How will bob decrypt the message?
Bob can easily decrypt the message as he knows the secret matrix. All he has to do is to just take the inverse of the secret matrix and multiply it with the first 3 entries then the next 3 and so on. By doing so he will get original decrypted message.
Can Eve decrypt the message without finding the inverse of the secret matrix?
No, she has to know the inverse of the matrix to decrypt the message as the (encrypted message*inverse of secret matrix=decrypted matrix)
on solving we find the inverse of the secret matrix to be
$ \begin{matrix} - \frac{2}{3} & 0 & \frac{2}{3} \\ 0 & 1 & 0 \\ 4 &0 & -1 \end{matrix} $
What is the decrypted message corresponding to (2,23,3)?
the decrypted message corresponding to (2,23,3) is BWE
$ (2,23,3) \to \begin{matrix} - \frac{2}{3} & 0 & \frac{2}{3} \\ 0 & 1 & 0 \\ 4 &0 & -1 \end{matrix} \to BWE $
