Communication security is key to widespread adoption of wireless sensor networks. Coupling this need with the typical resource constraints encountered, TinySec was introduced by a group from UC Berkeley for the open source and highly popular TinyOS - a platform designed for wireless embedded sensor networks. Prior to this project the TinySec security implementation was provided with 2 ciphers, namely Skipjack and RC5. The library being modular in design afforded the opportunity to replace the cipher subcomponent with one of our choice, Misty1 in our case. Misty1 is a block cipher based on nested Feistel network with variable number of rounds (typically 8). It uses a 128-bit sized cryptovariable and block size of 64 bits. Designed on the basis of the theory of provable security against differential and linear cryptanalysis, the cipher is intended to allow realization of high-speed encryption on hardware platforms as well as on software environments. The new cipher implementation within TinySec was tested using 2 sample applications and run inside TOSSIM - a discrete event simulator environment for TinyOS. With the addition of Misty1, TinySec library users now have an additional choice as regards the cipher.