In this paper, we analyze attacks on existing time synchronization protocols for wireless sensor networks. We propose a secure time synchronization toolbox to counter these attacks. This toolbox includes protocols for secure pairwise and group synchronization of nodes that lie in each other’s power ranges and of nodes that are separated by multiple hops. We provide an in-depth analysis of security and energy overhead of the proposed protocols. Categories and Subject Descriptors C.2.2 [Computer Systems Organization]: Computer Communication Networks – Network Protocols. General Terms Algorithms, Security, Verification. Keywords Sensor Networks, Security, Time Synchronization, Message Authentication Code, Delay.