Suppose $N$ is $L$-bit long and $k$ is also $L$-bit long. How to show that it takes at most O$(L^2)$ operations to compute it. For example $N = 1010$ and $k = 10$. Then $N^k = 6^2 = 100100$. But I am not familiar with how $100100$ comes up without going back to decimal and using the fact that $6*6 = 36$. Therefore, I also find it hard to realize that it takes at most O$(L^2)$ operations to compute the powering of a number. 
How many operations at maximum needed to compute powering of a number, i.e. $N^k$?
complexity theory;number theory