Section: 10.5.1.2 Ciphering Key Sequence Number
Content: In a GSM authentication challenge, the purpose of the Ciphering Key Sequence Number information element is to make it possible for the network to identify the ciphering key Kc which is stored in the mobile station without invoking the authentication procedure. The ciphering key sequence number is allocated by the network and sent with the AUTHENTICATION REQUEST or AUTHENTICATION AND CIPHERING REQUEST message to the mobile station where it is stored together with the calculated keys, e.g. Kc, CK, IK, Kc128, Kint. The Ciphering Key Sequence Number information element is coded as shown in figure 10.5.2/3GPP TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ]  and table 10.5.2/3GPP TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ] . In a UMTS authentication challenge, the purpose of the Ciphering Key Sequence Number information element is to make it possible for the network to identify the ciphering key CK and integrity key IK which are stored in the MS without invoking the authentication procedure. CK and IK form a Key Set Identifier (KSI) (see 3GPP TS 33.102[ 3G security; Security architecture ]  [5a]) which is encoded the same as the CKSN and is therefore included in the CKSN field. The ciphering key sequence number is a type 1 information element. Figure 10.5.2/3GPP TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ]  Ciphering Key Sequence Number information element Table 10.5.2/3GPP TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ] : Ciphering Key Sequence Number information element