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Ric0chet's solution

to Simple Cipher in the Scala Track

Published at Oct 17 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 cryptanalysis, but we are lucky that generally our little sisters are not cryptanalysts.

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 alphanumeric characters in length.


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.

The Scala exercises assume an SBT project scheme. The exercise solution source should be placed within the exercise directory/src/main/scala. The exercise unit tests can be found within the exercise directory/src/test/scala.

To run the tests simply run the command sbt test in the exercise directory.

Please see the learning and installation pages if you need any help.


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.


import org.scalatest.{Matchers, FunSuite}

/** @version created manually **/
class CipherTest extends FunSuite with Matchers {

  test("Random key cipher - can encode/decode") {
    // Here we take advantage of the fact that plaintext of "aaa..."
    // outputs the key. This is a critical problem with shift ciphers, some
    // characters will always output the key verbatim.

    val cipher = Cipher(None)
    cipher.encode("aaaaaaaaaa") should be (cipher.key.substring(0, 10))
    cipher.decode(cipher.key.substring(0, 10)) should be ("aaaaaaaaaa")

  test("Invalid key - contains caps") { 
    intercept[IllegalArgumentException] { 

  test("Invalid key - contains numerics") { 
    intercept[IllegalArgumentException] {

  test("Invalid key - is empty") { 
    intercept[IllegalArgumentException] {

  test("Substitution cipher - can encode") { 
    Cipher(Some("abcdefghij")).encode("aaaaaaaaaa") should be ("abcdefghij")

  test("Substitution cipher - can decode") { 
    Cipher(Some("abcdefghij")).decode("abcdefghij") should be ("aaaaaaaaaa")

  test("Substitution cipher - is reversible") { 
    val cipher = Cipher(Some("abcdefghij"))
    cipher.decode(cipher.encode("abcdefghij")) should be ("abcdefghij")

  test("Substitution cipher - can double shift") { 
    val cipher = Cipher(Some("iamapandabear"))
    cipher.encode("iamapandabear") should be ("qayaeaagaciai")

  test("Substitution cipher - can wrap") { 
    Cipher(Some("abcdefghij")).encode("zzzzzzzzzz") should be ("zabcdefghi")
import scala.util.Random
//  10-16-19

// 41 ms
case class Cipher(seed: Option[String]) {
  val key: String = seed match {
    case Some(txt) => txt
    case None      => genKey
  require(!key.isEmpty && key.forall(c => c.isLetter && c.isLower))

  def encode(plain: String): String =
    plain.zipWithIndex.foldLeft("") { (acc, tup) =>
      acc + ((tup._1 + key(tup._2) - 194) % 26 + 97).toChar  // -97 -97 = -194

  def decode(cipher: String): String =
    cipher.zipWithIndex.foldLeft("") { (acc, tup) =>
      acc + ((tup._1 - key(tup._2) + 26) % 26 + 97).toChar  // -97 +97 = 0

  private def genKey: String =         // 'a' = 97
    (1 to 100).map { _ => (Random.nextInt(26) + 97).toChar }.mkString

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Ric0chet's Reflection

Basically a port of my solution from another Track (with some math simplification).