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

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

i 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 $-g-g$ 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 $-e-g5$

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 $-d-g5$

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 $-e-g5$

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.

Handling user input

$ python$3$

vigenere.py $-$e

Enter the key: #abcd$1234$

Error: Keys must be at least $1$ character, and consist only of alphabetic characters.

Enter the key: #abcd

Enter the plaintext: #Hello, World!

Ciphertext is: Hfnoo, Xqule!

$ python$3$

vigenere.py $-$e

Enter the key: #

Error: Keys must be at least $1$ character, and consist only of alphabetic characters.

Enter the key: #abcd

Enter the plaintext: #Hello, World!

Ciphertext is: Hfnoo, Xqule!Handling user input

$ python$3$

vigenere.py $-$e

Enter the key: #abcd$1234$

Error: Keys must be at least $1$ character, and consist only of alphabetic characters.

Enter the key: #abcd

Enter the plaintext: #Hello, World!

Ciphertext is: Hfnoo, Xqule!

$ python$3$

vigenere.py $-$e

EnVigen$$re Cipher

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

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: rijvsuyvin

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.