Japan's purple machine [pdf]

Is that some student report, or a class handout, or what?

The classic work on this subject is "The Man who Broke Purple", a biography of William Friedman. Friedman is probably the greatest name in cryptanalysis. He changed the field from guessing to crunching. (See page 301 of [1]. That's where modern cryptanalysis began. Explanation at [2].) He was also the founder of the National Security Agency.

[1] http://marshallfoundation.org/library/wp-content/uploads/sit...

[2] https://en.wikipedia.org/wiki/Index_of_coincidence