Today’s computer systems and their owners fail to protect data. Exacerbating this are new threats stemming from the rise of cloud computing. The consequences are dire: sensitive information like financial statements, medical records, and private messages are disclosed to malicious parties. In my research at the intersection of security, cryptography, and systems, I work to change this by breaking and building efficient end-to-end (E2E) encrypted systems, which protect data by keeping it encrypted throughout processing and storage. In this talk, I’ll explain some of the flaws I’ve found in existing E2E-encrypted systems deployed to billions of users, how the flaws have led me to a new methodology for building these systems that’s rooted in co-design of cryptography and systems, and some of the new E2E-encrypted systems I’m building.