I understand what is going on in higher level. But I don't understand what's really going on in page 3 and 4 of this slide.
- How $S_0$ and $S_1$ is used? What goes in what goes out?
- How exactly do key bits determine which s-box is to be used at each position? Position refers to what?
- Is the function $f$ on page 4 exactly the same SP network on page 3?
- Why split plain text into left $L$ and right $R$?
- Why XOR $L$ with $R$ after applying $f$?
- Why rotate after applying last $f$?
- Why is XOR used? Why not some other bitwise operation?
Here are the preceding slides for reference.