# 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:

```text
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`

## Deprecation of String.lowercase
Depending on the version of OCaml you installed the use of `String.lowercase` is
frowned upon. Since version 4.03.0 `String.lowercase` is deprecated in favor of
`String.lowercase_ascii`. So instead of writing 

```ocaml
String.lowercase "Hello, World!"
```

use

```ocaml
String.lowercase_ascii "Hello, World!"
```

See [String documentation](http://caml.inria.fr/pub/docs/manual-ocaml/libref/String.html) 
for more useful functions.


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

## Installation
To work on the exercises, you will need `Opam` and `Core`. Consult [opam](https://opam.ocaml.org) website for instructions on how to install `opam` for your OS. Once `opam` is installed open a terminal window and run the following command to install core:

```bash
opam install core
```

To run the tests you will need `OUnit`. Install it using `opam`:

```bash
opam install ounit
```

## Running Tests
A Makefile is provided with a default target to compile your solution and run the tests. At the command line, type:

```bash
make
```

## Interactive Shell
`utop` is a command line program which allows you to run Ocaml code interactively. The easiest way to install it is via opam:
```bash
opam install utop
```
Consult [utop](https://github.com/diml/utop/blob/master/README.md) for more detail.

## Feedback, Issues, Pull Requests
The [exercism/ocaml](https://github.com/exercism/ocaml) repository on
GitHub is the home for all of the Ocaml 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

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.