Usable Security and Privacy for Secure Messaging Applications
The threat of government and corporate surveillance around the world, as well as the publicity surrounding major cybersecurity attacks, have increased interest in secure and private end-to-end communications. In response to this demand, numerous secure messaging applications have been developed in recent years. These applications have been welcomed and publically used not just by political activists and journalists but by everyday users as well. Most of these popular secure messaging applications are usable because they hide many of the details of how encryption is provided. The strength of the security properties of these applications relies on the authentication ceremony, wherein users validate the keys being used for encryption that is exchanged through the service providers. The validation process typically involves verifying the fingerprints of encryption keys to protect the communication from being intercepted.In this dissertation, we explore how to help users enhance the privacy of their communica- tions, with a particular focus on secure messaging applications. First, we explore whether secure messaging applications are meeting the security and privacy needs of their users, especially in countries that practice censorship and restrict civil liberties, including blocking access to social media and communication applications. Second, we studied existing popular secure messaging applications to explore how users interact with these applications and how well they are using the authentication ceremony during lab studies. Third, we applied design principles to improve the interfaces for the authentication ceremony, and also to help users find and perform the authentication ceremony faster. Forth, we applied the lessons from our interviews with participants in our user studies to help users comprehend the importance of authentication. As part of the effort, we developed an authentication ceremony using social media accounts to map key fingerprints to social features, pushing the ceremony to a more natural domain for users. We modified the Signal secure messaging application to include this social authentication ceremony and used a user study to compare this method to other common methods. We found that social authentication has some promising features, but that social media companies are too distrusted by users. Based on our results, we make several recommendations to improve the use of security and privacy features in secure messaging applications and outline areas for future work.