## 1 Introduction  
Wireless Sensor Networks (WSNs) comprises of a large number of sensor nodes that are densely deployed either inside the phenomenon or very close to it. In sensor networks, a source is defined as a sensor node that detects a stimulus, which is a target or an event of interest, and generates data to report the stimulus. A sink is defined as a user collecting these data reports from the sensor network. In many sensor network applications such as military, homeland security or environmental surveillance, it is necessary for users (sinks) to access sensor networks while they are moving. For example, in a battle- field a soldier with a PDA in hand might continuously collect an enemy tank's movement information while he is moving. Sink mobility brings new challenges to secure routing in large-scale sensor networks. To maintain a dissemination rout, a mobile sink has to constantly propagate its current location information to all sensor nodes. That introduces significantly communication overhead. Furthermore, intermediate nodes on the new rout have to discover their neighbors and exchange secret information to establish a secure communication. That also introduces more computation and communication overhead. A number of routing protocols have been proposed for WSNs such as LEACH [1], SecRout [2], SEEM [3], SeRINS [4], TTSR [23]. However, these works do not take into account of sink mobility and their routing paths remains static. So they are not suitable to work in such mobile sink circumstances. Several studies have considered the scalability and efficiency of data dissemination from multiple sources to mobile sinks such as TTDD [17], Directed Diffusion (DD) [18], SAFE [5], SEAD [6], Declarative routing protocol [7], and GRAB [8]. However, the authors mainly focused on energy efficiency. These approaches are very vulnerable from many attacks in sensor network routing such as spoofed, altered, or replayed routing information [9][16], selective forwarding, sinkhole [16], _Sybil_[10], wormhole [11], and HELLO flood (unidirectional link) attacks [16]. Also, most of existing routing protocols consider routing protocols and security schemes (like key management) separately. As a consequence, it is unsurprising that they are vulnerable to many attacks. It is not trivial to fix the problem since it is unlikely that a sensor network routing protocol can be made secure by incorporating security mechanisms after the design has completed. Studies and experiences have shown that considering security during design time is the best way to provide security and efficiency for sensor network routing. In this paper, we propose an energy-efficient secure routing protocol and key management scheme for mobile sinks in sensor networks, called SCODE_plus_. This scheme is a significant extension of our previous work [12][26] in five aspects:1. Key management scheme and routing protocol are considered during design time to increase security and efficiency. In our previous study, we focused on the efficiency for the secure routing protocol with an assumption that the sensor network achieves a key infrastructure from LEAP+ [14]. 2. The network topology is organized in a hexagonal plane. Compared with our previous square-grid topology, the hexagonal topology brings more efficiency. It decreases the number of cells, reduces transmission hops, and increases the number of sleeping nodes to save more energy. This is because the hexagonal geometry provides the best approximation to circle and covers the biggest area as compared to the rectangle or triangle. Also, a hexagon has the least neighboring cells (six) compared with a rectangle (eight) or a triangle (twelve). 3. The key management scheme can eliminate the impacts of node compromise attacks on links between non-compromised nodes. Our previous work has a weakness with impacts of nodes compromise attacks like most existing key management schemes. 4. Sensor node deployment conforms to _Gaussian_ distribution.