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to Simple Cipher in the Elixir Track

Published at Nov 19 2019 · 0 comments
Test suite

Implement a simple shift cipher like Caesar and a more secure substitution cipher.

Step 1

"If he had anything confidential to say, he wrote it in cipher, that is, by so changing the order of the letters of the alphabet, that not a word could be made out. If anyone wishes to decipher these, and get at their meaning, he must substitute the fourth letter of the alphabet, namely D, for A, and so with the others." —Suetonius, Life of Julius Caesar

Ciphers are very straight-forward algorithms that allow us to render text less readable while still allowing easy deciphering. They are vulnerable to many forms of cryptoanalysis, but we are lucky that generally our little sisters are not cryptoanalysts.

The Caesar Cipher was used for some messages from Julius Caesar that were sent afield. Now Caesar knew that the cipher wasn't very good, but he had one ally in that respect: almost nobody could read well. So even being a couple letters off was sufficient so that people couldn't recognize the few words that they did know.

Your task is to create a simple shift cipher like the Caesar Cipher. This image is a great example of the Caesar Cipher:

Caesar Cipher

For example:

Giving "iamapandabear" as input to the encode function returns the cipher "ldpdsdqgdehdu". Obscure enough to keep our message secret in transit.

When "ldpdsdqgdehdu" is put into the decode function it would return the original "iamapandabear" letting your friend read your original message.

Step 2

Shift ciphers are no fun though when your kid sister figures it out. Try amending the code to allow us to specify a key and use that for the shift distance. This is called a substitution cipher.

Here's an example:

Given the key "aaaaaaaaaaaaaaaaaa", encoding the string "iamapandabear" would return the original "iamapandabear".

Given the key "ddddddddddddddddd", encoding our string "iamapandabear" would return the obscured "ldpdsdqgdehdu"

In the example above, we've set a = 0 for the key value. So when the plaintext is added to the key, we end up with the same message coming out. So "aaaa" is not an ideal key. But if we set the key to "dddd", we would get the same thing as the Caesar Cipher.

Step 3

The weakest link in any cipher is the human being. Let's make your substitution cipher a little more fault tolerant by providing a source of randomness and ensuring that the key contains only lowercase letters.

If someone doesn't submit a key at all, generate a truly random key of at least 100 characters in length.

If the key submitted is not composed only of lowercase letters, your solution should handle the error in a language-appropriate way.


Shift ciphers work by making the text slightly odd, but are vulnerable to frequency analysis. Substitution ciphers help that, but are still very vulnerable when the key is short or if spaces are preserved. Later on you'll see one solution to this problem in the exercise "crypto-square".

If you want to go farther in this field, the questions begin to be about how we can exchange keys in a secure way. Take a look at Diffie-Hellman on Wikipedia for one of the first implementations of this scheme.

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!"

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.


Substitution Cipher at Wikipedia http://en.wikipedia.org/wiki/Substitution_cipher

Submitting Incomplete Solutions

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


defmodule SimpleCipherTest do
  use ExUnit.Case

  # @tag :pending
  test "encoding with 'a' returns the original text" do
    assert SimpleCipher.encode("a", "a") == "a"
    assert SimpleCipher.encode("b", "a") == "b"
    assert SimpleCipher.encode("c", "a") == "c"

  @tag :pending
  test "encoding with another key returns shifted text" do
    assert SimpleCipher.encode("a", "d") == "d"
    assert SimpleCipher.encode("b", "d") == "e"
    assert SimpleCipher.encode("c", "d") == "f"

  @tag :pending
  test "decoding with 'a' returns the original text" do
    assert SimpleCipher.decode("a", "a") == "a"
    assert SimpleCipher.decode("b", "a") == "b"
    assert SimpleCipher.decode("c", "a") == "c"

  @tag :pending
  test "decoding with another key returns unshifted text" do
    assert SimpleCipher.decode("d", "d") == "a"
    assert SimpleCipher.decode("e", "d") == "b"
    assert SimpleCipher.decode("f", "d") == "c"

  @tag :pending
  test "key uses per-letter translation for encoding" do
    key = "abc"

    assert SimpleCipher.encode("abc", key) == "ace"
    assert SimpleCipher.encode("bcd", key) == "bdf"
    assert SimpleCipher.encode("cde", key) == "ceg"
    assert SimpleCipher.encode("iamapandabear", "dddddddddddddd") == "ldpdsdqgdehdu"

  @tag :pending
  test "key uses per-letter translation for decoding" do
    key = "abc"

    assert SimpleCipher.decode("ace", key) == "abc"
    assert SimpleCipher.decode("bdf", key) == "bcd"
    assert SimpleCipher.decode("ceg", key) == "cde"
    assert SimpleCipher.decode("ldpdsdqgdehdu", "dddddddddddddd") == "iamapandabear"

  @tag :pending
  test "only lowercase a-z are translated, rest are passed through" do
    assert SimpleCipher.encode("this is a test!", "d") == "wklv lv d whvw!"
    assert SimpleCipher.decode("wklv lv d whvw!", "d") == "this is a test!"

  @tag :pending
  test "if key is shorter than text, repeat key" do
    assert SimpleCipher.encode("abc", "a") == "abc"
    assert SimpleCipher.encode("abcdefghi", "abc") == "acedfhgik"

  @tag :pending
  test "if key is longer than text, only use as much as needed" do
    key = "somewhatlongkey"

    assert SimpleCipher.encode("abc", key) == "spo"
    assert SimpleCipher.decode("abc", key) == "inq"

  @tag :pending
  test "if you know both the encoded and decoded text, you can figure out the key" do
    key = "supersecretkey"

    plaintext = "attackxatxdawn"
    ciphertext = SimpleCipher.encode(plaintext, key)

    assert SimpleCipher.decode(ciphertext, plaintext) == key


ExUnit.configure(exclude: :pending, trace: true)
defmodule SimpleCipher do
  def encode(plaintext, key) do
    translate(plaintext, key, &right_shift/1)

  def decode(ciphertext, key) do
    translate(ciphertext, key, &left_shift/1)

  defp translate(text, key, shift) do
    |> to_charlist()
    |> Stream.cycle()
    |> Enum.zip(to_charlist(text))
    |> Enum.map(shift)
    |> to_string()

  defp right_shift({key, letter}) when letter in ?a..?z,
    do: ?a + Integer.mod(letter - ?a + key - ?a, 26)
  defp right_shift({_, letter}), do: letter

  defp left_shift({key, letter}) when letter in ?a..?z,
    do: ?a + Integer.mod(letter - key, 26)
  defp left_shift({_, letter}), do: letter

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