Consider the matrix . Joe decides to send a message to Sam by encrypting the message with the matrix . He first takes his message and converts it to numbers by replacing A with 1, B with 2, C with 3, and so on till replacing Z with 26. He uses a 0 for spaces. After replacing the letters with numbers, he breaks the message up into chunks of 3 letters. He then multiplies each chunk of 3 by the matrix , resulting in a coded message. For example, to send the message “good job ben” he firsts converts the letters to the numbers and places them in a large matrix (top to bottom, left to right) To encode the matrix, he computes and then sends the numbers $[ [ 14, 20, 45], [ -2, -10, 10], [ 32, 79, 4], [ -5, -22, 24]] A$ to decode the message.

1. Find the inverse of .
2. Use to decode and show the message is “good job ben.”
3. Decode the message .

• Make remarks with a list.

problem.simple_matrix_encryption.tex

%Encryption using matrices (and inverses to decode things).
\begin{problem}
Consider the matrix 
$A =
\begin{bmatrix}
 2&1&-1\\
 5&2&-3\\
 0&2&1
\end{bmatrix}
$.  
Joe decides to send a message to Sam by encrypting the message with the matrix $A$. He first takes his message and converts it to numbers by replacing A with 1, B with 2, C with 3, and so on till replacing Z with 26.  He uses a 0 for spaces.  After replacing the letters with numbers, he breaks the message up into chunks of 3 letters.  He then multiplies each chunk of 3 by the matrix $A$, resulting in a coded message. For example, to send the message ``good job ben'' he firsts converts the letters to the numbers and places them in a large matrix $M$ (top to bottom, left to right) 
$$
\left[
\begin{bmatrix}g\\o\\o\end{bmatrix}, \begin{bmatrix}d\\ \ \\ j\end{bmatrix},\begin{bmatrix}o\\b\\\  \end{bmatrix},\begin{bmatrix}b\\e\\n\end{bmatrix}\right] 
\rightarrow
\left[\begin{bmatrix}7\\15\\15\end{bmatrix}, \begin{bmatrix}4\\0\\10\end{bmatrix},\begin{bmatrix}15\\2\\0\end{bmatrix},\begin{bmatrix}2\\5\\14\end{bmatrix}\right] 
= M=
\begin{bmatrix}
7 & 4 & 15 & 2 \\
15 & 0 & 2 & 5 \\
15 & 10 & 0 & 14  
\end{bmatrix}
.$$
To encode the matrix, he computes 
$$AM = 
\begin{bmatrix}
14 & -2 & 32 & -5 \\
20 & -10 & 79 & -22 \\
45 & 10 & 4 & 24
\end{bmatrix}.$$
and then sends the numbers 
$[
[ 14,  20,  45],
[ -2, -10,  10],
[ 32,  79,   4],
[ -5, -22,  24]]
$ to Sam. Sam uses the inverse of $A$ to decode the message. 
\begin{enumerate}
 \item Find the inverse of $A$. 
 \item Use $A^{-1}$ to decode $[
[ 14,  20,  45],
[ -2, -10,  10],
[ 32,  79,   4],
[ -5, -22,  24]]$ and show the message is ``good job ben''.
 \item Decode the message $[[39, 89, 22],[20, 48,  4],[39, 88, 33]]$.
\end{enumerate}
 
 
\end{problem}