# Simple Cipher

Implement a simple shift cipher like Caesar and a more secure substitution cipher.

## Step 1

"If he had anything confidential to say, he wrote it in cipher, that is,
by so changing the order of the letters of the alphabet, that not a word
could be made out. If anyone wishes to decipher these, and get at their
meaning, he must substitute the fourth letter of the alphabet, namely D,
for A, and so with the others."
—Suetonius, Life of Julius Caesar

Ciphers are very straight-forward algorithms that allow us to render
text less readable while still allowing easy deciphering. They are
vulnerable to many forms of cryptoanalysis, but we are lucky that
generally our little sisters are not cryptoanalysts.

The Caesar Cipher was used for some messages from Julius Caesar that
were sent afield. Now Caesar knew that the cipher wasn't very good, but
he had one ally in that respect: almost nobody could read well. So even
being a couple letters off was sufficient so that people couldn't
recognize the few words that they did know.

Your task is to create a simple shift cipher like the Caesar Cipher.
This image is a great example of the Caesar Cipher:

![Caesar Cipher][1]

For example:

Giving "iamapandabear" as input to the encode function returns the cipher "ldpdsdqgdehdu". Obscure enough to keep our message secret in transit.

When "ldpdsdqgdehdu" is put into the decode function it would return
the original "iamapandabear" letting your friend read your original
message.

## Step 2

Shift ciphers are no fun though when your kid sister figures it out. Try
amending the code to allow us to specify a key and use that for the
shift distance. This is called a substitution cipher.

Here's an example:

Given the key "aaaaaaaaaaaaaaaaaa", encoding the string "iamapandabear"
would return the original "iamapandabear".

Given the key "ddddddddddddddddd", encoding our string "iamapandabear"
would return the obscured "lpdsdqgdehdu"

In the example above, we've set a = 0 for the key value. So when the
plaintext is added to the key, we end up with the same message coming
out. So "aaaa" is not an ideal key. But if we set the key to "dddd", we
would get the same thing as the Caesar Cipher.

## Step 3

The weakest link in any cipher is the human being. Let's make your
substitution cipher a little more fault tolerant by providing a source
of randomness and ensuring that the key is not composed of numbers or
capital letters.

If someone doesn't submit a key at all, generate a truly random key of
at least 100 characters in length, accessible via Cipher#key (the #
syntax means instance variable)

If the key submitted has capital letters or numbers, throw an
ArgumentError with a message to that effect.

## Extensions

Shift ciphers work by making the text slightly odd, but are vulnerable
to frequency analysis. Substitution ciphers help that, but are still
very vulnerable when the key is short or if spaces are preserved. Later
on you'll see one solution to this problem in the exercise
"crypto-square".

If you want to go farther in this field, the questions begin to be about
how we can exchange keys in a secure way. Take a look at [Diffie-Hellman
on Wikipedia][dh] for one of the first implementations of this scheme.

[1]: https://upload.wikimedia.org/wikipedia/commons/thumb/4/4a/Caesar_cipher_left_shift_of_3.svg/320px-Caesar_cipher_left_shift_of_3.svg.png
[dh]: http://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange


## Getting Started

For installation and learning resources, refer to the
[exercism help page](http://exercism.io/languages/haskell).

## Running the tests

To run the test suite, execute the following command:

```bash
stack test
```

#### If you get an error message like this...

```
No .cabal file found in directory
```

You are probably running an old stack version and need
to upgrade it.

#### Otherwise, if you get an error message like this...

```
No compiler found, expected minor version match with...
Try running "stack setup" to install the correct GHC...
```

Just do as it says and it will download and install
the correct compiler version:

```bash
stack setup
```

## Running *GHCi*

If you want to play with your solution in GHCi, just run the command:

```bash
stack ghci
```

## Feedback, Issues, Pull Requests

The [exercism/haskell](https://github.com/exercism/haskell) repository on
GitHub is the home for all of the Haskell exercises.

If you have feedback about an exercise, or want to help implementing a new
one, head over there and create an issue.  We'll do our best to help you!

## Source

Substitution Cipher at Wikipedia [http://en.wikipedia.org/wiki/Substitution_cipher](http://en.wikipedia.org/wiki/Substitution_cipher)

## Submitting Incomplete Solutions
It's possible to submit an incomplete solution so you can see how others have completed the exercise.