During the last decade, authentication of sensor node and secure routing of data been remained an open challenges in Wireless Networks due to their applications in various vulnerable environments. These challenges become more signi cant when Wireless Sensor Networks (WSN) composed of tiny inexpensive nodes are consid- ered. This is due to the fact that the solutions proposed for the similar purposes in conventional wireless networks cannot be exploited for sensor networks because of high complexities and power consumptions involved in their algorithms. This thesis proposes a two-fold solution for the issues of node authentication and secure routing in Wireless Sensor Networks. In therst part of the thesis, a low complexity Sybil attack detection mechanism for Wireless Sensor Networks is pro- posed; while in the second part, a Secure Energy E cient Routing scheme called SEER is presented for the data security. Both of the proposed schemes are based on the Signed Response (SRes) authentication and voice encryption mechanism developed for Global System for Mobile (GSM) communications. The proposed Sybil attack detection scheme use pre-distributed key embedded in the sensor nodes. A modi ed version of A3 algorithm used in node authentication produces a SRes with the help of pre-distributed keys against a random challenge number sent by the sink or Cluster Head (CH). The 32 bit SRes is sent back to the sink or CH by the node to prove its legitimacy. The design of node authentication scheme is made exible so that it can be implemented in both hierarchical and centralized Wireless Sensor Networks. The scheme is analyzed for its performance under var- ious Sybil attacks. The scheme is evaluated for its probability of detecting Sybil nodes when di erent authentication key pool sizes are utilized. After extensive simulations, it is observed that the proposed scheme is able to counter Sybil at- tacks with higher probability as compared to notable existing schemes. Moreover, it has also been observed that the proposed Sybil detection scheme exhibits lesser computational cost and power consumption as compared to the existing schemes for the same Sybil attack detection performance.In the second part of the thesis, a secure mechanism for routing of data in Wire- less Sensor Networks; SEER is proposed. The proposed protocol is based on A5 encryption scheme developed for voice encryption in GSM. After successful au- thentication, a modi ed version of A5 algorithm is used to encrypt data during its routing from source to the sink or relay node. SEER uses GRACE (GRAdient Cost Establishment) routing protocol for transmission. For this purpose, a 64-bit ciphering key is used which is produced through a complicated process of pertur- bation in order to make it harder to be traced. SEER has been tested through simulations in MATLAB R by setting up hostile and vulnerable Wireless Sensor Network scenarios with respect to data integrity. The results obtained are then compared with two notable existing secure routing protocols. It is proved that the proposed mechanism SEER helps achieve the desired performance under dy- namically changing network conditions with various numbers of malicious nodes. Due to its linear complexity, lesser power consumption and more dynamic route updation, the proposed Sybil detection and SEER schemes can be easily extended to cater to the needs of emerging industrial wireless sensor networks, Dust Sen- sor Networks and IoT. Emerged from the conventional Wireless Sensor Networks, all the aforementioned networks have got the same nature of vulnerabilities and threats along with the inherited limitations with respect to their hardware and processing capabilities.