# A Survey of Public Key Infrastructure-Based Security for Mobile Communication Systems
## 3 A Survey of the Literature
### Review of PKI-Based Security for 2G-GSM Systems
In short, [PERSON]'s protocol is based on the idea that AuC/HLR can generate a temporal key that it can be used for the authentication between MSC/VLR and SIM/MS without involving the HLR/AuC in this process, and with zero knowledge of the secret pre-shared key. Therefore, it involves computational overhead, and the same temporary key is reused as long as the MS has stayed in the same VLR. The first and the second phases involve 5 message flows and 2 message flows respectively. [PERSON] et al. [21] proposed an improvement to [PERSON] et al. 's protocol in [20] such that providing mutual authentication between VLR and MS can be guaranteed all the time while MS sends an authentication request. In addition, this scheme also proposed a relatively new authentication protocol which can not only solve the weaknesses of the pervious works but also make the authentication more efficient. However, in this approach and for each new communication or data session, VLR computes the certificate to verify its identity even though the MS is in the same VLR. Thus, it includes more computational overhead due to the reuse of the same temporary key as long as the MS has remained in the same VLR, and hence the first phase involves 5 message flows and the second phase involves 2 message flows. [PERSON] et al. [22] came and proposed an authentication scheme with low handshaking processes and relatively low processing time. This protocol tried to solve the main GSM problems such communication overhead, and computational cost. However, the protocol is lack of mutual authentication property, and it has a high bandwidth consumption and space overhead especially between the network entities. [PERSON] and [PERSON] [23] followed and proposed an authentication and anonymous channel protocol to be applied over the GSM wireless system without changing the underlying structure. Provided that a PKI exists, [PERSON] and [PERSON]'s protocol can be applied over any wireless network. Prepaid tickets perform the anonymity generated by the HLR that can be used later to be authenticated anonymously by VLR. However, the objective of this scheme is to provide complete anonymity, in the sense that nobody (including the HLR) in the system can identify a user who is transmitting over the network. Following [PERSON] and [PERSON], [PERSON] [24] proposed an efficient authentication protocol that provides anonymous channels for the GSM system by the means of algorithms A3, A5, A8, and a combination between scheme [22,23]. This protocol covers all the features of Scheme [22] and the anonymity provided in scheme [23], and it is based on the generation of temporal keys defined in [22] the utilization of prepaid tickets [23]. However, the ticket is generated using A3 and A5 algorithms and maintains the general structure of scheme [22]. [PERSON] [25] followed and proposed a middleware security system that aims to protect the GSM communication channel. By minimizing the computational overhead of the provided authentication and cryptographic schemes of the network, this scheme provides an end-to-end secured communication between the GSM mobile devices and the GSM base stations; it insures compatibility between wireless GSM devices, and is easy to install without any modification of the basic GSM system. The advantage of this scheme comes from the fact it is based on the ECC and provides as much security as RSA but with less overhead in the processing of messages; therefore, to secretly communicate with each other, mobile terminals must have keys which are known only by the communicating terminals. A random public point, G, is chosen on the elliptic curve (EC), to generate a compressed public key.