There is a classic paper in usable security called, "Why Johnny Can't Encrypt", where a group of people were given a new graphical version of PGP, but could not successfully complete tasks set out in the task. Some of the issues here are that PGP is a complex piece of software with many different parts, and part is the complexity of encryption itself.
Since that paper, there have been some newer software that have made it easier to sign (signify) and encrypt messages (such as age) and sending information (e.g., magic wormhole). But most of these systems still are command-line only, which limit their usefulness.
The master would be looking at designing new interfaces (graphical, mobile, or otherwise) to make the new tools available and usable to a large audience. There should also be a component of requirements gathering and evaluating the prototypes, either in an experiment or as a user evaluation (e.g., recreating the scenario in from "Why Johnny Can't Encrypt".
Note that these masters do not look at the cryptography issues. These projects focus on the usability issues.