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65 lines
1.8 KiB
Markdown
65 lines
1.8 KiB
Markdown
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# Atbash Cipher
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Create an implementation of the atbash cipher, an ancient encryption system created in the Middle East.
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The Atbash cipher is a simple substitution cipher that relies on
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transposing all the letters in the alphabet such that the resulting
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alphabet is backwards. The first letter is replaced with the last
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letter, the second with the second-last, and so on.
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An Atbash cipher for the Latin alphabet would be as follows:
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```plain
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Plain: abcdefghijklmnopqrstuvwxyz
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Cipher: zyxwvutsrqponmlkjihgfedcba
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```
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It is a very weak cipher because it only has one possible key, and it is
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a simple monoalphabetic substitution cipher. However, this may not have
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been an issue in the cipher's time.
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Ciphertext is written out in groups of fixed length, the traditional group size
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being 5 letters, and punctuation is excluded. This is to make it harder to guess
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things based on word boundaries.
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## Examples
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- Encoding `test` gives `gvhg`
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- Decoding `gvhg` gives `test`
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- Decoding `gsvjf rxpyi ldmul cqfnk hlevi gsvoz abwlt` gives `thequickbrownfoxjumpsoverthelazydog`
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## Elm Installation
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Refer to the [Exercism help page](http://exercism.io/languages/elm) for Elm
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installation and learning resources.
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## Writing the Code
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The first time you start an exercise, you'll need to ensure you have the
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appropriate dependencies installed.
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```bash
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$ npm install
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```
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Execute the tests with:
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```bash
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$ npm test
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```
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Automatically run tests again when you save changes:
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```bash
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$ npm run watch
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```
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As you work your way through the test suite, be sure to remove the `skip <|`
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calls from each test until you get them all passing!
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## Source
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Wikipedia [http://en.wikipedia.org/wiki/Atbash](http://en.wikipedia.org/wiki/Atbash)
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## Submitting Incomplete Solutions
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It's possible to submit an incomplete solution so you can see how others have completed the exercise.
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