## 2 Related Work
### Communication-Based Validation  
The last approach to evaluate the trustworthiness of V2X reported position information uses the message conveying technology itself. First of all, a vehicle could compute the angle [22] or distance to the message sender using the received signal strength, the time-of-flight, and the angle-of-arrival of the V2X message. However, highly dynamic V2X channel conditions such as shadowing by blocking vehicles or multipaths created by metal-hulled vehicles make it hard to obtain reliable measurements. Moreover, the strict time synchronization between vehicles that is required for the computation would be hard to satisfy on the on-board units (OBUs) as these devices are not created for such purpose. Therefore, in this section, we will discuss only the schemes that rely on the most conservative constraints, i.e., the maximum possible communication distances. [PERSON] et al. [5] proposed a scheme where a vehicle's relative location is defined by its entanglement with other vehicles. Each vehicle regularly broadcasts its identity (a public key) along with its signature of a current timestamp. When a vehicle \(A\) receives such a broadcast from another vehicle \(B\), it signs \(B\)'s ID and rebroadcasts it. If a vehicle \(C\) on the opposite lane rebroadcast \(A\)'s identity before it rebroadcasts \(B\)'s identity, then \(B\) can conclude that \(A\) is ahead of him/her. As the authors noted, however, this scheme was a sketch of a possible solution and had strong assumptions to make it work. For one, it depends on the opposite-side traffic, a condition we cannot always rely on. However, the idea of entanglement within the communication range is viable, and we borrow it for our own proposal discussed later. [PERSON] et al. [12] proposed a method to evict a misbehaving vehicle from the trusted set of neighbors even before the certifying authority (CA) includes the attacker's certificates in the revocation list. In particular, if the attacker reports an implausible position that stands out from the observation of the honest majority, i.e., if messages are received beyond their expected area of propagation, the honest neighbors in the communication range of the attacker notice it. Then the honest majority begins to warn any new vehicle that comes into the communication range of the attacker to watch out for the potentially fabricated information from the suspected vehicle. Furthermore, if the accusation in the warning message collects enough supporting signatures from the honest majority, it promotes to the disregard message. When the disregard message with enough signatures is picked up by the roadside unit (RSU), it is forwarded to the CA so that it can revoke the certificates of the attacker. [PERSON] et al. [23] also exploited the maximum communication range limitation in addition to other behavioral anomalies of the vehicle in question to verify neighbors' position information. In case an observing vehicle \(M\) overhears communication between \(N\) and \(A\), it compares their positions and check if a possible attacker \(A\) can be within the maximum communication range of \(N\). If the previously claimed position of \(A\) contradicts the condition, \(M\) considers \(A\) sending out false position information when it is actually at a position closer to \(N\). Since this work is for multi-hop forwarding situation where the identity of the next forwarding node is known, we cannot directly apply it to the one-hop broadcast situation. Moreover, this checking method cannot be used against the remote attacker pretending to be close when it is actually far away. [PERSON] et al. [17] used a checking method that requires the neighboring vehicle should be heard during at least twice the maximum communication range \(d_{TX}\), which is called the minimum distance moved (MDM), in order to be trusted. This check is to cope with stationary attackers whose transmission range is \(d_{TX}\).