# A Secure Blockchain-Based Authentication and Key Agreement Scheme for 3 GPP 5G Networks
## 2 Related Work
### Blockchain in 5G Authentication  
Linking data blocks into a chain, blockchain technology is essentially a secure decentralized database solution that guarantees data immutability and practical consensus across multiple network nodes. There are three different types of blockchain platforms [8]: permission-less, permissioned, and consortium blockchains. Among all three types of blockchains, it is envisioned that private and consortium blockchains are the most suitable distributed solutions to solve the security challenges in 5G because of their high efficiency [6].  
In recent years, some proposals combining blockchains with 5G authentication have surfaced. For example, [PERSON] et al. [9] introduced the idea of a blockchain-based anonymous access (BAA) scheme that allows equipment manufacturers, network operators, and users to access the blockchain-based database and perform mutual authentication. However, there is no formally proved protocol presented in the proposal. [PERSON] et al. proposed a blockchain-based 5G authentication protocol based on a public blockchain in [10; 11]. They suggested that all 5G access points (APs) can use the UE public keys listed in the blockchain to perform mutual authentication between the AP and the UE. However, this misses a mechanism for UE to retrieve the public keys of the surrounding APs. [PERSON] et al. [12] proposed the use of redactable blockchains to store all subscriber's information. The redactable blockchain provides key deletion and revocation functions. It is beneficial for network operators to protect the privacy of their users. However, the proposal lacks an authentication protocol for UEs and core networks to secure user data using the keys in the blockchain. [PERSON] et al. [13] proposed a decentralized authentication scheme for 5G IoT devices. This protocol suggests that authentication entities in all domains can upload their device registration records to the same alliance blockchain. However, the protocol uses an identity-based cryptosystem. It introduces high computational overhead to mobile devices and edge servers, making them energy inefficient and prone to request flooding. [PERSON] et al. [14] proposed an efficient authentication protocol based on 5G extensible authentication protocol (5G EAP-AKA\({}^{\prime}\)) and a private blockchain. However, the security functionality of the proposed scheme has not been formally analyzed. Moreover, the EAP framework could also introduce more signaling overhead than the existing 5G-AKA scheme.  
Some recent solutions have been designed to accelerate handover authentication in 5G wireless networks by using blockchains [15; 16; 17; 18; 19]. While all of them are providing a fast way to share the secret keys among base stations, most of them did not discuss how to prevent DoS and DDoS attacks during the UE authentication phase in a fast and efficient way. As a result, this shows that most of the existing blockchain authentication works are incomplete, and almost all of them could not alleviate the threats of DoS and DDoS attacks effectively during the UE authentication. In other words, designing a blockchain-based authentication protocol that provides adequate attack prevention, is energy-efficient, and computationally fast at the same time is a challenging research work. Overall, Table 1 summarizes all recent blockchain-based 5G authentication schemes and their challenges.