VANETs are considered as sub-category from MANETs. They provide the vehicles with the ability of communication among each other to guarantee safety and provide services for drivers. VANETs have many network vulnerabilities like: Working on wireless media makes it vulnerable to many kinds of attacks and nodes can join or leave the network dynamically making change in its topology which affects communication links stability. In VANETs, each car works as a node and router, so if a malicious attacker joins the network, the attacker could send false messages to disrupt the network operation and that is why VANETs are vulnerable to many types of attacks. Denial of service, spoofing, ID disclosure, and Sybil attacks can be launched against VANETs. In this paper, we present cryptographic protocol for Sybil Attacks Prevention (Crypto-SAP) which is a new protocol. Crypto-SAP uses symmetric cryptography to defend VANETs against Sybil nodes. Simulations were done to investigate how Crypto-SAP protocol affects the network performance.