The open web is a critical platform for applications that handle highly sensitive data, from private communications to financial transactions and medical records. Traditionally, servers are trusted to deliver the appropriate code and resources for their web applications to browsers, who then provide a secure and isolated environment for their execution. In some circumstances, this trust model falls short.

Consider a browser-based messaging application, like Signal or WhatsApp, which uses end-to-end encryption. The browser depends on the server to provide a trustworthy javascript implementation of the app; which ensures the user’s messages and cryptographic keys are suitably protected. A malicious or compromised server could selectively serve modified code to some users, undermining their security with little risk of detection.…