Evaluate $17^{93} \mod 23$
\begin{align}e &= 93\\ &= 1 × 2^6 + 0 × 2^5 + 1 × 2^4+ 1 × 2^3 + 1 × 2^2 + 0 × 2^1 + 1 × 2^0\\ &= |\ 1011101\ |_2 \end{align} Then we have: \begin{align}17^{93} \mod 23 &= (((((((17^1 )^2 17^0 )^2 17^1 )^2 17^1 )^2 17^1 )^2 17^0 )^2 17^1\\ &= (((17^4 17)^2 17)^2 17)^4 17 \text{ $\leftarrow$ step 3}\\ &= 21 \end{align}
How do we go from step 3 to final answer? What are the theorems used?