## 1 Introduction  
Several recent researches have shown how a set of autonomous mobile agents and sensors, able to self-adapt and self-configure, can be used in several complex scenarios [1]. The collaborative use of multiple sensors is in fact of great advantage thanks to the resulting flexibility and robustness in the accomplishment of tasks. For instance, exploration of partially known or unknown environments can effectively be performed by a team of cooperating autonomous vehicles with an optimized use of the available resources and consequent saving of time and money [2]. As another example, the use of sensor networks for continuous monitoring of vital areas allows for disaster prevention and for a prompt reaction against unexpected situations [3]. One scenario in which the use of multiple vehicles or sensor nodes presents critical aspects and peculiar characteristics is the underwater one. Many relevant infrastructures are placed very close to the sea or directly in the water, opening new scenarios for the use of Autonomous Underwater Vehicles (AUVs) and the development of Autonomous Ocean Sampling Networks (AOSN), where multiple nodes can cooperate as a group to achieve some common goals [4]. Any cooperative mission necessarily involves communication among multiple agents. When the cooperating sensors are used in the underwater domain, communication issues become of paramount importance and in fact the agent spatial locations and mutual separation has a direct influence on the capability to communicate [5,6]. In the last years some studies have been carried out to include communication constraints into the development of cooperative strategies for set of vehicles [7,8]. However the impact of limited and/or unreliable communication has not been fully characterized. Most of the cooperation strategies proposed in the literature have been focused on cooperation of aerial or terrestrial vehicles, but these algorithms are not directly applicable in the underwater case due to the strong variation in space and time of the communication medium. Acoustic propagation, the main means of underwater communication, is strongly dependent on local environmental conditions, and during the evolution of the mission each vehicle can experience abrupt changes in the channel, with a consequent variation in communication performance. Moreover, acoustic communication is severely band-limited and range-limited. Sudden reduction of the channel capacity and bandwidth, or even a temporary loss of connectivity with the rest of the team, is a frequent condition for underwater communications. In operative scenarios, not only is it necessary to share information to achieve the mission objectives, the ability to securely communicate also becomes a key issue so that the correct data is transmitted and received by the right agents, and only among the desired group. The possibility to share in a secure way the necessary information may in fact determine the success or the failure of the mission as a whole. Listening to private messages, or modification or injection of fake data are all usual threatsin communication networks. They become even more critical in the context of distributed agents since cooperation may be achieved only when all the components receive the expected data from the legitimate peers. The underwater environment poses unique challenges also as far as network security is concerned, and again the traditional security mechanism successfully used and implemented on radio-based network suddenly becomes infeasible. With this work we tackle the problem of secure cooperation of a team of mobile underwater sensors or AUVs within surveillance and patrolling applications. The contribution of the paper is twofolds:  
* A novel cooperative adaptive algorithm for mobile agents is proposed, with the goal of protecting an asset (e.g., a critical infrastructure such as a power plant placed on the shore or directly in the water) using detection sonars mounted on each agent.