I wish someone makes a question: "Definition of $x^u \bmod k$", with mention of uses of that notation in RSA for $C=M^e \bmod N$ and $d=e^{-1} \bmod Ļ(N)$, and thus an RSA tag. Teachers and reference material often skip that, and it's complex when we want to cover uā¤0 as is often the case. It causes a lot of confusion.