“…Following this strategy, no third parties or external entities need to be trusted by the users to preserve their privacy. Existing approaches use different techniques and mechanisms and could be categorized mainly into three categories: cryptographic-based techniques [6,21,40,74,172], differential privacy-based approaches [66,76,89,120,128,130,166,197,198], and perturbation-based techniques [75,118,146,147,148,151,153,158,183] A group of works focus on providing cryptographic solutions to the problem of secure recommender systems. The approaches do not let the single trusted party have access to everyone's data [6,21,40,74,172].…”