pattern recognition, and data mining. In the literature, many techniques have been proposed to address the SkNN problem, which can be classified into two categories based on whether the data are encrypted or not: centralized and distributed. Centralized Methods: In the centralized methods, the data owner is assumed to outsource his/her database and DBMS functionalities (e.g., kNN query) to an untrusted external service provider, which manages the data on behalf of the data owner, where only the trusted