st with the mobile number but also with a SUPI in the 5GS. It was known as International Mobile Subscriber Identity (IMSI) in the 4G world. End-to-End Architecture Components, Concepts, Security, and Transport The SUPI should not be sent over the air in cleartext Subscriber identity de-concealing function Asymmetric Home network Home network Encryption public key private key Algorithm -MCC and MNC in Cleartext -MSIN encrypted Figure 2.21 - 5G identifiers The authentication of the UE and the network is based on symmetric keys and is dependent on the identification of the user. However, if the identification is sent in cleartext over the air from the UE to the network, it is susceptible to being imitated by malicious entities, who can pose as legitimate users. To avoid this, mobile networks use several temporary identifiers called Globally Unique Temporary Identifiers (GUTIs). These identifiers are frequently changed and used for identifying a UE over the air. However, in some situations, like the first user registration, the SUPI is used, and then from thereon, a GUTI is used: GUAMI - Globally Unique AMF ID GUAMI - Globally Unique AMF ID 5G-TMSI AMF Region ID AMF Set ID AMF Pointer GUAMI - Uniquely identifies the AMF 5G-TMSI - Uniquely identifies the UE within the AMF AMF Region ID - Identifies the Region AMF set ID - Identifies a specific AMF set within the Region AMF Pointer - Uniquely identifies the AMF within the AMF set Figure 2.22 - 5G GUTIs 5G identifiers The GUAMI serves as a globally unique identifier for each AMF within the 5G network. Between different regions, AMF set IDs can be duplicated: GUAMI - Globally Unique AMF ID AMF Region ID AMF Set ID AMF Pointer AMF Region 2 AMF Set 1 AMF Region 1 AMF Set 1 AMF 2 AMF 1 AMF Set 2 AMF 3 AMF 4 Mobile Country Code Mobile Network Code Figure 2.23 - The GUAMI Now, in 5G, a SUPI can be an IMSI, as in 4G, or a Network Access Identifier (NAI), which can be used in SIM-less devices. A SUPI is a string of 15 decimal digits: 3 digits for the Mobile Country Code (MCC), 