elm/exercises/atbash-cipher/README.md

# Atbash Cipher

Create an implementation of the atbash cipher, an ancient encryption system created in the Middle East.

The Atbash cipher is a simple substitution cipher that relies on
transposing all the letters in the alphabet such that the resulting
alphabet is backwards. The first letter is replaced with the last
letter, the second with the second-last, and so on.

An Atbash cipher for the Latin alphabet would be as follows:

```plain
Plain:  abcdefghijklmnopqrstuvwxyz
Cipher: zyxwvutsrqponmlkjihgfedcba
```

It is a very weak cipher because it only has one possible key, and it is
a simple monoalphabetic substitution cipher. However, this may not have
been an issue in the cipher's time.

Ciphertext is written out in groups of fixed length, the traditional group size
being 5 letters, and punctuation is excluded. This is to make it harder to guess
things based on word boundaries.

## Examples
- Encoding `test` gives `gvhg`
- Decoding `gvhg` gives `test`
- Decoding `gsvjf rxpyi ldmul cqfnk hlevi gsvoz abwlt` gives `thequickbrownfoxjumpsoverthelazydog`

## Elm Installation

Refer to the [Exercism help page](http://exercism.io/languages/elm) for Elm
installation and learning resources.

## Writing the Code

The first time you start an exercise, you'll need to ensure you have the
appropriate dependencies installed.

```bash
$ npm install
```

Execute the tests with:

```bash
$ npm test
```

Automatically run tests again when you save changes:

```bash
$ npm run watch
```

As you work your way through the test suite, be sure to remove the `skip <|`
calls from each test until you get them all passing!

## Source

Wikipedia [http://en.wikipedia.org/wiki/Atbash](http://en.wikipedia.org/wiki/Atbash)

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