AND CIPHERING REQUEST message. The new GPRS GSM Kint shall be calculated from the new UMTS security context established in the same procedure as described in the subclause 4.7.7.3b. The AUTHENTICATION AND CIPHERING REQUEST message shall include the calculated message authentication code for integrity protection. In A/Gb mode, if an established UMTS security context context is available in the network and if the MS has indicated support for integrity protection in the MS network capability IE to the network, and if integrity protection is in use, then if only a change of the integrity algorithm or ciphering algorithm or both, or a change of the ciphering mode, is requested in the authentication and ciphering procedure, then the AUTHENTICATION AND CIPHERING REQUEST message shall include the new GPRS GSM integrity algorithm or the new setting of the ciphering mode and the new GPRS GSM ciphering algorithm; that shall be taken into use. NOTE: The AUTHENTICATION AND CIPHERING REQUEST message is protected by a message authentication code in the LLC layer when no authentication takes place in the authentication and ciphering procedure.Therefore, no message authentication code shall be calculated and included by the GMM layer into this message when only an algorithm change takes place. In addition, there is no need to replay the MS network capability IE and the MS Radio Access Capability IE in the AUTHENTICATION AND CIPHERING REQUEST message when no authentication takes place as a valid UMTS security context is already established between the MS and network. The network includes the A&C reference number information element in the AUTHENTICATION AND CIPHERING REQUEST message. Its value is chosen in order to link an AUTHENTICATION AND CIPHERING REQUEST in a RA with its RESPONSE. The A&C reference number value might be based on the RA Colour Code value. Additionally, the network may request the MS to include its IMEISV in the AUTHENTICATION AND CIPHERING RESPONSE message.