Um Security Features
GSM 02.09 defines the following security features on Um:
- authentication of subscribers by the network,
- encryption on the channel,
- anonymization of transactions (at least partially)
Um also supports frequency hopping (GSM 05.01 Section 6), which is not specifically intended as a security feature but has the practical effect of adding significant complexity to passive interception of the Um link.
Authentication and encryption both rely on a secret key, Ki, that is unique to the subscriber. Copies of Ki are held in the SIM and in the Authentication Center (AuC), a component of the HLR. Ki is never transmitted across Um. An important and well-known shortcoming of GSM security is that it does not provide a means for subscribers to authenticate the network. This oversight allows for false basestation attacks, such as those implemented in an IMSI catcher.
Read more about this topic: Um Interface
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