In this paper, we propose a pairing-based signcryption scheme for authenticated broadcasting, which requires less computation than a previously proposed scheme suggesting identity-based cryptosystem for ad hoc networks. Due to the dynamic nature of ad hoc networks, we allow nodes to generate their own broadcast keys for different groups in the network and change those when associated groups are changed. However, we ensure through our signcryption scheme that broadcast keys would be implicitly controlled by the Trusted Authority (TA), and can be used for as long the private keys are issued. Our keys are based on identities and do not use certificates. We also propose a non-probabilistic method for computing unique broadcast keys for different groups. We use identity-based pairwise symmetric keys as the building block for our broadcast scheme. Such keys are computed non-interactively by the nodes, which reduces communication overhead and simplifies key management in pairwise communication.