Protocols for Secure Communication in Wireless Sensor Networks

56 Chapter 2. Wireless Sensor Networks and Their Security
establish pairwise secret keys between arbitrary pairs of nodes.
• Entity authentication – By means of a challenge/response protocol, a key
can prove its knowledge of a certain secret key and thus demonstrate its
identity and liveness.
Once a key has been established between two communicating entities (either
two sensor nodes or a node and the base station), this key can serve as a master
key for deriving actual communication keys that are only used for a certain
time span. This makes cryptographic attacks on the communication harder,
since only a limited amount of data is available for cryptanalysis. Also, if
a communication key happens to be exposed to the attacker, only a limited
amount of data is compromised. In sensor networks, long-term relationships
between pairs of nodes exist mainly between neighbouring nodes. Generating
a fresh communication key for their communication is desirable. Long-range
communication between nodes is usually sparse and not bound to distinct nodes
but happens rather between node clusters or groups. Frequent updates to these
communication keys may not be necessary in many cases.
Key revocation in sensor networks has the goal of excluding specific nodes
from future communication after it has been detected that the key material of
these nodes has been exposed to the attacker, for example after a node capture.
Such nodes must not be allowed to further participate in the operation of the
network. Additionally, the key material shared between compromised nodes
and others should not be used anymore and keys that have been established
based on this material may have to be renewed. This is especially important
in cases where it has to be assumed that the attacker has recorded all previous
traffic.
Revoking keys is an expensive operation in a sensor network as it cannot
be expected that nodes regularly check a central repository of revoked keys.
Thus, revocations have to be actively distributed throughout the network to be
effective. These messages may become large if a large amount of key material
is affected such as in pool-based schemes. The follow-up key re-negotiations
put further load on the nodes.
Another problem is the detection of compromised nodes, which is necessary
to initiate a key recovation procedure. In general, this is only possible through
aberrant behaviour of nodes or through external means such as surveillance.
A sophisticated attacker might avoid detection by not substantially altering the
behaviour of captured nodes, and surveillance may not be possible or too expensive.
Yet another problem of revocation is possible abuse by an attacker. It may