As much as I’ve listed some bits and pieces, I’d recommend anyone find a reference/textbook on probability and work from that versus anything found online.
Ironically, you can access one of these, online.
It’ll fill in the gaps and the more we have in our arsenal against Kryptos, the better off we are.
Kryptos - Beyond K4 
There’s a lot of gaps in what I know about probability
There’s a lot of gaps in what you know in general pecansandy