The SUPI (Subscription Permanent Identifier) is the 5G equivalent of the LTE IMSI — a globally unique identity stored in the USIM. However, unlike IMSI which was sent in cleartext over the air in LTE (a known vulnerability), 5G encrypts the SUPI into a SUCI (Subscription Concealed Identifier) using the home network's public key before transmission. This is a major privacy improvement in 5G.