How can Bob decrypt the message?
Easy peasy, all Bob has to do is break the encrypted message vector into little vectors of three elements each, multiply each of those vectors by the inverse of the encryption matrix. Afterwards, he will just have to reassemble the output into the message.
Can Eve decrypt the message without finding the inverse of the secret matrix?
Without finding the inverse of the secret matrix, I don't think it is possible. However, she technically can translate anything though because she has all the things she needs to find the inverse matrix, with which she can decrypt messages.
What is the decrypted message?
$ \left[\begin{array}{ccc}2 & 0 & 0 \\0 & 1 & 0 \\0 & 0 & 3 \end{array}\right]\times\left[\begin{array}{ccc}1 & 0 & 1 \\0 & 1 & 0 \\4 & 0 & 1 \end{array}\right]^{-1}=\left[\begin{array}{ccc}-\frac{2}{3} & 0 & \frac{2}{3} \\0 & 1 & 0 \\4 & 0 & -1 \end{array}\right]\! $
$ \left[\begin{array}{ccc}-\frac{2}{3} & 0 & \frac{2}{3} \\0 & 1 & 0 \\4 & 0 & -1 \end{array} \right]^{-1}=\left[ \begin{array}{ccc}-\frac{1}{2} & 0 & \frac{1}{3} \\0 & 1 & 0 \\2 & 0 & \frac{1}{3} \end{array}\right] $
$ \left[\begin{array}{ccc}-\frac{1}{2} & 0 & \frac{1}{3} \\0 & 1 & 0 \\2 & 0 & \frac{1}{3}\end{array} \right] \left[ \begin{array}{ccc} 2 \\23\\3 \end{array}\right]=\left[ \begin{array}{ccc} 2 \\23\\5 \end{array}\right] $
The numbers translate to the letters BWE. Come on, you could've at least had a real message in the end. Now, I just wasted a bunch of time translating jibberish. You can't see me right now, but I am uber angry.
