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Security of Wireless Sensor Networks (WSNs) is a very crucial factor when deployed for reconnaissance in sensitive areas. The existing symmetric encryption schemes provide a good amount of security, but maintenance of keys is difficult. When asymmetric schemes are used, maintenance of keys is easier, but they provide a lesser degree of security when compared to symmetric encryption schemes. In order to cope up with these shortcomings, we propose to use an improved version of the hybrid encryption scheme, which is a combination of Advanced Encryption Standard (AES) and Elliptical Curve Cryptography (ECC) with cross encrypted keys for secure key exchange and node authentication and hybrid encryption for enhanced cipher-text security. In case of transmission in WSNs, Statistical Cooperative Diversity based on Alamouti code is the most commonly used transmission scheme. However, for an arbitrary number of sensors, Alamouti code limits the BER performance and energy is not distributed equally, thereby creating the energy-hole problem which leads to early dysfunction of the sensors and may eventually lead to dysfunction of the Wireless Sensor Network (WSN). Extended Cooperative Space-time Block Codes (ECBSTBCs), which are obtained from Alamouti code, have the same characteristics of Alamouti code with the energy being distributed equally among the active sensors. With these factors in mind, we propose to use ECBSTBC as the transmission scheme. The improved hybrid scheme is ideal for ECBSTBC based WSN due to the speed of operation and higher degree of security that it offers.