Section: 4.3.2.12.1 PDN connection with integrity protection
Content: At PS to CS domain change from Iu mode to A/Gb mode due to SRVCC handover of a PDN connection for which integrity protection has been activated, ciphering may be started (see 3GPP TS 44.018[ None ]  [84]) without any new authentication procedure. Deduction of the appropriate security key for ciphering in A/Gb mode, depends on the current GSM or UMTS security context for the PS domain stored in the MS and the network. The ME shall handle the GSM ciphering key according to table 4.3.2.12.1. Table 4.3.2.12.1/3GPP TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ] : SRVCC handover from Iu mode to A/Gb mode The network shall replace an already established GSM or UMTS security context for the CS domain, if any, when the SRVCC handover from Iu mode to A/Gb mode has been completed successfully. If the SRVCC handover from Iu mode to A/Gb mode has not been completed successfully, the MS and the network shall delete the new derived GSM or UMTS security context for the CS domain. Additionally, the network shall delete the already established GSM or UMTS security context for the CS domain, if the CKSN of the already established GSM or UMTS security context is equal to the CKSN of the new derived GSM or UMTS security context for the CS domain.