Many distributed systems assume participants are both performant and secure, characteristics offered by many cloud-based systems. However, scaling distributed techniques down to highly resource- or power-constrained contexts may require alternative approaches. One such context is the deployment of ad hoc distributed systems in insecure or uncontrolled areas, for example during disaster response activities. Providing reliable and secure service is exacerbated by the computational and power constraints imposed on these devices. In this work, we first introduce the concept of on-demand topic-channels. Then, we describe three message authentication protocols which provide secure, authenticated communication between participants and a coordinator, while also providing resilience from adversarial or accidental disruption. We leverage homomorphic hashing primitives to trade message secrecy against communication and computational costs. Finally, we assess these protocols, and show that our hash-based protocols provide significant efficiency improvements over traditional encryption-based approaches.