  Security is a critical and vital task in wireless sensor networks, therefore
different key management systems have been proposed, many of which are based on
symmetric cryptography. Such systems are very energy efficient, but they lack
some other desirable characteristics. On the other hand, systems based on
public key cryptography have those desirable characteristics, but they consume
more energy. Recently based on authenticated messages from base station a new
PKC based key agreement protocol was proposed. We show this method is
susceptible to a form of denial of service attack where resources of the
network can be exhausted with bogus messages. Then, we propose two different
improvements to solve this vulnerability. Simulation results show that these
new protocols retain desirable characteristics of the basic method and solve
its deficiencies.
