Wireless Sensor Networks, part 2: Limitations - 3. Limited Resources
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Moving on, the main obstacle for adopting complex security mechanisms is the limited resources that sensor nodes have. Although their limited size makes them attractable for use in a number of situations, at the same time their size affects resources such as the energy, computational power, and storage available.
3.1 Power restrictions
The power restrictions of sensor nodes are raised due to their small physical size and lack of wires. Since the absence of wires results in lack of a constant power supply, not many power options exist. Sensor nodes are typically battery-driven. However, because a sensor network contains hundreds to thousands of nodes, and because often WSN are deployed in remote or hostile environments, it is difficult to replace or recharge batteries. The power is used for various operations in each node, such as running the sensors, processing the information gathered and data communication.
Keep in mind that communication between sensor nodes consumes most of the available power, much more than sensing and computation. Power limitations greatly affect security, since encryption algorithms introduce a communication overhead between the nodes; more messages must be exchanged, i.e. for key management purposes, but also messages become larger as authentication, initialization and encryption data must be included.
3.2 Limited Computational power
In the case of computational power, computations are linked with the available amount of power. As you may understand, since there is a limited amount of power, computations are constrained also. Although it is acknowledged that sensors are not expected to have the computing power of workstations or even mobile handheld devices, researchers and developers are greatly concerned with the issue.
As I mentioned in the previous section, more power is used for communication than computations. Therefore, since the power for computations is even more constrained than the total quantity of power, complex security solutions are prohibited. The limitation of computational power limits the adoption of strong cryptographic algorithms such as the RSA public key algorithm, which is computationally expensive.
Instead, symmetric encryption algorithms are used to secure sensor nodes' communication, since symmetric encryption doesn’t have as demanding computational requirements as asymmetric encryption. However, with asymmetric encryption, features like digital signatures are not supported. Therefore, another challenge for researchers and developers is to design appropriate algorithms to establish and verify trust among the nodes participating in a communication. Furthermore, other security solutions must be adopted to cover the weaknesses of symmetric encryption; when an adversary compromises a node, he could retrieve the shared key used to encrypt the messages and then compromise the entire communication of the sensor network.
3.3 Storage Restrictions
The limited capability for storage affects the storage of cryptographic keys as well. According to the encryption scheme used, each sensor node may need to know a number of keys for each other node in the network to secure communication, and thus store the keys in the nodes’ storage space. However, the large number of sensor nodes requires a lot of memory, which may not be provided. As I mentioned previously, having a single encryption key common to all nodes allows an adversary to compromise the whole network by compromising only a single node. The challenge of storage restriction is for researchers to design security protocols in a way that a minimum number of encryption keys must be used to provide adequate protection to the network.
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