# Negotiation-Free Encryption for Securing Vehicular Unicasting Communication
## 7 Conclusions  
This paper proposes the negotiation-free encryption method based on VANETs. In NFEM, the existing vehicle status information stored on the cloud can be used to generate the encryption key for each vehicle communication session, and the vehicle cloud is regarded as a key distribution center, with no need to build a new key distribution server. Moreover, the performance of cloud is also far superior to that of traditional centralized key distribution servers.  
From the process point of view, the traditional KDC generates the encryption key for the communication session, and encrypt and send it to the requestor. Then the requestor transfers the key to the respondent. When the respondent receives the encryption key, it also needs to send a reply to the requestor to verify that the received communication request is true. During this period, there are several interaction steps between the two sides of communication, and there exists a risk of being attacked by a third party. In the NFEM method, the cloud server generates the encryption  
Figure 8: Beacon Transmission Frequency vs. delay (a).  
Figure 9: Beacon Transmission Frequency vs. delay (b).  
key by hashing the vehicle status information of the respondent at a certain time, and returns the key to the both communicating parties separately. Based on the timestamp, the respondent obtains the encryption key directly from the vehicle cloud, decrypts the ciphertext, which avoids the extra interaction steps of KDC and the third party attack risk.  
Compared with the traditional encryption methods of self-organizing network, NFEM mainly has the following characteristics. (1) In the transmission encryption process, it can reduce the key agreement, delay of encryption and overhead of key management. (2) The state value of vehicle attribute is used as a key, rather than by calculation and consultation with some of the key independent attributes of the vehicle itself, which greatly reduces the key generation time and meets the application requirements of VANETs. (3) In contrast with the traditional consultative mechanisms, NFEM reduces bandwidth consumption. In complex distributed vehicles network environments, as the vehicle density increases, the minimized bandwidth usage will be more favourable to network transmission. (4) In terms of security, the invariability of vehicle cloud and timestamp can ensure the security and privacy of data communication and control transmission for VANETs. Hence, our next work will be focused on transportation security.  
[PERSON], [PERSON], and [PERSON] designed the study. [PERSON], [PERSON], and [PERSON] developed the computer simulations and performed the data analysis. [PERSON], [PERSON] prepared the figures and tables. [PERSON] and [PERSON] wrote the final manuscript. All authors read and approved the final manuscript.  
This research was funded by the National Nature Science Foundation of China Grant Number 61373123, 61572229, U1564211, 6171101066 and 6187060366, by the Jilin Provincial Science and Technology Development Foundation Grant Number 20170204074 GX and 20180201068 GX, by the Jilin Provincial International Cooperation Foundation Grant Number 20180414015 GH, by CERNET Innovation Project Grant Number NGII20180701.  
The authors would like to thank the Associate Editor and the reviewers for their valuable comments that helped to improve the quality of this paper.  
The authors declare no conflict of interest.