Swarm robotics is the study of how a large number of relatively simple robots can be designed so that a desired collective behaviour emerges from the local interactions among robots and between the robots and their environment. While many aspects of a swarm may be modelled as various types of ad hoc networks, and accordingly many aspects of security of the swarm may be achieved by conventional means, here we will focus on swarm emergent behaviour as something that most distinguishes swarm robotics from ad hoc networks. We discuss the challenges emergent behaviour poses on communications security, and by classifying a swarm by types of robots, types of communication channels, and types of adversaries, we examine what classes may be secured by traditional methods and focus on aspects that are most relevant to allowing emergent behaviour. We will examine how this can be secured by ensuring that communication is secure. We propose a simple solution using hash chains, and by modelling swarm communications using a series of random graphs, we show that this allows us to identify rogue robots with a high probability.