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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: ```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` ## Setup There are two different methods of getting set up to run the tests with Objective-C: - Create an Xcode project with a test target which will run the tests. - Use the ruby gem `objc` as a test runner utility. Both are described in more detail here: http://exercism.io/languages/objective-c ### Submitting Exercises When submitting an exercise, make sure your solution file is in the same directory as the test code. The submit command will look something like: ```shell exercism submit <path-to-exercism-workspace>/objective-c/atbash-cipher/AtbashCipher.m ``` You can find the Exercism workspace by running `exercism debug` and looking for the line beginning with Workspace. ## 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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