# A V2V Identity Authentication and Key Agreement Scheme Based on Identity-Based Cryptograph
## 4 Identity-Based Cryptograph Technology  
Asymmetric cryptography uses different keys to encrypt and decrypt; the private key is saved locally, and the public key is public and can be transmitted in plaintext, solving the problem of key secure transmission in symmetric cryptography. Compared with symmetric cryptography, asymmetric cryptography has same the security, but it is easier to distribute and manage the key. In asymmetric cryptography, how to securely publish the public key and how to associate the user identity information with his key is the key problem. The PKI (public key infrastructure) system is generally used to generate, manage, store, distribute, and revoke certificates. Due to the complexity of certificate management, the PKI system is often large in the practical application with a high cost and low efficiency [11].  
In the security system based on PKI, each equipment needs to apply and install a certificate to ensure communication security, which makes the equipment more complex, especially for equipment with poor computing power. The PKI system needs to establish a CA to uniformly manage the generation, issuance, storage, and revocation of the certificate; the CA is a complex system. The equipment needs to verify the opposite equipment certificate to ensure the opposite equipment is registered equipment, which requires large computing overhead. Vehicles may be running in high speed on the highway. V2V communication may transmit highly timely information, such as traffic conditions and safety accidents. The communication delay should be low, and the computational complexity should be small. The method that requires complex computation may not be suitable for many V2V communication scenarios. To reduce the complexity of key management in asymmetric cryptography system, [PERSON] proposed identity-based cryptography in 1984 [12; 13]. The user identity can be used as the user public key, or the user public key can be calculated from the user identity by an appointment algorithm.  
Identity-based cryptography is the same as traditional asymmetric cryptography. Each user has both an associated public key and a private key. In the identity-based cryptography system, the user identity, such as name, IP address, e-mail address, mobile phone number, and so on, is used as the public key, and the corresponding user private key is calculated with the public key. The user ID is the user public key, which does not need generation and storage. It only needs to publish the public key, and the private key is saved secretly by the user. The user private key is calculated by the KGC (key generation center) according to the system master key and user ID. The user public key is uniquely determined by the user ID so it does not need a third party to ensure the authenticity of the public key. The identity-based cryptography does not need key infrastructure and has better security and convenience than the PKI.  
In 1999, [PERSON], [PERSON], and [PERSON] proposed an identity-based key-sharing scheme using elliptic curve pairing. In 2001, [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] proposed a method of constructing identity-based cryptography with elliptic curve pair. These works have promoted the new development of identity-based cryptography, and a number of identity-based cryptography implemented with elliptic curve pairs have emerged, including digital signature algorithm, key exchange protocol, key encapsulation mechanism, and public key encryption algorithm. For identity-based cryptography as an asymmetric cryptography, the most prominent feature is the user does not need a certificate; the user private key is generated by the KGC based on the system master key and user identity [14]. SM9 is an identity-based cryptography standard which is recommended by the government of China. Identity-based cryptography does not need a certificate, is simple and easy to use, and meets the high real-time application environment, such as the C-V2X system [15; 16].