Example of a Vigen$$re Cipher

Consider the following

Plaintext: helloworld

Key: key

$========================$

Ciphertext: rijvsuyvjn

But how did we get from the plaintext to the ciphertext?

Since the key in our cipher is smaller than our plaintext, we will need to expand it$.$ We can do this by just repeating it$,$ which we can call a keystream.

Plaintext: helloworld

Keystream: keykeykeyk

$========================$

Ciphertext: rijvsuyvjn

Now $($refer to the mappings from letters to numbers above$),$ if we shift h by k$,$ we get r$,$ e by e we get i and so on$.$

Noting that characters will wrap around $($next character after Z is A$),$ the ciphertext character at some position

$$

i is given by$,$

$$

$$

$=$

$$

$$

$+$

$$

$$

m

o

d

$$

$$

$$

c

i

$$

$=$p

i

$$

$+$k

imod$$k$$

$$

Where

$$

c is our ciphertext,

$$

p is our plaintext and

$$

k is our key.

$$

$$

$$

$$k$$ is the length of our key.

To obtain plaintext

$$

p we can just reverse the shift, so

$$

$$

$=$

$$

$$

$$

$$

$$

m

o

d

$$

$$

$$

p

i

$$

$=$c

i

$$

$$k

imod$$k$$

$$

The task: Write a program that can both encrypt and decrypt text using the Vigen$$re cipher

Requirements

Your program will be required to both encrypt and decrypt ciphertext. The mode is specified by a program argument flag, $-$e for encrypt mode, and $-$d for decrypt mode.

Your program should work for both upper and lowercase plaintext$/$ciphertext$/$keys$,$ even if the key and plaintext$/$ciphertext do not have the same case as eachother.

Your program should also have another option for grouping output characters together. This can be specified by the $-$g flag. What this will do is strip the ciphertext or plaintext of any non$-$alphabetic characters and group the output into space$-$separated groups consisting of uppercase characters.

Grouping by $0$ means the string will have no spaces. If the grouping number is larger than the amount of characters in the input, then the resulting output will also have no spaces.

The arguments can be in any order $(-$g can be either before or after the $-$e or $-$d flags$).$

If both $-$e and $-$d flags are provided, the program should print an error.

The precedence of argument error handling follows the order of: program mode, and then grouping. That is$,$ you only print one error even if there is more than one, following that order.