Protocols for Secure Communication in Wireless Sensor Networks

160 Chapter 6. Integrity-Preserving Communications
possible to authenticate the altered message with regard to the original statement
of origin.
6.1.2 Identity, Integrity, and Authentication in WSNs
Identities of nodes in a communication network may exist on different system
levels. For example, serial numbers or structured, unique addresses such as
the 6-byte MAC layer identifiers used in Ethernet networks, simple random
numbers (with a high probability of uniqueness), or public/private key pairs are
being used. Often, identities are the foundation for security measures such as
access control, which could be based for example on ID filtering or public key
signatures.
The key distribution schemes of Chapter 4 assume that a (random) identifier
exists for each node. This identifier is used to create a pseudo-random number
sequence that determines, which keys are assigned to a node. Additionally,
the assigned subset of keys can also be considered as an identifier for a node,
although it is only unique with a certain probability. However, a key subset
has the advantage that its validity can be checked by other parties by testing
whether the node actually knows the keys that have been assigned to it. In fact,
the major purpose of this kind of identifier is not to corroborate some unique
name for each node, but to prove that the node is legitimately participating in
the sensor network.
As mentioned above, the knowledge of the identity of a message’s source
does not imply any trust in that source or the truthfulness of a message’s content
that is attributed to that source. This trust has to be established through
some means that is usually outside of the scope of a direct communication relationship.
In a WSN, trust is usually assumed based on the fact that nodes
belong to the same WSN deployment, or are operated within the same administrative
domain. Membership in the same deployment can be established, as
mentioned in the previous paragraph, by using a key predistribution scheme.
Alternatively, a list of valid identifiers could be predistributed to all nodes, or
certificates could be used.
A system where trust is assumed if a valid key set is presented may be vulnerable
to the so-called Sybil attack [58]. This attack is based on the creation
of new identities. By recombining the key sets of captured nodes, new key sets
can be created, which can then be used to simulate a potentially large number
of virtual (fake) nodes and influence the result of a WSN’s operation. An intrusion
detection system [137] may help to detect such virtual identities, but it
may involve a significant overhead.