Protocols for Secure Communication in Wireless Sensor Networks

2.9. Existing Approaches to Wireless Sensor Network Security 59
A more flexible technique has been proposed first by Eschenauer and
Gligor [64]. It is based on random key predistribution and provides stochastic
security against node capture attacks, i.e. if only a small number of nodes
are captured, there is a high probability that a key exchanged between two
nodes remains secret. There have been several extensions and improvements
proposed to that basic technique [39, 143, 204, 38].
The earlier proposals for group key management by Blom [24] and Blundo
et al. [25] were the foundation for other key agreement schemes [117, 60, 161].
These exhibit a threshold property: unless a certain number of nodes have been
captured, all pairwise keys remain secure with high probability; if the threshold
is exceeded, all pairwise keys are almost immediately compromised.
Other variations are based on combinatorial designs [110] (where the assignment
of keys is deterministic, but the resilience against capture is nevertheless
stochastic), or assume a slightly different attacker model [5]. In the latter case,
it is assumed that the attacker can monitor only a small fraction of message
exchanges during an initial time frame after deployment. It is shown that although
keys are transmitted in clear text during that phase, only a small fraction
of keys is actually compromised.
2.9.2 Secure Communication
Key agreement as described in the previous subsection is a prerequisite for
general secure node-to-node communication, either on the link level between
neighbouring nodes, or between remote nodes that are separated by multiple
hops. For more constrained communication patterns, more economical techniques
are conceivable.
One of the most light-weight protocols is µTESLA [142]. It is intended to
be used in base station-centric networks, where the most prevalent communication
patterns are point-to-point between a base station and a node (e.g. for
queries and reporting sensor readings), and broadcasts from the base station
to all nodes (e.g. for queries or reprogramming the entire network). It only
assumes that every node shares a unique key with the base station (for pointto-point
messages between the base station and the node). Using this key, the
broadcast authentication mechanism can be bootstrapped.
This broadcast authentication mechanism is based on hash chains and loose
time synchronization. For bootstrapping the mechanism, the base station sends
an authenticated (using the pairwise key) element of the hash chain to a node,
together with timing information. The hash chain element is a symmetric key
that is valid only during a certain period in time. During this period, it is used