# Distributed Reputation for Accurate Vehicle Misbehavior Reporting (DRAMBR)
## 2 Background and Foundations
### Misbehavior Authority in the SCMS  
The SCMS comprises different communication channels due to the interaction between multiple entities, the Registration Authority (RA), the Certificate Authority (CA), the Pseudonym Certificate Authority (PCA), and the (MA), each with a distinct responsibility in managing certificates, such as handling digital certificate issuance, renewal, and revocation functions. In Europe, the European Telecommunications Standards Institute (ETSI) develops standards for the security of ITS, serving similar purposes to the SCMS, which provides certificates for messages and privacy protection for users using PKI. While both SCMS and ETSI ITS-G5 are designed for securing vehicular communications, SCMS has several key advantages compared with the alternative: broad applicability, robust misbehavior reporting, hierarchical trust management, and strong privacy from  
Figure 3: Peak district environment extracted from Google Maps. pseudonym certificates [10; 11]. These features make SCMS more suitable for the mitigation of malicious behavior, hence closely aligning with the research objectives in ensuring trust. In the SCMS, the MA places a strong emphasis on the identification and reporting of misbehavior. The main role of the MA is to collect the misbehavior reports generated locally by vehicles, which provide the SCMS with information that can be used to determine whether a vehicle is not performing at the appropriate level of communication. Misbehavior detection entails the (MA) being capable of discerning if multiple misbehavior reports reference the exact vehicle [12]. This process also necessitates the MA to compile information for publication in a CRL to invalidate a vehicle's certificates. Furthermore, the MA must inform the RA with the necessary data to enable blacklisting, preventing the misbehaving vehicle from obtaining new certificates. The SCMS architecture mandates that the following components collaborate to support misbehavior detection and introduce a system of checks and balances:  
* Step 1:The MA, PCA, and one of the Linkage Authorities (LA) must collaborate to reconstruct linkage information. * Step 2: The MA, PCA, RA, and both LAs must collaborate to generate revocation information for the CRL. * Step 3: The MA, PCA, and RA must collaborate to identify the enrollment certificate of the misbehaving vehicle, which the RA will then include in its blacklist. The MA conducts step 1 during the misbehavior investigation to ascertain whether a vehicle or set of vehicles engaged in misbehavior. Subsequently, after marking a vehicle as misbehaving, the MA proceeds with steps 2 and 3 during revocation to deduce the revocation information for the CRL and the enrollment certificate to be added to RA's internal blacklist. As shown in Figure 4, the process starts when a certificate is flagged as compromised or otherwise untrusted. The MA responsible for SCMS revokes the certificate and publishes its details to the CRL. Linkage IDs and timing information are published on the CRL and distributed to revoke all PCs of a given vehicle over a given period. The list is then broadcast across the entire vehicular network so that all vehicles and infrastructure units know these revoked certificates. Upon connectivity, the updated CRL is periodically downloaded by vehicles from the RSU or other secure distribution points. Every time a vehicle tries to communicate across the network, it validates certificates of the other parties against this CRL. This communication is blocked if the certificate appears on a list of compromised or unordered certificates, defending against possible influence from malicious and/or fraudulent sources out to communicate with other vehicles. Figure 4: SCMS pseudonym certificate revocation. Source: [3]. The current standard for distributing CRLs involves sending them to each vehicle through various channels such as RSEs, cellular networks, satellite communications, or customer Wi-Fi [13].