The Vigen$$re cipher is a method of encrypting alphabetic text using a series of interwoven Caesar ciphers, based on letters of a keyword.

Notice how in a caesar cipher, you shifted the alphabet by some integer value. Consider a mapping that maps alphabetic letters into numbers $($and vice versa$).$ The main difference between a Vigen$$re cipher and the Caesar cipher, is that the Vigen$$re cipher changes the shift value for every character, depending on a key.

Example of a Vigen$$re Cipher

Consider the following Example of a Vigen$$re Cipher

Consider the following$\backslash$end$\{$array$\}]$

Ciphertext:

rijvsuyvin

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.

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$,$

$c_i=p_i+k_{\text{i}\text{m}\text{o}\text{d}|k|}$

Where $c$ is our ciphertext, $p$ is our plaintext and $k$ is our key. $|k|$ is the length of our key. 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 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 $-d-e-d$ 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.

Examples

The # symbol denotes user input

$ python$3$

vigenere.py $-$e

Enter the key: #helloworld

Enter the plaintext: #The quick brown fox jumps over the lazy dog.

Ciphertext is: Alp bieqb muvay qct xlxsz sgpf pvv wdgc ozu.

$ python$3$

vigenere.py $-$d

Enter the key: #helloworld

Enter the ciphertext: #Alp bieqb muvay qct xlxsz sgpf pvv wdgc ozu.

Plaintext is: The quick brown fox jumps over the lazy dog.

Grouping

$ python$3$

vigenere.py $-$e $-$g $5$

Enter the key: #helloworld

Enter the plaintext: #The quick brown fox jumps over the lazy dog.

Ciphertext is: ALPBI EQBMU VAYQC TXLXS ZSGPF PVVWD GCOZU

$ python$3$

vigenere.py $-$d $-$g $5$

Enter the key: #helloworld $ python$3$

vigenere.py $-$d $-$g $5$

Enter the key: #helloworld

Enter the ciphertext: #Alp bieqb muvay qct xlxsz sgpf pvv wdgc ozu.

Plaintext is: THEQU ICKBR OWNFO XJUMP SOVER THELA ZYDOG

$ python$3$

vigenere.py $-$e $-$g $5$

Enter the key: #helloworld

Enter the plaintext: #The quick brown fox.

Ciphertext is: ALPBI EQBMU VAYQC T

In the third example, since we didn't have enough characters to make a full group, our last group

just wont have the full amount.

Errors

$ python$3$

vigenere.py $-$e $-$d

Error: Cannot have both encrypt and decrypt mode.

$ python$3$

vigenere.py $-$g $4$

Error: Program mode was not specified. Use $-$e for encrypt and $-$d for decrypt.

$ python$3$

vigenere.py

Error: Program mode was not specified. Use $-$e for encrypt and $-$d for decrypt.

$ python$3$

vigenere.py $-g-e$

Error: You must supply an integer of at least zero with the grouping flag.It is okay for the user to not enter a message. The key is required, though. You should keep asking

for the key until a valid one is provided by the user.