Answer: The 2G GSM systems were the first digital systems that performed the authentication of subscriber. The mobile was assigned a temporary identifier after the authentication so that it does not has to send the mobile subscriber’s permanent identity ‘IMSI’ on the air interface. And the voice was encrypted. However, 2G systems suffered from two major flaws:
1: The IMSI was sent over air interface, the first time mobile connects with the network after power up. And as a result this IMSI could be hacked using IMSI catcher.
2. A fake BTS could pose as a genuine 2G basestation and fool the mobile in revealing it’s information.
The 3G and 4G solved the second problem by introducing bidirectional authentication, thus network needed to authenticate itself with the mobile(UE). Hence, a fake BTS would fail it’s authentication with the mobile(UE).
However, the first problem still remained and IMSI catcher could still hack the permanent subscriber identity.
5G tried to address this first problem, the IMSI in 5G is called as SUPI and it is never sent over the air interface unencrypted, first it is encrypted using public key as SUCI and then sent over the air interface. The network can decrypt it using its private key that is never known to the UEs, once a UE is registered after authentication, it is assigned a temporary identifier called 5G GUTI and which is often changed.