  Although the security properties of 3G and 4G mobile networks have
significantly improved by comparison with 2G (GSM), significant shortcomings
remain with respect to user privacy. A number of possible modifications to 2G,
3G and 4G protocols have been proposed designed to provide greater user
privacy; however, they all require significant modifications to existing
deployed infrastructures, which are almost certainly impractical to achieve in
practice. In this article we propose an approach which does not require any
changes to the existing deployed network infrastructures or mobile devices, but
offers improved user identity protection over the air interface. The proposed
scheme makes use of multiple IMSIs for an individual USIM to offer a degree of
pseudonymity for a user. The only changes required are to the operation of the
authentication centre in the home network and to the USIM, and the scheme could
be deployed immediately since it is completely transparent to the existing
mobile telephony infrastructure. We present two different approaches to the use
and management of multiple IMSIs.
