GBA procedure

Posted: July 14, 2010 in technical
Tags: , , , , , , , , ,

The bootstrapping procedure is described in the picture below. The UE includes the “3gpp-gba-tmpi” token in the Request message it sends to the BSF over the Ub interface, then the BSF includes this token as well in the Response.

gba_exchange

GBA procedure

  1. UE sends the HTTP request to the BSF, inserting an user identity, either its IMPI or its TMPI, if it has a temporary ID available;
  2. The BSF identifies whether it received a TMPI or an IMPI; if it was a TMPI, it looks for the corresponding IMPI in its cache, and if it’s not found, it gives an error to the UE, requesting the IMPI, otherwise it continues authenticating the UE to the HSS.

Then the BSF tries to locate the HSS and retrieve the GUSS and the AV from it, where AV=(RAND||AUTN||XRES||CK||IK), over Zh;

  1. BSF forwards the RAND and AUTN to the UE, in order to authenticate it;
  2. The UE uses AUTN to authenticate the network, then computes the XRES, CK and IK and
  3. sends the XRES to the BSF, in order to be authenticated by this entity and
  4. the BSF verifies the XRES against its already computed RES; if they match, the UE is authenticated;
  5. The BSF obtains the Ks by concatenating CK and IK, same as the UE and
  6. replies to the UE with a B-TID in the 200 OK message;
  7. The UE also obtains the Ks by concatenating its CK and IK

At this point, both the UE and the BSF derive the Ks_NAF key, the actual key that will be used to secure the communication between the UE and the NAF.

Ks_NAF = KDF(Ks, “gba-me”, RAND, IMPI, NAF_Id), where KDF is the key derivation function and the NAF_Id looks like this: NAF_Id = FQDN of the NAF || Ua security protocol identifier. All the values possible and structure of these components are defined in annexes to [13].

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