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From: Pekka Pessi <Pekka.Pessi(a)nokia.com>
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+GBA Authentication
+==================
+
+ GBA authentication uses SIM card to establish a unique
+ identity and a shared secret for a network service. The
+ shared secret is computed from the domain name of the
+ service and a special boostrap key, Ks.
+
+ GBA is used to authenticate the mobile with the IMS XCAP
+ servers.
+
+ The mobile obtains the bootstrap key and a temporary
+ identity used with network services from a special
+ boostraps bootstrap server, BSF. Mobile authenticates
+ itself with BSF using AKA authentication and the BSF
+ returns it the identity to use with network services
+ (known as NAF). The bootstrap key is formed from the AKA
+ authentication results, too.
+
+ In the initial phase the client will send its user
+ identity, the IP Multimedia Private Identity (IMPI) to
+ the BSF server. The BSF server fetches an AKA challenge
+ from operator's subscriber database along with correct
+ response: RAND, AUTN, XRES, CK and IK. Server returns
+ the AKA challenge RAND and AUTN to mobile.
+
+ The client uses SIM to calculate the AKA RES, CK and IK
+ from the challenge, and uses RES to generate AKA Digest
+ credentials sent back to the bootstrap server.
+
+ The bootstrapping server verifies the response using
+ XRES, and if accepted, it will return an XML document
+ containing the bootstrapping transaction identifier
+ (B-TID) and the lifetime of the KS_NAF keys.
+
+ The CK and IK parameters from AKA authentication are
+ used for Ks. The Ks can now be used to derive unique
+ key for each NSF service.
+
+ For further reference on GBA, see 3GPP TS 33.220
+ and 3GPP TS 24.109.
+
+GBA Variants
+------------
+
+ There are three different GBA variants: 2G GBA, GBA_ME
+ and GBA_U. The variant is selected based on SIM card
+ capabilities.The 2G GBA is used if there is no 3G USIM
+ available. The GBA_ME is used if there is USIM or ISIM
+ application on SIM card (also known as UICC), but there
+ is no support for GBA service.
+
+ In the GBA_ME variant the Ks key is stored outside SIM
+ card. It is implemented using ordinary AKA
+ authentication.
+
+ In the 2G variant, there is no AKA authentication, but
+ it is rather simulated using 2G A8 SIM authentication
+ algorithm. The 2G variant is described in the 3GPP
+ 33.220 Annex I.
+
+ The UICC-based GBA variant, GBA_U, keeps the Ks key
+ stored on the SIM card (UICC) and lets the SIM card
+ calculate the NAF keys.
+
+ Note that the device cannot downgrade the protocol:
+ subscriber database has recorded if the SIM card
+ supports GBA_U andthe information is included in the AKA
+ challenge. The SIM card will reject a GBA_U-related AKA
+ challenge if it is used with ordinary AKA service.
--
1.7.1