At present, the sensor-cloud infrastructure is gaining popularity, since it offers a flexible, open and reconfigurable configuration over monitoring and controlling application. It handles mainly the user data, which is quite sensitive and hence the data protection in terms of integrity and authenticity is of greater concern. Thus, security is a major concern in such system, inclusive of intruders, who tries to access the infrastructure. In this paper, an improved encryption protocol for secured session keying between the users using a trusted services proposed over sensor-cloud architecture. This technique uses modified Elliptical Curve Cryptography (ECC) algorithm to improve the authentication of sensor nodes in the network. Further, Abelian group theory is designed to convert intruder deduction problem to linear deduction problem to resolve the complexity associated with manipulation of finding the intruders in the network.This helps to reduce the computational complexity of generating a secured message transmission and increased possibility to find the intruders in the network. The experimental validation with the proposed ECC in terms of computational cost proves that the proposed method attains lower computational cost and improved detection of intruders in the network. Also, the technique seems efficient and can be applied on practical cases, where other ECC algorithms fails while implementing it on a real time basis.