Info from question
$ \left[ \begin{array}{ccc} 1 & 0 & 4 \\ 0 & 1 & 0 \\ 1 & 0 & 1 \end{array} \right] \times \left[ \begin{array}{ccc} X \end{array} \right] = \left[ \begin{array}{ccc} 2 & 0 & 0 \\ 0 & 1 & 0 \\ 0 & 0 & 3 \end{array} \right] $
1. How can Bob decrypt the message?
Since Bob already knows the matrix [X] and the encrypted matrix, all he has to do to find the original matrix is to invert [X] and multiply with the encrypted matrix.
