John Pryor spent most of World War II in a Nazi POW camp. Because of good behavior, they allowed him to write letters home—letters that contained unbelievably complicated code.
John Pryor survived the war, but he forgot the encoding method he used to use. So researchers re-cracked it.
Cryptography is a multi-disciplinary field, often the purview of mathematicians and logicians. I have to admit, even though I was fascinated with codes and ciphers as a kid, this system is too complicated for me to wrap my head around.