Wireless Security.pdf - PDF Archive

Security in Traditional Wireless Networks 375
microwave (wireless links). GSM does not specify how to secure this link, thus making it
susceptible to attacks.
There is another important characteristic of the GSM authentication process that is worth
discussing. In GSM, the authenticating entity is the SIM and not the subscriber per se. In
other words, the network authenticates the SIM card and not the subscriber of the SIM
card. Remember that the authentication process relies on a preshared secret (K i ) between
the SIM and the AuC. During the authentication process, the MSC validates that the SIM
trying to access the network has a valid K i . What happens if a ME is stolen and is used
for making calls (and using other GSM services)?
GSM does have some countermeasures to protect against equipment theft. For one, the
GSM core network maintains a database of all valid mobile equipment 7 on the network.
This database is called the Equipment Identity Register (EIR). If a subscriber loses their
ME, it is their responsibility to report it to the service provider. Before authenticating
the ME into the network, the MSC also ensures that the ME that is trying to authenticate
in to the network has not been compromised. Extrapolating this approach, a service
provider may also maintain a list of compromised SIMs. When a SIM is reported stolen,
the service provider marks the IMSI and the corresponding K i
8
as compromised. If a
compromised SIM tries to access the network, it is denied access.
Note that when the GSM authentication process completes, it has also established a
security context: the session key K c which can then be used for providing confidentiality
in the network. It is the preshared secret key (K i ) between the SIM and the AuC that
forms the basis of generating the session key. GSM uses the A8 algorithm to derive a
session key K c from the preshared secret key K i as shown in Figure 17.4 .
Compare Figure 17.4 with Figure 17.3 . The purpose of the A8 algorithm is to derive a 64-bit
session key (K c ) given the 128-bit K i and the 128-bit RAND. On the other hand, the purpose
of the A3 algorithm is to derive a 32-bit SRES given the same two inputs (the K i and the
RAND). The important thing to note here is that A3 and A8 are not algorithms per se: they are
just labels (reference names) for algorithms. In other words, a service provider is free to use
7 Each ME in the GSM network is uniquely identified by the international mobile equipment
identity (IMEI).
8 There is a one-to-one mapping between the IMSI and the K i .
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