Protocols for Secure Communication in Wireless Sensor Networks

94 Chapter 3. A Security Model for Wireless Sensor Networks
we consider the ability of a network to reach consensus useful as a means for
comparing authentication schemes. Authentication based on pairwise keys provides
the highest security level in this framework, since it prevents the adversary
from manipulating messages. This allows secure communication among
all pairs of uncompromised nodes. Alternative authentication schemes, as described
in chapters 6 and 5, provide secure communication only for a fraction
of the node pairs. This reduces the ability of the uncompromised nodes to
reach a network-wide consensus. The fraction of nodes still able to participate
in this network-wide consensus yields a quantitative measure for the provided
security.
Bibliographic notes The problem of Byzantine agreement has been defined by
Lamport, Shostak, and Pease [138, 108]. Protocols and complexity bounds for
distributed consensus are presented in an accessible way in the book by Nancy
Lynch [119].
3.6 Related Work
The standard attacker model in cryptographic research has been defined by
Dolev and Yao [57]. It assumes a distributed system in which hosts communicate
by exchanging messages. It considers two (or more) honest parties that
are trying to communicate, while the attacker tries to tamper with this communication.
The attacker is assumed to be nearly omnipotent, having access to all
communications and being able to suppress or fabricate messages. He is only
limited by cryptography, which is assumed to be secure. This model has proven
to be useful for the analysis of cryptographic protocols. However, as discussed
in [45] within the context of ubiquitous computing, often other threat models
are more useful for the analysis of security protocols. In such settings, additional
security assumptions are being made that allow to relax the Dolev-Yao
model by going beyond the availability of unbreakable cryptographic primitives.
One condition, which is often fulfilled in practice, is the existence of a lowbandwidth
but secure channel that can be used for a short period of time. One
example where this is exploited is the “pairing” of consumer Bluetooth devices.
Here, the (human) user enters the same random code on both devices. This code
is then used as a seed for creating a secret key [106] that allows future secure
communication between these devices. The underlying assumption is that the
attacker does not have access to the codes entered by the user.