# Vigenere Cipher

> Blue Cosmo | 08/02/21

## Overview:

```
a python shell script to encrypt and decrypt vigenere
```

## Resources:

* python3

## Components:

* encryption
* decryption
* advanced
  * encryption
  * decryption
* key decoding
  * advanced key decoding

## Roadmap:

* implementing unkownkey
* implement key wordlist \[bruteforce]

## Extraneous:

* aka polyalphabetic cipher
* invented by Giovan Battisa Bellaso in 1553
  * a more advanced version was published byBlaise de Vigenere
* relies on the index of the letter withing the key

***

### Example:

we start with our indexing chart, a little different than the one for the caesar cipher

```
Indexing:
/---------------------------------------------------\
| A | B | C | D | E | F | G | H | I | J | K | L | M |
|---+---+---+---+---+---+---+---+---+---+---+---+---|
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10| 11| 12|
|---+---+---+---+---+---+---+---+---+---+---+---+---|
| N | O | P | Q | R | S | T | U | V | W | X | Y | Z |
|---+---+---+---+---+---+---+---+---+---+---+---+---|
| 13| 14| 15| 16| 17| 18| 19| 20| 21| 22| 23| 24| 25|
\---------------------------------------------------/
```

for this example, we will be using "KEY" as our key - our key has the following indexes \[these will be important]

```
/-----------\
| K | E | Y |
|---+---+---|
| 10| 4 | 24|
\-----------/
```

* now we can start to encrypt some plaintext, we will choose the text "ENCRYPT"
* we must first find the indexes of the plaintext's characters

```
/---------------------------\
| E | N | C | R | Y | P | T |
|---+---+---+---+---+---+---|
| 4 | 13| 2 | 17| 24| 15| 19|
\---------------------------/
```

we can now comibne them to the respective index's of our key

```
/---------------------------\
| E | N | C | R | Y | P | T | <-- PLAINTEXT
|---+---+---+---+---+---+---| 
| 4 | 13| 2 | 17| 24| 15| 19| <-- PLAINTEXT INDEXES
|---+---+---+---+---+---+---|
|+10| +4|+24|+10| +4|+24|+10| <-- KEY
|---+---+---+---+---+---+---|
| 14| 17| 0 | 1 | 2 | 13| 3 | <-- CIPHERTEXT INDEXES
|---+---+---+---+---+---+---|
| O | R | A | B | C | N | D | <-- CIPHERTEXT
\---------------------------/
```

now we can just reverse the process by subtracting the key values from the plaintext

```
/---------------------------\
| O | R | A | B | C | N | D | <-- CIPHERTEXT
|---+---+---+---+---+---+---| 
| 14| 17| 0 | 1 | 2 | 13| 3 | <-- CIPHERTEXT INDEXES
|---+---+---+---+---+---+---|
|-10| -4|-24|-10| -4|-24|-10| <-- KEY
|---+---+---+---+---+---+---|
| 4 | 13| 2 | 17| 24| 15| 19| <-- PLAINTEXT INDEXES
|---+---+---+---+---+---+---|
| E | N | C | R | Y | P | T | <-- PLAINTEXT
\---------------------------/
```


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