 # paulfioravanti's solution

## to Affine Cipher in the Ruby Track

Published at May 12 2019 · 0 comments
Instructions
Test suite
Solution

Create an implementation of the affine cipher, an ancient encryption system created in the Middle East.

The affine cipher is a type of monoalphabetic substitution cipher. Each character is mapped to its numeric equivalent, encrypted with a mathematical function and then converted to the letter relating to its new numeric value. Although all monoalphabetic ciphers are weak, the affine cypher is much stronger than the atbash cipher, because it has many more keys.

the encryption function is:

`E(x) = (ax + b) mod m`

• where `x` is the letter's index from 0 - length of alphabet - 1
• `m` is the length of the alphabet. For the roman alphabet `m == 26`.
• and `a` and `b` make the key

the decryption function is:

`D(y) = a^-1(y - b) mod m`

• where `y` is the numeric value of an encrypted letter, ie. `y = E(x)`
• it is important to note that `a^-1` is the modular multiplicative inverse of `a mod m`
• the modular multiplicative inverse of `a` only exists if `a` and `m` are coprime.

To find the MMI of `a`:

`an mod m = 1`

• where `n` is the modular multiplicative inverse of `a mod m`

More information regarding how to find a Modular Multiplicative Inverse and what it means can be found here.

Because automatic decryption fails if `a` is not coprime to `m` your program should return status 1 and `"Error: a and m must be coprime."` if they are not. Otherwise it should encode or decode with the provided key.

The Caesar (shift) cipher is a simple affine cipher where `a` is 1 and `b` as the magnitude results in a static displacement of the letters. This is much less secure than a full implementation of the affine cipher.

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 `ybty` with the key a=5 b=7
• Decoding `ybty` gives `test` with the key a=5 b=7
• Decoding `ybty` gives `lqul` with the wrong key a=11 b=7
• Decoding `kqlfd jzvgy tpaet icdhm rtwly kqlon ubstx`
• gives `thequickbrownfoxjumpsoverthelazydog` with the key a=19 b=13
• Encoding `test` with the key a=18 b=13
• gives `Error: a and m must be coprime.`
• because a and m are not relatively prime

### Examples of finding a Modular Multiplicative Inverse (MMI)

• simple example:
• `9 mod 26 = 9`
• `9 * 3 mod 26 = 27 mod 26 = 1`
• `3` is the MMI of `9 mod 26`
• a more complicated example:
• `15 mod 26 = 15`
• `15 * 7 mod 26 = 105 mod 26 = 1`
• `7` is the MMI of `15 mod 26`

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 affine_cipher_test.rb
``````

To include color from the command line:

``````ruby -r minitest/pride affine_cipher_test.rb
``````

## Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

### affine_cipher_test.rb

``````require 'minitest/autorun'
require_relative 'affine_cipher'

# Common test data version: 2.0.0 8026923
class AffineCipherTest < Minitest::Test
def test_encode_yes
# skip
cipher = Affine.new(5, 7)
plaintext = 'yes'
ciphertext = 'xbt'
assert_equal ciphertext, cipher.encode(plaintext)
end

def test_encode_no
skip
cipher = Affine.new(15, 18)
plaintext = 'no'
ciphertext = 'fu'
assert_equal ciphertext, cipher.encode(plaintext)
end

def test_encode_omg
skip
cipher = Affine.new(21, 3)
plaintext = 'OMG'
ciphertext = 'lvz'
assert_equal ciphertext, cipher.encode(plaintext)
end

def test_encode_o_m_g
skip
cipher = Affine.new(25, 47)
plaintext = 'O M G'
ciphertext = 'hjp'
assert_equal ciphertext, cipher.encode(plaintext)
end

def test_encode_mindblowingly
skip
cipher = Affine.new(11, 15)
plaintext = 'mindblowingly'
ciphertext = 'rzcwa gnxzc dgt'
assert_equal ciphertext, cipher.encode(plaintext)
end

