# A Low-Overhead Message Authentication and Secure Message Dissemination Scheme for VANETs
## 3 Proposed Approach
### Proposed Method
IEEE 1609.2, the standard for Wireless Access in Vehicular Environments (WAVE)--Security Services for Applications and Management Messages [22], has specified that the vehicle must be authenticated using certificates issued by the \(TA\) and defined the \(CRL\) that contains the list of the revoked certificates that are updated timely and disseminated in the vehicular network. Once the \(CRL\) is distributed to the vehicles, it can compare the certificate of a vehicle with the list and determine if it has been revoked. In our scheme, the \(DMV\) will manage and maintain the updated \(CRL\). The \(DMV\) will distribute the \(CRL\) to the \(RSU\)s, which, in turn, will distribute them to all vehicles in their region directly or through the \(GL\)s, if the \(GL\)s have been selected. The \(RSU\)s and \(GL\)s always check the authenticity of the vehicles using the \(CRL\). If a vehicle is found to be malicious, the \(RSU\) sends the certificate information of the vehicle to the \(DMV\). Then, the \(DMV\) adds the certificate to the \(CRL\) and distributes the updated \(CRL\) to all \(RSU\)s. Note that vehicles only communicate either with the \(RSU\) or the \(GL\) and that no communication between themselves occurs, which reduces the communication and computation overhead. We do not address the problem of detecting malicious vehicles. Many researchers have addressed the malicious vehicle detection problem in VANETs [23, 24]. Any of those schemes can be used to detect malicious vehicles. ### Some Optimizations for Our Approach

In our scheme, when a vehicle \(v\) enters the region of an \(RSU\), it obtains a symmetric key \(K\) through the _Accept_ message \(M_{2}=(``\emph{Accept}\",K,ts)\) from the \(RSU\) for establishing secure communication between \(v\) and the \(RSU\) (please see Algorithm 1). This key \(K\) is used by \(v\) to encrypt messages and send them to the \(RSU\) in the absence of \(GLs\); this key is also used by the \(RSU\) to send messages, as well as \(CRLs\), securely to \(v\), in the absence of \(GLs\). To reduce this overhead caused by sending unicast messages, the \(RSU\) can attach a group key \(GK\) to the accept message as \(M_{2}=(``\emph{Accept}\",GK,K,ts)\); then, \(GK\) can be used by the \(RSU\) to broadcast (instead of unicasting) securely the \(CRLs\) as well as other messages to all vehicles in its region. Similar optimizations can be performed in Algorithm 3 when a \(GL\) assigns a symmetric key \(K\) to a vehicle \(v\) through the message \(M_{2}=(``\emph{Accept}\",K,ts)\). ## 4 Results

In our scheme, the encryption and the signature are fundamental security mechanisms used to resist impersonation, eavesdropping, replay, and modification attacks. The message that is sent by a vehicle \(v\) to its \(GL\) to be modified must be decrypted, modified, and then encrypted by an attacker using the \(v\)'s shared symmetric key. To decrypt the message, the attacker needs the symmetric key shared between the \(v\) and \(GL\), which is not available to the attacker, thus making it impossible to modify the message. Replay attacks are prevented using timestamps. In our scheme, an attacker cannot generate a valid signature of other vehicles because the attacker does not know the private key of the vehicle. As a result, an attacker cannot send a malicious signed message without being detected. **Our scheme is secure against impersonation attacks:** To perform an impersonation attack, the attacker should be able to obtain the private key \(PR_{v}\) of a legitimate vehicle \(v\), which the attacker does not possess. In addition, an attacker cannot impersonate a vehicle \(v\), as the message encrypted using a shared symmetric key \(K\) between \(v\) and \(GL\) (or between \(v\) and the \(RSU\)) cannot be decrypted without using \(K\), which the attacker does not possess. **Our scheme preserves privacy--an attacker cannot discover the vehicle's identity:** Vehicles are assigned pseudo-IDs. A vehicle never uses its real ID in any communication.