GSM authentication is based on a challenge-response mechanism.  The
   A3/A8 authentication and key derivation algorithms that run on the
   SIM can be given a 128-bit random number (RAND) as a challenge.  The
   SIM runs operator-specific algorithms, which take the RAND and a
   secret key Ki (stored on the SIM) as input, and produce a 32-bit
   response (SRES) and a 64-bit long key Kc as output.  The Kc key is
   originally intended to be used as an encryption key over the air
   interface, but in this protocol, it is used for deriving keying
   material and is not directly used.  Hence, the secrecy of Kc is
   critical to the security of this protocol.  For more information
   about GSM authentication, see [GSM-03.20].  See Section 12.1 for more
   discussion about the GSM algorithms used in EAP-SIM.

http://tools.ietf.org/html/rfc4186

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