The SUPI or Subscription Permanent Identifier is the 5G equivalent of the IMSI and serves as the globally unique subscriber identity stored on the USIM. Unlike earlier generations where the IMSI was transmitted in cleartext over the air, 5G mandates that the SUPI be encrypted into a SUCI before any radio transmission. The encryption uses ECIES with the home network's public key, and only the SIDF function within the home UDM can perform decryption. Even the visited network never sees the SUPI in cleartext, which eliminates the IMSI catcher attacks that plagued 2G through 4G networks.