Hill Cipher with unequal matrix How can we encrypt a text in Hill Cipher with a key matrix that is 3x3 when the plaintext is even like "ATTACKS"? Do we need to add padding like "x" or "z" to make the matrix equal? Thanks.
To make the plaintext matrices equal, you could use padding with a null character, such as using "X". For example with "ATTACKS", you would get the following vectors:

$$
\begin{bmatrix}A\\T\\T\end{bmatrix}\ \begin{bmatrix}A\\C\\K\end{bmatrix}\ 
\begin{bmatrix}S\\X\\X\end{bmatrix}\ 
$$