Protocols for Secure Communication in Wireless Sensor Networks

176 Chapter 6. Integrity-Preserving Communications
6.3.1 Single Message Overhead
For each message being transmitted from the source to a target node, the Canvas
scheme requires that each node on the path checks k authentication codes
(MAC) and generates another k MACs addressed to nodes further down the
path. Nodes close to the source will check fewer MACs, while nodes close to
the target will generate fewer of them. In general, on each link, T = k(k +1)/2
MACs are transmitted. For links that are close to the source or the sink, some
of these MACs are “missing”, and we can adjust the number of MACs being
transmitted by:
(k − d)(k − d + 1)
δ(d) = ,d < k
2
where d is the distance of the link from the source/target, starting with d = 1
for links that are adjacent to the source/target. For each link with d < k, T is
reduced by δ(d) in order to determine the number of transmitted MACs.
At each node, checking and generating the MACs consumes time, which
delays the relaying of the message. If the HMAC construction is used, the
complete message has to be processed separately for each MAC being verified
or generated. From a performance point of view, the secret suffix construction
is therefore preferred as it requires the message to be hashed only once. The
hash value generated is then used for MAC verification and generation.
When a digital signature scheme is being used, the following information
has to be transmitted in order to authenticate a message: the public key of
the source node, a certificate stating that the public key is authorized, and a
signature of the message. For each pair of communicating nodes, the public key
and the certificate have to be exchanged only once. The encoding format for
keys, signatures, and certificates may induce additional overhead. For example,
a certificate format such as X.509 [81] contains information about the validity
of the certificate and other facts that need not to be explicitly represented in the
context of wireless sensor networks. Therefore, we can restrict ourselves here
to the minimum amount of data required. An ECDSA signature is a pair (r,s).
The size of both r and s is governed by the parameter n, which should be at
least 160 bits large according to [88]. Thus the size of a signature is at least
320 bits. The public key is the result of a multiplication of the private key with
a point. Using a standardized elliptic curve with key length 192 bits, this yields
a public key size of around 600 bits.
The effort of generating and verifying the signature is only induced at the
source and the target nodes. Intermediate nodes on the path do not have to perform
any computation but need to simply relay the message. Thus, no further
delay is introduced by this scheme.