  As has been widely discussed, the GSM mobile telephony system only offers
unilateral authentication of the mobile phone to the network; this limitation
permits a range of attacks. While adding support for mutual authentication
would be highly beneficial, changing the way GSM serving networks operate is
not practical. This paper proposes a novel modification to the relationship
between a Subscriber Identity Module (SIM) and its home network which allows
mutual authentication without changing any of the existing mobile
infrastructure, including the phones; the only necessary changes are to the
authentication centres and the SIMs. This enhancement, which could be deployed
piecemeal in a completely transparent way, not only addresses a number of
serious vulnerabilities in GSM but is also the first proposal for enhancing GSM
authentication that possesses such transparency properties.
