Protocols for Secure Communication in Wireless Sensor Networks

6.3. Performance Evaluation 177
We compare the 2-Canvas scheme with elliptic curve signatures. The comparison
is illustrated along three measures: the overall single-message bandwidth
overhead, i.e. the sum of the authentication data transmitted by all nodes;
the time overhead for a single message; and the bandwidth overhead if multiple
messages are being exchanged.
The parameters are fixed as follows: the message sizes used are 64 byte and
512 byte, which makes a difference for the Canvas scheme as the message must
be hashed by each node. For the Canvas authentication codes we use either all
20 bytes that are output by the SHA-1 algorithm, or the truncated version with
7 bytes, which correspondes to the security level of DES. The key length for EC
signatures is assumed to be 192 bits, which provides a lower security level than
the 160 bits of the SHA-1 output. The times required for performing operations
are taken from Tables 3.1 and 2.2 whereby the time for EC signature generation
is estimated to be 600 ms.
Figures 6.7 and 6.8 show the time overhead induced by each of the authentication
schemes. The delay of the signature scheme is constant as only the
source and the target nodes have to perform additional operations. With the
Canvas scheme, each intermediate node has to perform operations that pile up
over the path. At some path length, the overhead produced by Canvas will be
larger than that of a signature scheme. The graphs show clearly the disadvantage
of HMAC, which requires the message to be hashed for each MAC. The
time overhead increases the end-to-end latency of the message, in addition of
the transmission time.
Figure 6.9 illustrates the bandwidth overhead of the authentication schemes.
In each hop, authentication information must be transmitted that adds to the
overall amount of transmitted data. It shows the clear advantage of the Canvas
scheme, which produces less data overhead than a signature scheme. Especially
when the output of the MAC generating function is truncated, only a
small overhead is produced. This option is not available for signature schemes,
where the signature always has to be transmitted in full length. However, MAC
truncation means that the security level is effectively reduced.
6.3.2 Multiple Messages Overhead
When using a signature scheme, the certificate data only has to be exchanged
once between a pair of nodes. This allows to amortize this additional overhead
if multiple messages are being exchanged. This is illustrated in Figure 6.10. In
this example, if more than seven messages are exchanged, the signature scheme
is advantageous over Canvas when MACs are transmitted in full length. If