Our perception of reality is under constant threat from ever-improving video manipulation techniques, including deep-fakes and generative AI. Therefore, proving authenticity of videos is increasingly important, especially in legal and news contexts. However, it is very challenging to prove it based on post-factum video content analysis.

Demonstrating veracity of videos is a longstanding problem that has recently become more urgent and acute. It is extremely hard to accurately detect manipulated videos using content analysis, especially in the face of subtle, yet effective, manipulations, such as frame rate changes or skin tone adjustments.

An increasing number of mobile devices are incorporating cameras, allowing users to record videos at any time, anywhere. This opens up a wide variety of applications, most notably security-critical ones, where videos are used as evidence or include sensitive content. Examples of such applications include (but are not limited to): (i) citizen journalists recording important events (e.g., protests), (ii) courts using videos as evidence, and (iii) electronic legal contract-signing platforms using videos to identify signing users [1].

An increasing number of mobile devices are incorporating cameras, allowing users to record videos at any time, anywhere. This opens up a wide variety of applications, most notably security-critical ones, where videos are used as evidence or include sensitive content. Examples of such applications include (but are not limited to): (i) citizen journalists recording important events (e.g., protests), (ii) courts using videos as evidence, and (iii) electronic legal contract-signing platforms using videos to identify signing users [1].

A legal contract is an agreement between two or more parties as to something that is to be done in the future. Forming contracts electronically is desirable since it is convenient. However, existing electronic contract platforms have a critical shortcoming. They do not provide strong evidence that a contract has been legally and validly created. More specifically, they do not provide strong evidence that an electronic signature is authentic, that there was mutual assent, and that the parties had an opportunity to read the contract. We present Tabellion, a system for forming legal contracts on mobile devices, such as smartphones and tablets, that addresses the above shortcoming. We define four secure primitives and use them in Tabellion to introduce self-evident contracts, the validity of which can be verified by independent inspectors. We show how these primitives can be implemented securely in the Trusted Execution Environment (TEE) of mobile devices as well as a secure enclave in a centralized server, all with a small Trusted Computing Base (TCB). Moreover, we demonstrate that it is feasible to build a fully functional contract platform on top of these primitives. We develop ~15,000 lines of code (LoC) for our prototype, only ~1,000 of which need to be trusted. Through analysis, prototype measurements, and a 30-person user study, we show that Tabellion is secure, achieves acceptable performance, and provides slightly better usability than the state-of-the-art electronic contract platform, DocuSign, for viewing and signing contracts.