# Positioning Information Privacy in Intelligent Transportation Systems:
## 3 Vehicle Location Protocols Using Additional Information
Now, it is presumed that such a link has already been established and the data exchange between the station and the vehicle takes place via the secure medium. It is also presumed that a shared symmetric key was delivered after the devices have established a secure link employing, for example, the well-known Diffie-Hellmann protocol. Thus, common pairwise key \(K_{VS}\) could be utilized for the message authentication for both vehicle and BS. Basically, we state that correct deciphering of \(e=E_{K_{VS}}(m)\) and obtaining \(m\) is only feasible for the vehicle and the BS. The first step of the protocol for determining the distance between the base station and the vehicle is the generation of two random nonces (\(N_{V}\), \(N_{V}^{\prime}\)). Next, it in necessary to calculate their hash function \(h\) (\(commit\)) on the vehicle side and to send the result to the infrastructure. In turn, the BS generates one nonce (\(N_{S}\)) and sends it to \(V\). The vehicle must calculate \(N_{S}\oplus N_{V}\) and return it. The key point of the protocol is that \(S\) measures the time between sending \(N_{S}\) and receiving \(N_{S}\oplus N_{V}\). Using this time, the infrastructure can estimate the distance \(d_{VS}\) to the vehicle. After that, \(V\) communicates with the base station, sending the identification number \(id\), \(N_{V}^{\prime}\) and the signature of \(id\) and \(N_{V}^{\prime}\), using their pairwise symmetric key \(K_{VS}\). Base station \(S\) verifies if the signature and \(commit=h(N_{V},N_{V}^{\prime})\) are correct. If all checks are correct, the base station considers the distance \(d_{VS}\) computed at the previous step to be reliable. Further, the received value \(d_{VS}\) can either be sent to the vehicle or used on a dedicated server to calculate the vehicle's location. In 2006, this protocol was improved in [63], further referred as P-06 protocol. The modified strategy is shown in Figure 4. In this improved version of the protocol, only the last two steps were modified. \(V\) sends encrypted information, its \(id\) number, \(N_{S}\) and \(N_{V}^{\prime}\) with the symmetric key \(K_{VS}\). \(S\) decrypts the message and verifies if \(commit=h(N_{V},N_{V}^{\prime})\). Figure 3: The distance-bounding protocol P-04 [86]. Carrying out a XOR operation imposes extra computational cost; to address this problem, we propose to modify the protocol slightly. Proposed modification is shown in Figure 5 and detailed in [87]. This modification eliminates the necessity for XOR calculation. Should both the vehicle and the infrastructure node require to determine vehicle location, an extra step can be added to the protocol, namely, as \(S\to V:E_{K_{VS}}(t_{VS},N_{V})\). The above-listed protocols assume that the vehicle is a location-unaware data transmission initiator. We note that discussed protocols suggest vehicle authentication via Public Key Infrastructure,  
Figure 4: An improved distance-bounding P-06-M protocol. Figure 5: Potential improvement of distance-bounding protocol. while mutual authentication is not present. The protocols should be improved to allow mutual authentication since a malicious BS attack could be executed, thus compromising the localization process. We propose our improvement in the next section.