That letter describes perfectly Nash personality. Here is a quote from when he Einsten:

'I remember talking to him about theories in physics. He was not at all impressed by my words and just told me that, whatever topic I am working on, I have to do in-depth research in order to be successful

He unfortunately didn't research cryptography to confront his ideas. First of all he would have follow the Kerckhoffs's principle. He would have base himself on information theory proposed in 1948. He could have put "numbers" on his conjecture et build a very secure practical machine. Too bad such a genius didn't have such mindset. I like that guy.