🎉 Exercism Research is now launched. Help Exercism, help science and have some fun at research.exercism.io 🎉
Avatar of thekeele

thekeele's solution

to Atbash Cipher in the Elixir Track

Published at Dec 01 2019 · 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

Running tests

Execute the tests with:

$ mix test

Pending tests

In the test suites, all but the first test have been skipped.

Once you get a test passing, you can unskip the next one by commenting out the relevant @tag :pending with a # symbol.

For example:

# @tag :pending
test "shouting" do
  assert Bob.hey("WATCH OUT!") == "Whoa, chill out!"
end

Or, you can enable all the tests by commenting out the ExUnit.configure line in the test suite.

# ExUnit.configure exclude: :pending, trace: true

If you're stuck on something, it may help to look at some of the available resources out there where answers might be found.

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_test.exs

defmodule AtbashTest do
  use ExUnit.Case

  describe "encode" do
    # @tag :pending
    test "yes" do
      assert Atbash.encode("yes") == "bvh"
    end

    @tag :pending
    test "no" do
      assert Atbash.encode("no") == "ml"
    end

    @tag :pending
    test "OMG" do
      assert Atbash.encode("OMG") == "lnt"
    end

    @tag :pending
    test "O M G" do
      assert Atbash.encode("O M G") == "lnt"
    end

    @tag :pending
    test "mindblowingly" do
      assert Atbash.encode("mindblowingly") == "nrmwy oldrm tob"
    end

    @tag :pending
    test "numbers" do
      assert Atbash.encode("Testing, 1 2 3, testing.") == "gvhgr mt123 gvhgr mt"
    end

    @tag :pending
    test "deep thought" do
      assert Atbash.encode("Truth is fiction.") == "gifgs rhurx grlm"
    end

    @tag :pending
    test "all the letters" do
      plaintext = "The quick brown fox jumps over the lazy dog."
      cipher = "gsvjf rxpyi ldmul cqfnk hlevi gsvoz abwlt"
      assert Atbash.encode(plaintext) == cipher
    end
  end

  describe "decode" do
    @tag :pending
    test "exercism" do
      cipher = "vcvix rhn"
      plaintext = "exercism"
      assert Atbash.decode(cipher) == plaintext
    end

    @tag :pending
    test "a sentence" do
      cipher = "zmlyh gzxov rhlug vmzhg vkkrm thglm v"
      plaintext = "anobstacleisoftenasteppingstone"
      assert Atbash.decode(cipher) == plaintext
    end

    @tag :pending
    test "numbers" do
      cipher = "gvhgr mt123 gvhgr mt"
      plaintext = "testing123testing"
      assert Atbash.decode(cipher) == plaintext
    end

    @tag :pending
    test "all the letters" do
      cipher = "gsvjf rxpyi ldmul cqfnk hlevi gsvoz abwlt"
      plaintext = "thequickbrownfoxjumpsoverthelazydog"
      assert Atbash.decode(cipher) == plaintext
    end

    @tag :pending
    test "with too many spaces" do
      cipher = "vc vix    r hn"
      plaintext = "exercism"
      assert Atbash.decode(cipher) == plaintext
    end

    @tag :pending
    test "with no spaces" do
      cipher = "zmlyhgzxovrhlugvmzhgvkkrmthglmv"
      plaintext = "anobstacleisoftenasteppingstone"
      assert Atbash.decode(cipher) == plaintext
    end
  end
end

test_helper.exs

ExUnit.start()
ExUnit.configure(exclude: :pending, trace: true)
defmodule Atbash do
  @spec encode(String.t()) :: String.t()
  def encode(plaintext) do
    plaintext |> substitute() |> Enum.chunk_every(5) |> Enum.join(" ")
  end

  @spec decode(String.t()) :: String.t()
  def decode(cipher) do
    cipher |> substitute() |> to_string()
  end

  defp substitute(text) when is_binary(text),
    do: text |> String.downcase() |> to_charlist |> Enum.reduce([], &substitute/2)
  defp substitute(letter, cipher) when letter in ?a..?z,
    do: cipher ++ [(?a + ?z) - letter]
  defp substitute(number, cipher) when number in ?0..?9,
    do: cipher ++ [number]
  defp substitute(_, cipher),
    do: cipher
end

Community comments

Find this solution interesting? Ask the author a question to learn more.

What can you learn from this solution?

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

  • What compromises have been made?
  • Are there new concepts here that you could read more about to improve your understanding?