## 1 Introduction  
Underwater Acoustic Sensor Networks (UASNs) are constituted by underwater sensor nodes and autonomous underwater vehicles (AUVs) interconnected by a wireless acoustic underwater communication network. UASNs can be used for many purposes including ocean sampling, environment monitoring, undersea explorations, distributed tactical surveillance and disaster prevention [1]. Co-ordination and sharing of information among sensor nodes and AUVs require secure communication [2]. Since the acoustic channel is an open medium, an attacker conveniently equippedby an acoustic modem can easily eavesdrop on the messages traversing the network. This could be very dangerous, for example, in distributed tactical surveillance applications where messages must be secret. Furthermore, the attacker can also modify or inject fake messages so compromising the integrity of the system at both the application and the network layer. The level of damage that may ensue depends on the specific case. However, it may include an alteration of the positives/negatives rate, forms of denial of service deriving from the violation of the integrity of the routing service [3], and even the loss or the damage of AUVs and sensor nodes. All these considerations show the urgency of establishing secure channels among underwater nodes. The unique characteristics of the underwater acoustic channel, and the differences between UASNs and their ground counterpart, namely Wireless Sensor Networks (WSNs) [4], require the development of efficient security mechanisms. Radio waves do not propagate well underwater due to the high energy absorption of water. Therefore, underwater communication is based on acoustic waves that are characterised by large propagation delays. The propagation speed of sound in water is typically 1,500 m/s, five orders of magnitude lower than light speed. In addition, acoustic links have low bandwidth and low quality due to the chemical-physical properties of the water medium such as temperature, salinity, density and spatio-temporal variations. Furthermore, underwater hardware is more expensive than WSNs nodes, and thus underwater nodes are sparsely deployed. It follows that underwater communication have more stringent power requirements than terrestrial systems because acoustic communication are more energy expensive, distances between underwater nodes are greater and thus higher power is required to ensure coverage [1]. In this paper we present the secure communication suite for UASNs we have designed and experimented within the European project called \"Underwater Acoustic Network\" (UAN) [5]. We have considered a UASN composed of both AUVs and fixed underwater sensor nodes that cooperate towards the same application. Nodes may move as well as dynamically join and leave the system. A node joins the system when its mission starts and leaves when its missions finishes. A node may also be forced to leave when suspected of being compromised. The reference application scenario was a distributed surveillance system. The secure communication suite is composed of two integrated components--a _secure routing protocol_ and a set of _cryptographic primitives_ (cipher, digest, and re-keying)--and aims at protecting confidentiality and integrity of the application and network control messages while taking into account the peculiarities of the underwater acoustic medium. The paper provides the following novel contributions. First of all, the paper presents a practical and efficient solution to secure communication in UASNs. The proposed suite (i) protects end-to-end confidentiality and integrity; (ii) supports both the one-to-one and one-to-many communication paradigm; (iii) allows secure reconfiguration due to nodes mobility, joining, and leaving. The resulting suite introduces limited communication overhead and negligible energy consumption. We show that these claims are well founded by means of sea trials and simulations based on real data.