Ever liked Nancy Drew? This is the 007 of applied mathematical fields, where you develop strategies of sending secret messages across the internet without hackers or members of the public being able to see their content. This field is not only hugely relevant to all modern tech, but is rapidly evolving in the dawn on quantum mechanics.
Cryptography I and II by Stanford
Cybersecurity Basics by IBM
Unlocking Information Security I: From Cryptography to Buffer Overflows (better for if you want to learn cryptography within the wider scope of Cybersecurity