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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` ## Running the tests To run the tests, run the command `busted` from within the exercise directory. ## Further information For more detailed information about the Lua track, including how to get help if you're having trouble, please visit the exercism.io [Lua language page](http://exercism.io/languages/lua/about). ## 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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63 entries across 63 versions & 1 rubygems