Section: 4.7.7.5.1 Authentication not accepted by the MS
Content: In a UMTS authentication challenge, the authentication procedure is extended to allow the MS to check the authenticity of the core network. Thus allowing, for instance, detection of false base station. Following a UMTS authentication challenge, the MS may reject the core network, on the grounds of an incorrect AUTN parameter (see 3GPP TS 33.102[ 3G security; Security architecture ]  [5a]). This parameter contains two possible causes for authentication failure: a) MAC code failure If the MS considers the MAC code (supplied by the core network in the AUTN parameter) to be invalid, it shall send a AUTHENTICATION AND CIPHERING FAILURE message to the network, with the GMM cause 'MAC failure'. The MS shall then follow the procedure described in subclause 4.7.7.6 (f). b) SQN failure If the MS considers the SQN (supplied by the core network in the AUTN parameter) to be out of range, it shall send a AUTHENTICATION AND CIPHERING FAILURE message to the network, with the GMM cause 'Synch failure' and the re-synchronization token AUTS provided by the USIM (see 3GPP TS 33.102[ 3G security; Security architecture ]  [5a]). The MS shall then follow the procedure described in subclause 4.7.7.6 (g). In Iu mode, an MSwith a USIM inserted shall reject the authentication challenge if no Authentication Parameter AUTN IE was present in the AUTHENTICATION REQUEST message (i.e. a GSM authentication challenge has been received when the MS expects a UMTS authentication challenge). In such a case, the MS shall send the AUTHENTICATION AND CIPHERING FAILURE message to the network, with the GMM cause 'GSM authentication unacceptable’. The MS shall then follow the procedure described in subclause 4.7.7.6 (f). If the MS returns an AUTHENTICATION_AND_CIPHERING_FAILURE message to the network, the MS shall delete any previously stored RAND and RES and shall stop timer T3316, if running. If the MS has a PDN connection for emergency bearer services established or is establishing such a PDN connection, additional MS requirements are specified in subclause 4.7.7.6, under "for items f and g".