$1$ Introduction

In this assignment, you will write a program that performs encryption$/$decryption using a

very simple transposition technique inspired by the Rail Fence Cipher. Let's see an example

to illustrate how it works.

$1\mathrm{.}1$ A Simple Example

Assume that we are going to encrypt the following plain$-$text message before sending it out

to provide some level of security and avoid the eavesdropping of malicious actors.

Encryption Steps

Here are the steps that we need to follow to encrypt this message:

First step: We get rid of non$-$alphabetical letters like punctuation and white spaces.

Second step: Then, we replace the uppercase letters with their corresponding lowercase

equivalents. Let's assume that there are $l$ letters in this step.

Third step: We place all letters in a rectangular $2-$D array of size $nm$ by filling it row$-$

by$-$row from left to right and top to bottom so that all rows $($with a possible exception

of the last row$)$ are full of alphabetical characters. If the last row is not full, we need to

fill it with a sequence of adjacent alphabetical letters ending at letter $\text{'}z\text{'}.$ The $2$D array

dimensions are calculated in the following way: number of columns, $m,$ is the length

of encryption key $($it will be introduced later$)$; and number of rows, $n,$ is calculated by

the rounding the following value up to the closest integer: $\frac{l}{m}$ where $l$ is the numbmer

of characters left in the original message after removing non$-$alphabetical characters.

Fourth step: The key to this encryption is a certain permutation of columns. In the final

step, we use such permutation to make the encrypted text by concatenating letters

column$-$by$-$column from top to bottom and in the order specified by the key.