Christopher Michel · CC BY-SA 4.0

Designed the Signal Protocol, the end-to-end encryption scheme now running underneath Signal, WhatsApp, Google Messages and, for a time, Facebook Messenger — quietly making strong encryption the default for a meaningful share of the world's private messages. Co-founded Whisper Systems, which Twitter acquired in 2011, then left to found Open Whisper Systems and build Signal itself, serving as its first CEO until stepping down in 2022. Came to cryptography from a hacker and activist background rather than a conventional corporate one.