Authentication

Unsuccessful authentication If the response (sres or RES) provided by the MS in the authentication response differs from the sres or RES stored in the VLR, the following actions can be mad e, depending on the identity type used in the transaction. When the MS identifie s itself with the TMSI, the network uses the IDENTITY_REQUEST procedure to enqui re the IMSI of the MS for the mobile-originating transactions. If the IMSI diffe rs from the IMSI the TMSI referred to, authentication is retried and, if authent ication succeeds, the transaction is continued. If the transaction type is mobil e-terminating, and the MS is identified with the TMSI, the connection towards th e MS is released and the MS is paged with the IMSI. If the MS has already been i dentified with the IMSI in a mobile terminated case, no retry is done. If the re sponse is not valid and the IMSI is used, the network sends the message AUTHENTI CATION_REJECT in the cases of the MO and MT CM connection establishments, and th e message LOCATION_UPDATING_REJECT in the case of location updatings. If authent ication fails, all parallel CM connections are released. In a UMTS authentication challenge, the authentication procedure is extended to allow the MS to check the authenticity of the core network. This allows, for ins tance, the detection of a false base station. In this case, the MS sends an AUTH ENTICATION FAILURE message to the network. What are Ki, Kc, RAND, and SRES? Ki is the 128-bit Individual Subscriber Authentication Key utilized as a secret key shared between the Mobile Station and the Home Location Register of the subs criber s home network. RAND is 128-bit random challenge generated by the Home Location Register. SRES is the 32-bit Signed Response generated by the Mobile Station and the Mobil e Services Switching Center. Kc is the 64-bit ciphering key used as a Session Key for encryption of the overthe-air channel. Kc is generated by the Mobile Station from the random challenge presented by the GSM network and the Ki from the SIM utilizing the A8 algorithm .