name: KSI
full_form: Key Set Identifier
context: The security parameters for authentication and ciphering are tied together in sets. In a GSM authentication challenge, from a challenge parameter RAND both the authentication response parameter SRES and the GSM ciphering key can be computed given the secret key associated to the IMSI. In a UMTS authentication challenge, from a challenge parameter RAND, the authentication response parameter RES and the UMTS ciphering key and the UMTS integrity key can be computed given the secret key associated to the IMSI. In addition, in the USIM a GSM ciphering key can be computed from the UMTS ciphering key and the UMTS integrity key by means of an unkeyed conversion function. Furthermore, in A/Gb mode if an A5 ciphering algorithm that requires a 128-bit ciphering key is taken into use, then a GSM Kc128 shall also be calculated as described in the subclause 4.3.2.3a. In order to allow start of ciphering on a RR connection without authentication, the ciphering key sequence numbers are introduced. The ciphering key sequence number is managed by the network in the way that the AUTHENTICATION REQUEST message contains the ciphering key sequence number allocated to the GSM ciphering key (in case of a GSM authentication challenge) or the UMTS ciphering key and the UMTS integrity key (in case of a UMTS authentication challenge) which may be computed from the RAND parameter carried in that message. If an authentication procedure has been completed successfully and a ciphering key sequence number is stored in the network, the network shall include a different ciphering key sequence number in the AUTHENTICATION REQUEST message when it intiates a new authentication procedure. The mobile station stores the ciphering key sequence number with the GSM ciphering key (in case of a GSM authentication challenge) and the UMTS ciphering key and the UMTS integrity key (in case of a UMTS authentication challenge) and indicates to the network in the first message (LOCATION UPDATING REQUEST, CM SERVICE REQUEST, PAGING RESPONSE, CM RE-ESTABLISHMENT REQUEST) which ciphering key sequence number the stored GSM ciphering key (in case of a GSM authentication challenge) or set of UMTS ciphering, UMTS integrity, derived GSM ciphering key, and potentially the derived GSM Kc128 (in case of a UMTS authentication challenge) has. When the deletion of the ciphering key sequence number is described this also means that the associated GSM ciphering key, the UMTS ciphering key and the UMTS integrity key shall be considered as invalid and also the GSM Kc128 shall be deleted if any (i.e. the established GSM security context or the UMTS security context is no longer valid). In A/Gb mode, the network may choose to start ciphering with the stored GSM ciphering key or GSM Kc128 (under the restrictions given in 3GPP TS 42.009 [5]) if the stored ciphering key sequence number and the one given from the mobile station are equal. NOTE 1: The decision of starting ciphering with the GSM ciphering key or the GSM Kc128 depends on whether the network indicates in the CIPHERING MODE COMMAND message an A5 ciphering algorithm which requires a 64 or 128-bit ciphering key as specified in 3GPP TS 33.102 [5a]. In Iu mode, the network may choose to start ciphering and integrity with the stored UMTS ciphering key and UMTS integrity key (under the restrictions given in 3GPP TS 42.009 [5] and 3GPP TS 33.102 [5a]) if the stored ciphering key sequence number and the one given from the mobile station are equal. NOTE 2: In some specifications the term KSI (Key Set Identifier) might be used instead of the term ciphering key sequence number.
description: An alternative term used in some specifications instead of Ciphering Key Sequence Number.
explanation: Classified as a Parameter because it is used for identifying specific key sets.
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