Chapter 2.8, Problem 5E

In the $27$-letter alphabet $A$ described in Example $2$, use the affine cipher with key $a=7\text{ and }b=5$ to encipher the following message. all systems go

What is the inverse mapping that will decipher the ciphertext?

Example 2 Translation Cipher Associate the $n$ letters of the "alphabet" with the integers $0,1,2,3\mathrm{.....}n-1$. Let $A=\left\{0,1,2,3\mathrm{.....}n\text{ - }1\right\}$ and define the mapping

$f:A\to A$ by

$f\left(x\right)=x+kmodn$ where $k$ is the key, the number of positions from the plaintext to the ciphertext. If our alphabet consists of $a$ through $z$, in natural order, followed by a blank, then we have $27$ "letters" that we associate with the integers $0,1,2,\mathrm{...},26$ as follows:

$\begin{array}{cccccccccccccc}\text{Alphabet:}& \text{a}& \text{b}& \text{c}& \text{d}& \text{e}& \text{f}& \mathrm{...}& \text{v}& \text{w}& \text{x}& \text{y}& \text{z}& \text{"blank"}\\ \text{A:}& \text{0}& \text{1}& \text{2}& \text{3}& \text{4}& \text{5}& & \text{21}& \text{22}& \text{23}& \text{24}& \text{25}& \text{26}\end{array}$