Page 454 - From GMS to LTE

440 From GSM to LTE-Advanced Pro and 5G

user. To simplify the process, the EAP‐SIM protocol was specified in RFC 5216. Here,
the authentication is performed with data contained on the SIM card and no user inter-
action is required.
EAP‐SIM uses the same authentication method as was described in the Sections 6.7.2
and 6.7.3. Figure 6.25 shows the messages that are exchanged during the authentication
process between a mobile device and the authentication server over an EAP‐SIM‐com-
patible AP. After an open system authentication and an association procedure, the net-
work initiates the EAP procedure by sending an EAP Identity Request message which
the mobile device answers with an EAP Identity Response message. The identity that is
returned in this message consists of the Identity Type Identifier, the IMSI read from the
SIM card and a specific postfix of the mobile network operator.
Alternatively, the mobile device can also send a temporary identity that has been
assigned to it during a previous authentication procedure to the network. This tempo-
rary identity is similar to the Temporary Mobile Subscriber Identity (TMSI) used in
GSM and UMTS and hides the user’s identity from potential eavesdroppers on the air
interface.
In the next step, the network sends an EAP‐SIM Start Request message. This message
contains information on the supported EAP‐SIM authentication algorithms. The
mobile device selects one of them and answers with an EAP‐SIM Start Response mes-
sage. This message contains a random number, which is used later in the network
together with the secret key Kc for a number of calculations. As the secret GSM key Kc
is stored in the network and on the SIM card, it is possible to use it as a basis to authen-
ticate the device toward the network and vice versa.
At this point, the authentication server uses the subscriber’s IMSI to contact the
Home Location Register (HLR)/Authentication Center (AuC), as described in Chapter 1,

Access Figure 6.25 EAP‐SIM authentication.
Client
point
Open system auth. and association procedure
EAP identity request
EAP identity response
IMSI, pseudonym
EAP SIM start request
Version list
EAP SIM start response
EAP-SIM Version, random value

EAP SIM Challenge request
GSM random numbers, etc.
EAP SIM Challenge response
Message authentication code
EAP Success