Vehicular ad-hoc networks (VANETs) are an important step in vehicular development. Especially broadcast communication is an important matter to exchange data about traffic flow or dangerous spots. Given that these data should be available for everyone, encryption does not matter. However, it should be possible to determine the source of the transmission to ensure that only messages from authorized members are accepted. This way attackers are not able to disturb the network and propagate bogus messages. Generally, this is done by using digital certificates and public-key encryption, which is very resource intensive. A much better approach for source authentication is the usage of one-way hash chains: A key chain computed by repeatedly hashing a start value to generate a chain of self authenticating values, which are used in reverse direction of generation. In the disclosure of these keys, time synchronization of the sender and the receiver is of great importance to ensure security. This book first presents related protocols for source authentication in ad-hoc networks. Subsequent they are compared to each other and a new protocol is going to be presented and implemented in C.