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JaeHyoLee's solution

to Atbash Cipher in the Lua Track

Published at Jul 13 2018 · 1 comment
Test suite

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:  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.


  • 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.


Wikipedia 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.


local encode = require('atbash-cipher').encode

describe('atbash-cipher', function()
  it('should encode single letter plaintexts', function()
    assert.are.equal('m', encode('n'))

  it('should encode single-chunk plaintexts', function()
    assert.are.equal('svool', encode('hello'))

  it('should encode multi-chunk plaintexts', function()
    assert.are.equal('nrmwy oldrm tob', encode('mindblowingly'))

  it('should encode all letters as lower-case', function()
    assert.are.equal('svool', encode('HeLLo'))

  it('should not encode whitespace', function()
    assert.are.equal('svool', encode('h e l l o'))

  it('should not encode punctuation', function()
    assert.are.equal('svool', encode('h,e,l,l,o'))

  it('should not encode numbers', function()
    assert.are.equal('sv11l', encode('he11o'))

  it('should encode all letters', function()
      'gsvjf rxpyi ldmul cqfnk hlevi gsvoz abwlt',
      encode('The quick brown fox jumps over the lazy dog.')
local function encode_worker(raw)
  return string.char(string.byte('z') - raw:byte() + string.byte('a'))

local function encode(input)
  local norm = input:lower()
  local inter = norm:gsub("%a", encode_worker)
  inter = inter:gsub("%W", '')
  if #inter <= 5 then return inter end
  local encoded = ''
  for i=1, #inter, 5 do
    local j = i + 4 
    if j > #inter then j = #inter end
    encoded = encoded .. inter:sub(i,j) .. ' '
  return encoded:sub(1,-2)

return {
  encode = encode

Community comments

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Avatar of ryanplusplus

How about making some helper functions so that it is more clear what your algorithm is doing? For example, how about splitting the chunking part out to a helper?

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