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tom-mi's solution

to Atbash Cipher in the Ruby Track

Published at Jul 30 2020 · 0 comments
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

For installation and learning resources, refer to the Ruby resources page.

For running the tests provided, you will need the Minitest gem. Open a terminal window and run the following command to install minitest:

gem install minitest

If you would like color output, you can require 'minitest/pride' in the test file, or note the alternative instruction, below, for running the test file.

Run the tests from the exercise directory using the following command:

ruby atbash_cipher_test.rb

To include color from the command line:

ruby -r minitest/pride atbash_cipher_test.rb

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_test.rb

require 'minitest/autorun'
require_relative 'atbash_cipher'

# Common test data version: 1.2.0 d5238bd
class AtbashCipherTest < Minitest::Test
  def test_encode_yes
    # skip
    plaintext = 'yes'
    ciphertext = 'bvh'
    assert_equal ciphertext, Atbash.encode(plaintext)
  end

  def test_encode_no
    skip
    plaintext = 'no'
    ciphertext = 'ml'
    assert_equal ciphertext, Atbash.encode(plaintext)
  end

  def test_encode_omg
    skip
    plaintext = 'OMG'
    ciphertext = 'lnt'
    assert_equal ciphertext, Atbash.encode(plaintext)
  end

  def test_encode_spaces
    skip
    plaintext = 'O M G'
    ciphertext = 'lnt'
    assert_equal ciphertext, Atbash.encode(plaintext)
  end

  def test_encode_mindblowingly
    skip
    plaintext = 'mindblowingly'
    ciphertext = 'nrmwy oldrm tob'
    assert_equal ciphertext, Atbash.encode(plaintext)
  end

  def test_encode_numbers
    skip
    plaintext = 'Testing,1 2 3, testing.'
    ciphertext = 'gvhgr mt123 gvhgr mt'
    assert_equal ciphertext, Atbash.encode(plaintext)
  end

  def test_encode_deep_thought
    skip
    plaintext = 'Truth is fiction.'
    ciphertext = 'gifgs rhurx grlm'
    assert_equal ciphertext, Atbash.encode(plaintext)
  end

  def test_encode_all_the_letters
    skip
    plaintext = 'The quick brown fox jumps over the lazy dog.'
    ciphertext = 'gsvjf rxpyi ldmul cqfnk hlevi gsvoz abwlt'
    assert_equal ciphertext, Atbash.encode(plaintext)
  end

  def test_decode_exercism
    skip
    ciphertext = 'vcvix rhn'
    plaintext = 'exercism'
    assert_equal plaintext, Atbash.decode(ciphertext)
  end

  def test_decode_a_sentence
    skip
    ciphertext = 'zmlyh gzxov rhlug vmzhg vkkrm thglm v'
    plaintext = 'anobstacleisoftenasteppingstone'
    assert_equal plaintext, Atbash.decode(ciphertext)
  end

  def test_decode_numbers
    skip
    ciphertext = 'gvhgr mt123 gvhgr mt'
    plaintext = 'testing123testing'
    assert_equal plaintext, Atbash.decode(ciphertext)
  end

  def test_decode_all_the_letters
    skip
    ciphertext = 'gsvjf rxpyi ldmul cqfnk hlevi gsvoz abwlt'
    plaintext = 'thequickbrownfoxjumpsoverthelazydog'
    assert_equal plaintext, Atbash.decode(ciphertext)
  end

  def test_decode_with_too_many_spaces
    skip
    ciphertext = 'vc vix    r hn'
    plaintext = 'exercism'
    assert_equal plaintext, Atbash.decode(ciphertext)
  end

  def test_decode_with_no_spaces
    skip
    ciphertext = 'zmlyhgzxovrhlugvmzhgvkkrmthglmv'
    plaintext = 'anobstacleisoftenasteppingstone'
    assert_equal plaintext, Atbash.decode(ciphertext)
  end
end
=begin
Write your code for the 'Atbash Cipher' exercise in this file. Make the tests in
`atbash_cipher_test.rb` pass.

To get started with TDD, see the `README.md` file in your
`ruby/atbash-cipher` directory.
=end

module Atbash
  def self.encode(plaintext)
    plaintext
        .downcase
        .gsub(/[^a-z0-9]/, '')
        .tr(('a'..'z').to_a.join, ('a'..'z').to_a.reverse.join)
        .chars
        .each_slice(5)
        .map(&:join)
        .join(' ')
  end

  def self.decode(ciphertext)
    encode(ciphertext).gsub(' ', '')
  end
end

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