Cryptography is commonly used to provide link-layer message authentication in wireless sensor networks. However, keys are susceptible to compromise and introduce management requirements. Avoiding keys can therefore deliver security and management benefits. Our paper introduces and discusses the feasibility of RTTMAP, a protocol that uses radio frequency ranging for message authentication. RTTMAP uses secure round-trip-time with hash functions to determine the minimum distance of a transmitter. Transmissions from outside of a defined radius are rejected without requiring keys. We provide our motivation, an evaluation of our findings and continuing research challenges. We find RTTMAP offers higher security, costs about twice the energy of keyed message authentication but complicates MAC protocol selection.