Wireless sensor networks and other power-efficient devices fill increasingly important roles in modern society. At the same time, they also face increasing internal and external threats, such as node capture or protocol disruption by adversarial agents. Providing reliable and secure service in the face of these challenges remains an ongoing problem, and one that is only exacerbated by the computational and power constraints imposed on these devices. In this paper, we first introduce the concept of on-demand topic channels in the context of ephemeral wireless sensor networks. Then, building on this concept, we introduce three novel messaging protocols to provide secure, authenticated communication between a sensor network and an authorized user while also providing resilience from accidental or adversarial disruption. These protocols leverage homomorphic hashing in innovative ways to trade secrecy against network and computational costs in on-demand topic channel authentication. Finally, we compare and contrast the costs of these protocols, and show that hash-based protocols provide significant implementation-independent improvements to network resilience.