WirelessHART is the first open standard for Wireless Sensor Networks designed specifically for industrial process automation and control systems. WirelessHART is a secure protocol; however, it relies on a Security Manager for the management of the security keys and the authentication of new devices. The WirelessHART standard does not provide specifications and design of the Security Manager. Also, the security specifications in the standard are not well organized and are dispersed throughout the standard. The lack of Security Manager design and ambiguous security specifications impede implementation of the standard since it requires close knowledge of all the core specifications on the part of the developer. In this paper we provide the detailed specifications, design, and implementation of the Security Manager for the WirelessHART standard. We evaluate our Security Manager against different cryptographic algorithms and measure the latency between the Network Manager and the Security Manager. Our evaluation shows that the proposed Security Manager meets the WirelessHART requirements. Our analysis shows that the provided Security Manager is capable of securing both the wireless and wired part of the WirelessHART network.