## 3 Privacy Enhancement Proposals for 5G Core Networks  
With the understanding that 5G technology comprises a powerful and complex system, and the goal of maintaining user identity and privacy protections within these systems is an increasingly important and complex issue, researchers have made strides in suggesting mechanisms to create a more private experience for users of 5G networks. For example, the study shown in [38] presents different proposed solutions to conceal the long-term identity of a user across network operations, especially communications between the UE Roaming Network and the HN. The work reviews the schemes using various technologies: pseudonyms [39; 40; 41; 42], Certificate-Based Public-Key Cryptography, Root-Key-Based Encryption, and Identity-Based Encryption. Privacy Enhanced Fast Mutual Authentication (PEFMA) is introduced as a way to conceal the user's identity over time in normal and roaming scenarios. This approach is similar to how the SUPI is encrypted over the air but differs in that it uses different cryptographic principles. However, both solutions still require trust in the HN. The authors in [43] present mechanisms to protect privacy in Vehicular Networks, giving users conditional anonymity. Traceability to identities is possible via threshold cryptography, requiring collaboration between authorities only for misbehavior or legal abuses. In this mechanism, the HN is considered to be the Trusted Authority, which is responsible for generating and managing the cryptographic secrets. In [44], a pseudonym approach is presented that enhances the 5G-AKA (Authentication and Key Agreement) protocol to never utilize the subscriber's personal identifier, even during the first attachment. Research conducted in [45] breaks down the vulnerabilities to user privacy in the 5G-AKA protocol. This occurs when a UE has initial authentication communications with the CN. The authors state that the AKA protocol is vulnerable to 'guess' and'replay' attacks. An attacker that successfully completes these attacks could generate SUCIs using the HN's public key, testing these against the network's response of success or failure, thus leading to the possibility of an attacker tracking a user. The authors of [45] propose three methods to strengthen the 5G-AKA protocol, with the recommended solution relying on the user's secret key stored in the SIM card, along with using the UE's ephemeral public key that is generated during the creation of the SUCI. In their approach, the SUPI guess attack is thwarted by including a hash of the user's secret key into the HMAC (Hash-Based Message Authentication Code), which stops hackers from replicating the HMAC process for guessed SUPIs. The replay attack is patched by storing the user's ephemeral public key in the CN's database, allowing for replayed SUCIs to be rejected if their public key already exists. Research in [46, 47] covers aspects of anonymous IoT data in a cloud network environment. The devices are associated into groups, and each group's elected leader is responsible for aggregating the collected data and abstracting individual identities. However, all of the 5G-specific studies still rely on the existence of a trust relationship with the 5G Core Network. No research has yet addressed the scenarios arising when the trust between users and the 5G CN is eroded.