8. Conclusion
In this paper, we studied various ways to protect the privacy of the SSC and the MDOs in different MCS scenarios. We first proposed PPSQ to secure the privacy of the summation query of the SSC regarding the real target MDOs. Next, we put forward PPDRC to protect the data privacy of the MDOs to retrieve the difference rank in the multi-party computation process. As the signs of polynomials can be derived without disclosing the numeric values of the data records of the involved MDOs, given a baseline value d, the difference between the sensing reading of any MDO and d can be compared against a designated proportion of the summation of sensing reading differences of a group of MDOs without revealing the values of d and any sensing readings. Subsequently, we elaborated how to identify K-nearest neighbors around the POO issued by the SSC with minimum error, while keeping the POO and the locations of all involved MDOs secrete. In this solution, all the possible distance values between the PPO and each MDO location are included in a distance range, and all the MDOs agree on the privacy window of the smallest size such that the privacy level regarding the distances of some MDOs is still acceptable even in the worst case. Security analysis are given and the performance evaluations are done at the end.