Protocols for Secure Communication in Wireless Sensor Networks

76 Chapter 3. A Security Model for Wireless Sensor Networks
Messages Wireless radio communication is based on a shared medium, which
makes it easy for a passive attacker to overhear the traffic in a network. Sometimes,
directed radio links or optical communication make eavesdropping harder,
but a determined attacker is likely to be able to record all traffic. Ultimately,
the confidentiality of messages can only be preserved through encryption. Setting
up keys for securing point-to-point links is a well-understood problem. In
sensor networks, key agreement schemes as described in chapter 4 can be used.
This renders pure passive attacks more or less useless. Still, it might be possible
to extract useful information from the traffic patterns that occur in the network.
Of course, link-level encryption is not able to effectively hide the content
of messages from an active attacker. All messages received by compromised
nodes can be read by the attacker. In a strong sense, the only way to preserve
confidentiality is end-to-end encryption. The same applies to message authentication
(and thus integrity): only if there is a common cryptographic context
between the sender and the receiver, the origin of a message can be verified in a
strong sense. However, due to the resource restrictions that apply to sensor networks,
end-to-end security may not be a practical approach. Other approaches
that approximate the properties of end-to-end security are the main topic of this
thesis.
In a large sensor network, messages often have to be transmitted over several
links until they arrive at their destination. Generally, we can not assume
a common cryptographic context between the sender and the destination. It is
therefore possible that a compromised node changes the contents of a message
it is relaying. Such changes could be detected by nodes that overhear the incoming
and outgoing messages. However, these potential guards may be asleep
at that time. Link-level encryption makes overhearing ineffective as well. We
propose interleaved authentication (see chapter 6) to this end.
The injection of fabricated messages is another way of manipulating the
operation of a sensor network. If a node emits messages in its own name, it
may not be possible for other nodes to decide whether these messages are the
product of correct operation, or if they are forged. Certain intrusion detection
techniques may be able to isolate such nodes if their behaviour deviates significantly
from ordinary operation. Alternatively, nodes may forge messages and
attach a different ID as their origin to them. This attack is commonly called
spoofing. If the used ID does not exist, this behaviour is subject to detection if
the ID is challenged. If the ID exists and another, non-compromised node with
the same ID exists in the network, it may raise an alarm if it detects a message
that was sent by the malicious node.