I imagine the purpose of the riddle(is it a riddle?) and keeping it a mystery is making you think for yourself what your answer would be. His masterkey(alternatively purpose in life(?)) would only work for him because he came up with it for himself. You won't be able to make use of it because it's made by his own introspection.
One could even argue the process of finding the masterkey is part of the masterkey itself and thus can't be told. By such reasoning maybe you technically could use his answer, but without the process getting there you won't really be using it.