# A Low-Overhead Message Authentication and Secure Message Dissemination Scheme for VANETs
## 3 Proposed Approach
### Proposed Method  
In our scheme, _RSUs_ are responsible for verifying the authenticity and integrity of messages sent by vehicles before disseminating them to other vehicles or _RSUs_. If traffic is heavy in the region of an _RSU_, the _RSU_ may not be able to receive messages from all vehicles in its region, process them, and disseminate them in a timely manner due to the authentication, aggregation, and communication overhead involved. To help _RSU_ minimize this overhead, the _RSU_ divides its region into sub-regions and selects one vehicle in each sub-region as the Group Leader (\(GL\)). These Group Leaders help the _RSU_ with receiving, authenticating, and aggregating messages from vehicles in its sub-regions and forwards them to the _RSU_. The _RSU_, in turn, is responsible for collecting, authenticating, and further aggregating the messages received from all the \(GL\)s in its region, and for disseminating them to all vehicles in its region through the \(GL\)s or to vehicles in other regions through other _RSUs_, as necessary. Thus, _RSUs_ incur less computation and communication overhead for collecting, authenticating, and disseminating messages. Following is the list of assumptions made in this paper:  
1. We assume that the clocks of _RSUs_, the \(DMV\), and the vehicles are loosely synchronized. This can be achieved using time received from a GPS. Messages are time-stamped using the local clock time to verify the freshness of the messages;
2. Certificates issued by the \(DMV\) for the vehicles and \(RSU\) are used for the authentication of vehicles and _RSUs_;
3. We do not address the issue of determining malicious vehicles or _RSUs_. Several approaches have been proposed in the literature to identify malicious entities in VANETs. Any of those approaches can be used for determining malicious vehicles. Once a vehicle is determined to be malicious, the \(DMV\) revokes its certificate and includes the certificate in the Certificate Revocation List (\(CRL\)). The \(DMV\) broadcasts the \(CRL\) to all _RSUs_ when it changes. The _RSUs_, in turn, broadcast the \(CRL\) to vehicles in its region;
4. When a vehicle \(v\) enters the region of an \(RSU\) (i.e., \(v\) is within the transmission range of an \(RSU\)), even though \(v\) will be able to receive messages sent by the \(RSU\), \(v\) may not be able to send messages directly to the \(RSU\) because the \(RSU\) may not be within the transmission range of \(v\). In this case \(v\) uses an underlying routing algorithm to send messages to the \(RSU\) through other vehicles. Any of the many routing algorithms proposed in the literature can be used for that purpose. Next, we describe our approach in detail. Figure 1: System model for VANETs. **When a vehicle \(v\) enters the region of an \(RSU\):** Each \(RSU\) periodically broadcasts its \(Cert_{RSU}\). When a vehicle \(v\) enters an area covered by an \(RSU\), \(v\) retrieves the public key of the \(RSU\) from \(Cert_{RSU}\) and checks its \(CRL\) to see if this \(RSU\)'s certificate has been revoked (the certificate of an \(RSU\) could be revoked if it is removed from the system). If not, then \(v\) sends a join request message \(M\) to the \(RSU\). The join request message \(M\) contains its currently used \(PID_{v}\), the corresponding certificate \(Cert_{v}\), and a timestamp (\(ts\)). After receiving this message, the \(RSU\) checks the freshness of the message using the \(ts\). Then, the \(RSU\) retrieves the public key \(PU_{v}\) and pseudo-ID \(PID_{v}\) of the vehicle from \(Cert_{v}\), and checks the \(CRL\) to determine if the vehicle's certificate has been revoked. If not, then the \(RSU\) sends an accept message to \(v\).