# A V2V Identity Authentication and Key Agreement Scheme Based on Identity-Based Cryptograph
## 6 Security Analysis and Experimental Validation  
### Security Analysis  
This paper proposes a V2V lightweight identity authentication and key agreement scheme. The security of cryptographic protocols depends on the security strength of the cryptographic algorithms and the rigorousness of the protocol logic. With this scheme, an equipment registration center (ERC) and a key management center (KMC) are set up in the C-V2X system. Identity authentication is implemented based on identity-based cryptography between the equipment to ensure the opposite equipment is registered equipment and the opposite equipment is trusted. During the authentication process between the equipment, the working key is synchronously negotiated and used to encrypt the communication data to ensure the confidentiality and integrity of the communication data.  
Initially, the equipment registers in the ERC, obtains the VID, and obtains the private key from the KMC. The equipment send secret data encrypted with the counterpart public key to each other, verify whether the data received from the opposite equipment is the private data previously sent, implement challenge/response authentication to verify whether the identity of the opposite equipment is consistent with its VID, and confirm the opposite equipment is trusted. The private data transmitted in the authentication process can be used as the material for generating the working key, and the working key can be calculated using the agreed algorithm.  
The V2V identity authentication process mainly faces two security risks: first, malicious equipment try to fake registered equipment to access the C-V2X system and try to establish secure communication with other equipment in the system. Second, malicious equipment monitor the authentication data in the open environment and try to replay the authentication response to access the system.  
For the first case, the equipment generates private data, uses the opposite equipment public key to encrypt the private data, and sends the encrypted private data to the opposite equipment. The equipment verifies whether the opposite equipment has a private key consistent with its VID by verifying the opposite equipment response. Only the equipment with the private key corresponding with its VID can decrypt the encrypted private data and feedback the correct response message to verify whether the opposite equipment is registered equipment and successfully obtain the private key from the KMC.  
For the second case, the authentication request and response data include the system time, \(T\), and the authentication data is encrypted by the public key of the opposite equipment to ensure the \(T\) has not been tampered. The equipment determines whether it is replay data by verifying the effectiveness of \(T\). The equipment rejects the authentication when \(T\) is reused or times out. When the V2V identity authentication process completes successful execution, the authentication data is invalid and cannot be reused for preventing replay attack.  
The identity authentication between the equipment confirms the identity of the opposite equipment and synchronously negotiates the working key, \(wk\), which used to protect the confidentiality and integrity of the communication data. The \(wk\) is time effective. When \(wk\) times out or the amount of encrypted data with the \(wk\) exceeds the set value, the equipment need to re-authenticate and negotiate the working key. It can ensure the working key security and prevent attacks, such as cracking the working key.