To establish secure (point-to-point and or broadcast) communication channels among the nodes of a wireless sensor network is a fundamental task. To this end, a plethora of (socalled) key pre-distribution schemes have been proposed in the past. All these schemes, however, rely on shared secret(s), which are assumed to be somehow pre-loaded onto the sensor nodes. In this paper, we propose a novel method for secure initialization of sensor nodes based on a visual out-of-band channel. Using the proposed method, the administrator of a sensor network can distribute keys onto the sensor nodes, necessary to bootstrap key pre-distribution. Our secure initialization method requires only a little extra cost, is efficient and scalable with respect to the number of sensor nodes. Moreover, based on a usability study that we conducted, the method turns out to be quite user-friendly and easy to use by naive human users.
The problem of secure sensor node initialization has been considered only recently. Prior to MiB method of @cite_15 (which we reviewed in the previous section), the following schemes were proposed. The Shake-them-up'' @cite_5 scheme suggests a simple manual technique for pairing two sensor nodes that involves shaking and twirling them in very close proximity to each other, in order to prevent eavesdropping. While being shaken, two sensor nodes exchange packets and agree on a key one bit at a time, relying on the adversary's inability to determine the sending node. However, it turns out that the sender can be identified using radio fingerprinting @cite_14 and the security of this scheme is uncertain.
@cite_5 @cite_15 @cite_14
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This paper presents a new pairing protocol that allows twoCPU-constrained wireless devices Alice and Bob to establish ashared secret at a very low cost. To our knowledge, this is thefirst software pairing scheme that does not rely on expensivepublic-key cryptography, out-of-band channels (such as a keyboardor a display) or specific hardware, making it inexpensive andsuitable for CPU-constrained devices such as sensors. In the described protocol, Alice can send the secret bit 1 toBob by broadcasting an (empty) packet with the source field set toAlice. Similarly, Alice can send the secret bit 0 to Bob bybroadcasting an (empty) packet with the source field set to Bob.Only Bob can identify the real source of the packet (since it didnot send it, the source is Alice), and can recover the secret bit(1 if the source is set to Alice or 0 otherwise). An eavesdroppercannot retrieve the secret bit since it cannot figure out whetherthe packet was actually sent by Alice or Bob. By randomlygenerating n such packets Alice and Bob can agree on ann-bit secret key. Our scheme requires that the devices being paired, Alice andBob, are shaken during the key exchange protocol. This is toguarantee that an eavesdropper cannot identify the packets sent byAlice from those sent by Bob using data from the RSSI (ReceivedSignal Strength Indicator) registers available in commercialwireless cards. The proposed protocol works with off-the-shelf802.11 wireless cards and is secure against eavesdropping attacksthat use power analysis. It requires, however, some firmwarechanges to protect against attacks that attempt to identify thesource of packets from their transmission frequency. We demonstrate the feasibility of finger-printing the radio of wireless sensor nodes (Chipcon 1000 radio, 433MHz). We show that, with this type of devices, a receiver can create device radio finger-prints and subsequently identify origins of messages exchanged between the devices, even if message contents and device identifiers are hidden. We further analyze the implications of device fingerprinting on the security of sensor networking protocols, specifically, we propose two new mechanisms for the detection of wormholes in sensor networks.