Elliptic curve cryptosystems (ECC) provide the highest strength per bit of any cryptosystem known today, which makes them especially well suited to computation resource-restricted devices. However, at the ECC implementation stage, a major concern is securing ECC scalar multiplications against Side-channel attacks (SCA). Existing solutions reached the goal by inserting dummy operations, which largely increase the computational costs and prohibit the deployment of ECC in computation resource-restricted devices. In this paper, we propose an efficient and secure scalar multiplication method by partitioning the bit string of the scalar in half and extracting the common substring from the two parts based on propositional logic operations. The computations for common substring are thus saved. Computational results demonstrate the proposed method is approximately 50% more efficient than almost all existing secure solutions. The power measurement experiments prove that the proposed method is secure against SCA.