In cryptography, the safe isn't a physical object. It's like a mathematical cloak, covering up private information with random static transforming understandable and usable information into incomprehensible, useless garbage. Wit had come up with the idea of public key cryptography, but no way to do it. Now marty and witt had to figure out how to build these safes. The trick was, of course, they couldn't built them out of iron and steel. They needed to build them out of math.