Explore how WhatsApp successfully implemented end-to-end encryption for over a billion users while simultaneously reducing spam in this 18-minute conference talk from USENIX Enigma 2017. Delve into the techniques used for effective spam prevention without accessing message content, addressing concerns about the impact of encryption on spam detection systems. Learn about the challenges faced during implementation, including spam detection methods, reporting flows, and the use of behavioral features, reputation analysis, and conditional probability analysis. Gain insights into the process of training classifiers, gathering training data, and the overall results of WhatsApp's innovative approach to balancing user privacy with spam reduction.