The Internet is a major distribution platform for applications, but there are no effective transparency and audit mechanisms. Due to the ephemeral nature of web applications, a client visiting a website has no guarantee that the code it receives today is the same as yesterday, or the same as others receive. Despite advances in web security, it is thus challenging to audit web applications before they are rendered in the browser. We propose Accountable JS, a browser extension and opt-in protocol for accountable delivery of active content on a web page. We prototype our protocol, formally model its security properties with the Tamarin Prover, and evaluate its compatibility and performance impact with case studies including WhatsApp Web, AdSense and Nimiq.
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