Fill in the values in the algorithm of fast exponentiation:

$3^{2^0}$mod$7=3^1$mod$7=3$

$3^{2^1}$mod$7=,{?}^2$mod$7=Ex$: $8$

$3^{2^2}$mod$7=Ex$: $8,{?}^2$mod$7=4$

$3^{2^3}$mod$7=4^2$mod$7=2$

$3^{2^4}$mod$7=2^2$mod$7=4$