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Challenge: Secure Radio 📻🔒

This challenge is to create a secure radio, which returns an encrypted audio stream.

Part 1

To achieve this, make use of the Radio class from the previous challenge and add a Radio#play to it which returns a fake audio stream as a string value.

Add an @@audio_samples class attribute array with examples of streams to Radio, here's an example:

  @@audio_samples = [
    "Here Comes the Sun",
    "Like a Rolling Stone",
    "Cantaloupe Island",
    "a traffic report",
    "a news report"
  ]

Create a private Radio#audio_stream method where a random element of @@audio_samples is returned. The Array#sample method could be used here.

Edit or create Radio#play to return the Radio#audio_stream. For, example: puts "The radio plays: " + audio_stream.

Part 2

Use a shift cipher to encrypt the stream string. A shift cipher replaces each characters with a letter, a fixed number of positions down the alphabet.

In the example below a shift of 3 is applied so A will become D.

With a shift of 3: A => D, B => E, C => F, D => G

Create a ShiftCipher class to be used when encrypting the stream.

Add a ShiftCipher.encode(plain_string, 3) class method that encrypts the strings. It takes a plain string, in this case, it would be the SecureRadio#audio_stream, and an integer of how much a character should be shifted by.

Create a ShiftCipher.decode(cipher_string, 3) method that will revert an encrypted string to its original state. It should take the same number of arguments as ShiftCipher.encode.

Technically, the secure radio would only broadcast encrypted streams and would be up to the listener to decode it, however it is a good exercise to write both of them.

Hints

Click to expand contents
  • When an array of letters is needed, [*a..z] can be used to spread letters from 'a' to 'z' into an array.
  • There are a variety of ways to write the shift cipher code, but the following methods can be useful when doing it:
    • Array#map
    • Array#include?
    • Array#find_index
    • Array#join
  • A good start might be starting by shifting lowercase strings with String#downcase before trying to work out how to encrypt mixed cased strings.
  • The modulo operator might be useful to keep the shift from happening with a letter at the end of the alphabet. 28 % 26 returns 2

Part 3

Create a SecureRadio subclass which inherits from Radio and makes use of ShiftCipher to encrypt the audio stream.

Click to expand contents
  • Use inheritance.
  • Remember method access control: protected, private. Protected makes methods available for its class and subclass but private does not.
  • Try to use super after you override a method.