# Negotiation-Free Encryption for Securing Vehicular Unicasting Communication
## 1 Introduction  
Vehicular Ad hoc Networks (VANETs) are special wireless networks which are designed to meet the needs of vehicular communications. Since applications of different regions have different characteristics, VANETs should be designed according to the characteristics of the vehicle to let the ad hoc network achieve the best performance in vehicle communications. Vehicles have a strong mobility characteristic compared with traditional self-organizing networks, usual encryption and authentication methods are not suitable for VANETs. Because VANETs support safety services [1; 2; 3; 4; 5] which plays an important role in Intelligent Transport System (ITS), a high level of security is needed to ensure secure vehicular communications. Therefore, data transmissions between vehicles are faced with some security issues. For instance, these include how to ensure that the data are not lost, stolen, or tampered with [6; 7; 8]. Additionally, there are some security concerns about privacy [9; 10; 11]. According to the present stage, end-to-end network transmission uses the Internet Key Exchange (IKE) Protocol. Before the transmission process, two stages are required: security protocols and key negotiations. After the Security Association (SA) negotiation, the process uses the DH algorithm for the key exchange transmission and distribution and then uses the AH and ESP protocols to encapsulate the package to ensure data security. The IPSec protocol must negotiate all safety parameters, which contain encryption and identification algorithms [12], encryption and authentication keys, the key lifetime, etc. It costs a lot of overhead to ensure the establishment of a Security Association (SA). In addition, in the communication process, every time the packets are sent, the key negotiation will be reestablished. Thus, the process spends a significant amount of time negotiating and does not conform to the existing vehicle network environment. To achieve efficiency and safety for the vehicle's transmission network, we need to shorten the time spent on the negotiation before the data transmission and reduce the average delay and the bandwidth overhead of the data transmission as much as possible without compromising security. Thus, we propose a negotiation-free encryption method (NFEM) to reduce the preparatory work before the data transmission and then achieve high efficiency and safety. In today's vehicular environment, a trustworthy cloud and OBU (on-board unit) are easily to implement, which provides our work with a reliable premise. As for the NFEM, we use the vehicle state attribute value as a packet encryption key to replace the traditional IPSec key negotiation and management stage. How to address the vehicle properties is a key point in this paper. We use a trustworthy cloud to manage the real-time status of every vehicle. Before transmitting, the sender requests the cloud for the receiver's state value, which is used to encrypt the transmitted data. Compared with IPSec, this method reduces the overhead of the transmission process, such as policy consultations, DH value consultation, nonce exchange, authentication, the cryptographic algorithm, some of the necessary auxiliary data, etc. Negotiation and authentication are particularly complex processes. However, in the NFEM, the vehicle simply needs to request the historical state value of itself from the cloud, which is equivalent to obtaining the decryption key and does not require the parties to negotiate. Thus, it considerably reduces the negotiation and validation overhead. To prevent the vehicle state privacy from leaking, the cloud will hash the status value before the data are transmitted. Such measures will prevent attackers from intercepting the vehicle's state value and prevent malicious requests that aim to know other vehicles' status value. In the process of messaging, we use the timestamps as the time recording of the messaging.