def test_encode_numbers
skip
cipher = Affine.new(3, 4)
plaintext = 'Testing,1 2 3, testing.'
ciphertext = 'jqgjc rw123 jqgjc rw'
assert_equal ciphertext, cipher.encode(plaintext)
end

def test_encode_deep_thought
skip
cipher = Affine.new(5, 17)
plaintext = 'Truth is fiction.'
ciphertext = 'iynia fdqfb ifje'
assert_equal ciphertext, cipher.encode(plaintext)
end

def test_encode_all_the_letters
skip
cipher = Affine.new(17, 33)
plaintext = 'The quick brown fox jumps over the lazy dog.'
ciphertext = 'swxtj npvyk lruol iejdc blaxk swxmh qzglf'
assert_equal ciphertext, cipher.encode(plaintext)
end

def test_encode_with_a_not_coprime_to_m
skip
assert_raises(ArgumentError) { Affine.new(6, 17) }
end

def test_decode_exercism
skip
cipher = Affine.new(3, 7)
ciphertext = 'tytgn fjr'
plaintext = 'exercism'
assert_equal plaintext, cipher.decode(ciphertext)
end

def test_decode_a_sentence
skip
cipher = Affine.new(19, 16)
ciphertext = 'qdwju nqcro muwhn odqun oppmd aunwd o'
plaintext = 'anobstacleisoftenasteppingstone'
assert_equal plaintext, cipher.decode(ciphertext)
end

def test_decode_numbers
skip
cipher = Affine.new(25, 7)
ciphertext = 'odpoz ub123 odpoz ub'
plaintext = 'testing123testing'
assert_equal plaintext, cipher.decode(ciphertext)
end

def test_decode_all_the_letters
skip
cipher = Affine.new(17, 33)
ciphertext = 'swxtj npvyk lruol iejdc blaxk swxmh qzglf'
plaintext = 'thequickbrownfoxjumpsoverthelazydog'
assert_equal plaintext, cipher.decode(ciphertext)
end

def test_decode_with_no_spaces_in_input
skip
cipher = Affine.new(17, 33)
ciphertext = 'swxtjnpvyklruoliejdcblaxkswxmhqzglf'
plaintext = 'thequickbrownfoxjumpsoverthelazydog'
assert_equal plaintext, cipher.decode(ciphertext)
end

def test_decode_with_too_many_spaces
skip
cipher = Affine.new(15, 16)
ciphertext = 'vszzm    cly   yd cg    qdp'
plaintext = 'jollygreengiant'
assert_equal plaintext, cipher.decode(ciphertext)
end

def test_decode_with_a_not_coprime_to_m
skip
assert_raises(ArgumentError) { Affine.new(13, 5) }
end
end``````
``````# frozen_string_literal: true

class Affine
ALPHABET = "abcdefghijklmnopqrstuvwxyz"
private_constant :ALPHABET
CHUNKS_OF_5 = /.{1,5}/.freeze
private_constant :CHUNKS_OF_5
IS_COPRIME = ->(a_key) { a_key.gcd(ALPHABET.length) == 1 }
private_constant :IS_COPRIME
NON_WORD_CHARACTERS = /\W/.freeze
private_constant :NON_WORD_CHARACTERS

def initialize(a_key, b_key)
raise ArgumentError unless IS_COPRIME.call(a_key)

@key = generate_key(a_key, b_key)
end

def encode(plaintext)
plaintext
.downcase
.gsub(NON_WORD_CHARACTERS, "")
.tr(ALPHABET, key)
.scan(CHUNKS_OF_5)
.join(" ")
end

def decode(ciphertext)
ciphertext
.gsub(NON_WORD_CHARACTERS, "")
.tr(key, ALPHABET)
end

private

attr_reader :key

def generate_key(*keys)
ALPHABET
.chars
.each_index
.with_object(keys)
.reduce("", &method(:affine_character))
end

def affine_character(acc, (index, (a_key, b_key)))
# (ax + b) mod m
affine_index = (a_key * index + b_key) % ALPHABET.length
acc + ALPHABET[affine_index]
end
end``````

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