what you are saying is contradictory.

If it is asking for a password, it is encrypted.  The rar file can be encrypted in a way to show the contents, if that is where your confusion is.  Bottom line, treat it like a lottery ticket:  brute force it on a computer u don't use and leave it....  maybe it will come up with the pwd within your lifetime.  There are ways of optimizing, maybe multi core brute force, or maybe some type of GPU compute brute force.  but those increases 2x-20x? are not that much compared to potential time it will take.