Protocols for Secure Communication in Wireless Sensor Networks

3.4. The Cost of End-to-End Security 85
3.4.1 Connection Establishment
Before an end-to-end connection between two nodes can be set up, the initiator
must sort out which node should be on the other end. In a traditional
network setting, this is often done through external means such as a service directory,
from which the address of a service provider is obtained. Each change
of the current service provider requires an update of the service directory. If
such changes occur frequently, all service requests must be preceded by a new
look-up in the directory in order to retrieve the current provider address. The
Domain Name System in the Internet is an example of such a system. However,
since changes in this system occur infrequently, most of the information can be
cached in a hierarchy of directory servers.
In a WSN, it can be expected that the actual node providing a service changes
quite often due to the dynamic nature of phenomena being observed by the
network, and due to load balancing and failure recovery mechanisms. This
transient existence of service providers potentially facilitates a form of man-inthe-middle
attacks, i.e. malicious nodes posing as service providers and intercepting
messages targeted at a legitimate service provider. The use and maintenance
of a central service directory would be helpful, but also very costly due
to frequent directory updates and look-ups. Additionally, a central component
is a security risk since it would be a worthwhile target for an attacker.
Assuming that some mechanism for obtaining the identity and the address of
a service provider exists, there is still the need to establish a secure connection
between the initiator (client) and the provider (server), i.e. to engage in a key
agreement protocol. The standard protocol for doing this on the Internet is SSL.
There are three messages being exchanged before application data is being sent:
one “hello” message for initiating the connection, and two messages whereby
client and server exchange certificates and key information. This protocol can
be simplified to a two-step version if certain parameters, which are usually
communicated in the first message exchange, are fixed in advance. Thus, establishing
a SSL connection requires at least one message in each direction.
In the context of the Internet, these two messages induce a negligible overhead.
They use only a small fraction of the available bandwidth and the induced
delay is insignificant compared to the duration of the following session. The
key exchange protocol is based on public-key cryptography, which requires a
significant amount of computational power. However, the involved computational
overhead is an easy task for modern processors used in PDAs, workstations,
and servers.
In a sensor network, however, the overhead induced by connection setup