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# 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`
## Loading your exercise implementation in PolyML
```
$ poly --use {exercise}.sml
```
Or:
```
$ poly
> use "{exercise}.sml";
```
**Note:** You have to replace {exercise}.
## Running the tests
```
$ poly -q --use test.sml
```
## Feedback, Issues, Pull Requests
The [exercism/sml](https://github.com/exercism/sml) repository on
GitHub is the home for all of the Standard ML 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.
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