It’s just as arbitrary as any other number. You can pick at random, but starting with a dictionary attack of easy, non-random keys would probably be a better start.

I remember playing a game of dvd-deal or no deal, where you would pick random number from one to twenty and see if you won money. The people I was playing with were mad because I just started guessing 1, 2, 3, 4…

I told them statistically it was just as likely to be those numbers as anything else.