3.2 RELATED WORK
A review over the various techniques which used for key management in VANET is presented in this section.
Hao, Y., Cheng, et Al“A distributed key management framework with cooperative message authentication in VANET In  The OKD scheme is a centralized group key management protocol and uses periodic rekeying to decrease the communication cost. The OKD scheme constructs the key tree with one-way key approach and derives the new key from the old one to improve the rekeying efficiency. There are internal k-nodes, external k-nodes, and members in each group (u-nodes). In the tree, the internal k-node stores the auxiliary key and the external k-node stores the secret key for each u-node. The secret key is only known by the…show more content… In the group signature, public key of one group is connected with private keys of multiple groups. A message that is signed by any private keys of group can be verified with unique group public key. Therefore attacker cannot identify who is sender of the message. Author also proposed an efficient cooperative message authentication protocol, to reduce verification load where validity of safety message is examined by only those verifiers, who are selected on the basis of their position related to sender vehicles. This scheme helps to revoke malicious vehicles, maintaining system and various different security principles. If a verifier discovers an invalid regular broadcast message that is not verified as group signature, it broadcasts a warning message also called as cooperative authentication message to other vehicles.
Guo, M. H., Liaw et Al“Centralized group key management mechanism for VANET,”  In this paper the author has been proposed technique of binary search is used in RSU message verification phase. Bloom filter is used to replace hash value in notification message for reducing message overhead and enhancing effectiveness of verification phase. To enhance efficiency pairing operations are reduced from three to two so in the verification phase Vehicles can use its different