Interoperation of 2G and 3G networks will be an important issue when both kinds of networks will exist in parallel and need to interwork with each other. Among other things, this implies that 2G subscribers should be able to access 3G networks by means of a 2G SIM card and a dual-mode handset. However, it is well known that 3G networks will offer its users an increased level of security when compared to 2G networks. For example, in 3G networks there will be mutual authentication between user and network as well as integrity protection of signalling commands on the air interface, with an associated 128 bit integrity key. Also the 3G cipher key will be 128 bit long, as opposed to the 2G cipher key of 64 bit length. The paper concentrates on the security aspects of 2G and 3G interworking. We identify all relevant roaming and handover cases between 2G and 3G networks and describe the solution for handling the cipher and integrity keys in these cases developed by the 3GPP security group, which is the relevant standardisation body. The guiding principle was to provide 3G security to 3G users whenever possible. The solution is assessed with respect to the security it offers and the signalling load it causes.
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