It’s easy to forget that not that long ago, people had to pay money to send a text message. In 2013 alone, mobile operators were estimated to have charged people and businesses a total of more than $120 billion just for sending texts.

Making mobile messaging free has led to a dramatic shift in how people, businesses and in some cases government entities communicate. More than 100 billion messages per day are now sent on WhatsApp alone — about five times the volume handled by the entire global SMS system in 2016. 

That alone is something worth celebrating. But WhatsApp also delivers rock-solid privacy to its 2 billion users across the world, through end-to-end encryption that ensures every message, picture, video and voice call sent through WhatsApp can only be read by the person it’s intended for. 

This makes WhatsApp the biggest end-to-end encrypted communications system in the world.  A lot of hard work went into making strong privacy work at a massive scale, so this week I spoke with Scott Ryder, an engineering director on WhatsApp, about how they made it happen. I hope you enjoy it as much as I did!