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# JaeHyoLee's solution

## to Atbash Cipher in the Lua Track

Published at Jul 13 2018 · 1 comment
Instructions
Test suite
Solution

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.

## 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.

## Source

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.

### atbash-cipher_spec.lua

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

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

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

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

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

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

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

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

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

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) .. ' '
end
return encoded:sub(1,-2)
end

return {
encode = encode
}``````