# A Secure Blockchain-Based Authentication and Key Agreement Scheme for 3 GPP 5G Networks
## 5 Security Evaluation
### Security Analysis
Additionally, since the authentication request involves only the computationally inexpensive ECIES and hash functions, adversaries cannot exhaust the computational resources of gNBs easily;
* **DDoS Flooding Attack Prevention:** By decentralizing the authentication tasks from the AUSF/UDM to all gNBs, the 5 GSBA lessens the effects of flooding attacks that paralyze the network with authentic requests. In the 5G era, the inter-site distance (ISD) of gNBs is getting smaller, and the number of gNBs deployed keeps growing. Hence, the total computational power of gNBs is growing steadily, making it increasingly difficult to flood or even paralyze the entire 5G network. Furthermore, since the 5 GSBA shifts the authentication tasks from AUSF/UDM to gNBs, it can also prevent the single-point-of-failure. One gNB failure or one AUSF failure will not affect the entire 5G network. Thus, DDoS attacks in the 5G authentication can be prevented, and the quality of service (QoS) across the 5G network can be maintained. The performance analysis will show that the 5 GSBA can serve much more incoming authentication requests than the existing centralized schemes;
* **Linkability Attack Prevention:** Unlike conventional symmetric key-based protocols such as 5G-AKA, the 5 GSBA generates session keys using ECDH instead of the sequence number. Hence, the 5 GSBA does not have the MAC failure or synchronization failure commonly found in symmetric key-based AKA protocols. Adversaries can no longer use these error messages as a loophole to trace a specific device;
* **UE Impersonation Attack Prevention:** By the conventional 5G-AKA protocol, users have to trust the network operator implicitly. Since the network operator owns a copy of users' symmetric keys and sequence numbers, insider attackers in the network can impersonate the UE by abusing these keys. By the 5 GSBA, since the one-time hash secret is only stored at the USIM, there is no way for network operators to impersonate the UE using the data stored in the private blockchain. Hence, if the network operators cannot provide the one-time hash secret used in the authentication, users can simply deny all malicious behavior for that session;
* **Rogue Base Station Attack Prevention:** By the 5 GSBA, since only the legitimate gNBs can decrypt the SUCI, rogue base stations cannot produce genuine authentication responses by generating the correct HMAC code \(\sigma\). Thus, it can prevent UE from establishing connections with rogue base stations;
* **Replay Attack Prevention:** The authentication requests and responses by the 5 GSBA are all tagged with a timestamp TS, and the HMAC key \(K_{hmac}\) should also work once only. Therefore, by checking the timestamp in both UEs and gNBs, replayed messages can be easily identified and discarded;
* **Battery Depletion Attack Prevention:** With the 5G-AKA, UE must respond to identity requests from the serving network by generating a fresh SUCI. If some rogue base stations frequently send the identity requests, the batteries of UE devices could deplete faster. In the 5 GSBA, only UE can take the initiative to generate a fresh SUCI for authentication. Hence, the serving network cannot force the UE to create a fresh SUCI, and it prevents the battery depletion attacks effectively.