This document discusses hop-by-hop message authentication in wireless sensor networks. It discusses how symmetric-key based approaches have key management and scalability issues and how public-key based approaches have high computational overhead. It then summarizes a proposed source anonymous message authentication scheme that provides hop-by-hop node authentication without threshold limitations and has better performance than symmetric-key based schemes. The scheme is based on optimal modified ElGamal signature and provides source anonymity, compromise resilience, and flexible time authentication.