10.3 GMS Authentication

10.3 GMS Authentication

1. Network authenticates to the SIM (not phone) to protect against cloning

   • Authentication can be performed at call establishment allowing a new Kc to be used for each call
   • SIM can be moved from phone to phone

2. Challenge response protocol

   • Important that its infeasible for an intruder to obtain information about KI which could be used to clone the SIM by observing the auth protocol
   • Encryption Key Agreement
     • A key Kc for radio interface encryption is derived as part of the authentication protocol to encrypt data across a wireless connection
   • NW produces a challenge and SIM produces a response
     • SIM response demonstrates knowledge of KI

3. Prerequisites

   • Auc and Sim share
     • Ki
     • Authentication Algorithm
       • Auth Function A3
         • Must have the property that if you see RAND or XRES you must not get Ki
       • Key Gen Function A8
     • Auc has a Random Number Generator

4. Protocol

   • 3 Entities
     • SIM
     • Mobile Switching Centre (MSC) or Serving GPRS Support Node (SGSN)
     • HLR / Auc

5. Process - Phone wants to Authenticate

   • HLR /AUC uses its copy of KI with a new random value and passes it through the A3/A8 Algo
     • Pair is generated = XRES (expected response) and short term cipher key = Kc
       • Kc is derived from Ki using a random value
         • So in response to the auth request a Random Value, Xres (expected response to the challenge) and Kc are sent, this is known as a tripple
           • When phone gets Rand, Sim takes Rand and KI and passes them though A3 and A8
             • Sim should generate same Response = RES and cipher key KC
               • RESult with = Expected Result if Ki is the same and its an authenticated mobile
                 • Both parties will have Kc.
                   • Phone will then use Kc for encryption, SIM does not have the power.