# Negotiation-Free Encryption for Securing Vehicular Unicasting Communication
## 6 Performance Evaluation
### Comparison and Analysis of Two Methods  
Our experimental environment is the following: 64-bit 3.6 GHz Core i7 processor with 4G memory. The experimental simulation environment includes a GNS3. We select the AES256 encryption method for the comparisons. According to the transmission mechanism, this paper defines the average delay of the message as follows:  
\[Delay=\frac{1}{N}\sum_{i=1}^{n}(T_{KeyGen}+T_{KeyAuth}) \tag{13}\] \begin{table}
\begin{tabular}{c c c} \hline \hline
**Characteristic** & **NFEM** & **IPSec** \\ \hline Key distribution Center & Vehicle Cloud & KDC server \\ Channel setup & No need & IKE negotiation to establish SA \\ Key Generation & Hash Corresponding Vehicle Status & 3 DES192, AES128 or AES256 \\ Key distribution & Both vehicles get key from cloud separately & Requestor handover the key to respondent \\ Decryption consumption & Symmetric decryption algorithm & depends on key length \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Algorithm comparison. \begin{table}
\begin{tabular}{c c} \hline \hline
**Simulation Parameters** & **Value** \\ \hline _Car IDreq_ & 5 bytes \\ _Car IDres_ & 5 bytes \\ _State Value (hash)_ & 32 bytes \\ _Message size_ & 20 bytes \\ _Ciphertext size_ & 45 bytes \\ _Ciphertextsize_ & 2 bytes \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Simulation Parameters. N is the number of messages, \(T_{KeyGen}\) is the time which is spent to generate the encryption key and \(T_{KeyAuth}\) is the time that is spent to generate the decryption key. \[T_{KeyGen}=T_{req}+T_{hash}+T_{res} \tag{14}\]  
\(T_{req}\) is the time that the sender requests the state of the receiver. \(T_{hash}\) is the time that the cloud spends on hashing the state value, and \(T_{res}\) is the time that the cloud spends on sending the hash value of the receiver to the sender. \[T_{KeyAuth}=T_{req}+T_{res} \tag{15}\]  
\(T_{req}\) is the time that receiver requests its state value from the cloud. \(T_{res}\) is the time that the cloud spends on sending the hash value of the receiver's state value to the receiver. For the Negotiation-free encryption algorithm, the average delay of the transmission is 150 ms. The major consumption steps are the key initialization and the authentication process, which are respectively 61 ms and 62 ms. In the Negotiation-free encryption algorithm process, there are some other costs. For example, the average cost of the hash of the vehicle state using SHA256 is 0.016 ms. It encrypts the plaintext using AES and the 256-Bit hash value as the encryption key. The average overhead is 3 ms. Because they occupy a small portion of the total time, which can be negligible, this paper will only take two parameters (\(T_{KeyGen}\) and \(T_{KeyAuth}\)) as the main overhead, and the rest can be ignored. After many IPSec simulation experiments, the average delay is 310 ms. The main time-consuming processes are the D-H method calculation, the certification of digital signatures and the consultation process. The delay of these three parts is 35 ms, 80 ms and 170 ms, respectively. In the IKE encryption process, the consumption of other parts is among 0 to 1.5 ms, such as the encryption, decryption, hashing generation and authentication, and the digital signature authentication. All these can be neglected since they occupy only a small portion of these costs. According to the NFEM, the average time of key generation is 61 ms and the average time of decryption and authentication is 62 ms. Added together with the encryption and transmission delay, the total delay is 150 ms. Compared with the average delay of 310 ms of IPSec, our method saves half of the time. Therefore, on the aspect of the average delay, the NFEM is better than IPSec. According to the different encryption methods, Table 4 compares the average delay. We can summarize that the efficiency of the Negotiation-free encryption transmission is twice that of the negotiation mechanism of the traditional IPSec.