Class: RotorMachine::Machine

Inherits:
Object
  • Object
show all
Defined in:
lib/rotor_machine/machine.rb

Overview

The Machine class serves as the entrypoint and orchestrator for an Enigma machine.

Components of an Enigma machine

The Enigma machine, as represented by the RotorMachine module, consists of the following components:

  • One or more rotors, which perform the transposition ciphering and also rotate to produce a polyalphabetic (rather than simple substitution) cipher.

  • A reflector, which performs a simple symmetric substitution of letters

  • A plugboard, which allows pairs of letters to be transposed on a per-message basis.

On an actual Enigma machine, these components are all electromechanical, and the Enigma also included a keyboard, a grid of lights to show the results, and in some cases a printer. Since this is a simulated Enigma, obviously, no keyboard/printer are supplied here.

The polyalphabetic encryption of the Enigma comes from the fact that the rotors are linked (mechanically in a real Enigma) so that they rotate one or more “steps” after each character, changing the signal paths and transpositions. This means that a sequence of the same plaintext character will encipher to different ciphertext characters.

The rotors are designed to advance such that each time a rotor completes a full revolution, it will advance the rotor to its left once. The rotors allow you to configure how many positions they advance when they do. So, assuming all rotors are advancing one position at a time, if the rotors have position “AAZ”, their state after the next character is typed will be “ABA”.

To learn much more about the inner workings of actual Enigma machines, visit https://en.wikipedia.org/wiki/Enigma_machine.

The Signal Path of Letters

On a physical Enigma machine, the electrical signal from a keypress is routed through the plugboard, then through each of the rotors in sequence from left to right. The signal then passes through the reflector (where it is transposed again), then back through the rotors in reverse order, and finally back through the plugboard a second time before being displayed on the light grid and/or printer.

One important consequence of this signal path is that encryption and decryption are the same operation. That is to say, if you set the rotors and plugboard, and then type your plaintext into the machine, you’ll get a string of ciphertext. If you then reset the machine to its initial state and type the ciphertext characters into the machine, you’ll produce your original plaintext.

One consequence of the Enigma’s design is that a plaintext letter will never encipher to itself. The Allies were able to exploit this property to help break the Enigma’s encryption in World War II.

Usage

To use the RotorMachine Enigma machine, you need to perform the following steps:

  1. Create a new Machine object.

  2. Add one or more Rotors to the ‘rotors` array.

  3. Set the ‘reflector` to an instance of the Reflector class.

  4. Make any desired connections in the Plugboard.

  5. Optionally, set the rotor positions with #set_rotors.

You’re now ready to encipher and decipher your text using the #encipher method to encode/decode, and #set_rotors to reset the machine state.

The #default_machine and #empty_machine class methods are shortcut factory methods whcih set up, respectively, a fully configured machine with a default set of rotors and reflector, and an empty machine with no rotors or reflector.

Instance Attribute Summary collapse

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize ⇒ Machine

Initialize a RotorMachine object.

This object won’t be usable until you add rotors, a reflector and a plugboard. Using the #default_machine and #empty_machine helper class methods is the preferred way to initialize functioning machines.



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# File 'lib/rotor_machine/machine.rb', line 123

def initialize()
  @rotors = []
  @reflector = nil
  @plugboard = nil
end

Instance Attribute Details

#plugboard ⇒ Object

Returns the value of attribute plugboard.



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# File 'lib/rotor_machine/machine.rb', line 80

def plugboard
  @plugboard
end

#reflector ⇒ Object

Returns the value of attribute reflector.



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# File 'lib/rotor_machine/machine.rb', line 80

def reflector
  @reflector
end

#rotors ⇒ Object

Returns the value of attribute rotors.



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# File 'lib/rotor_machine/machine.rb', line 80

def rotors
  @rotors
end

Class Method Details

.default_machine ⇒ Object

Generates a default-configuration RotorMachine, with the following state:

  • Rotors I, II, III, each set to A and configured to advance a single step at a time

  • Reflector A

  • An empty plugboard with no connections



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# File 'lib/rotor_machine/machine.rb', line 90

def self.default_machine
   machine = self.empty_machine
   machine.rotors << RotorMachine::Rotor.new(RotorMachine::Rotor::ROTOR_I, "A", 1)
   machine.rotors << RotorMachine::Rotor.new(RotorMachine::Rotor::ROTOR_II, "A", 1)
   machine.rotors << RotorMachine::Rotor.new(RotorMachine::Rotor::ROTOR_III, "A", 1)
   machine.reflector = RotorMachine::Reflector.new(RotorMachine::Reflector::REFLECTOR_A)
   machine
end

.empty_machine ⇒ Object

Generates an empty-configuration RotorMachine, with the following state:

  • No rotors

  • No reflector

  • An empty plugboard with no connections

A RotorMachine in this state will raise an ArgumentError until you outfit it with at least one rotor and a reflector.



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# File 'lib/rotor_machine/machine.rb', line 109

def self.empty_machine
   machine = RotorMachine::Machine.new()
   machine.rotors = []
   machine.reflector = nil
   machine.plugboard = RotorMachine::Plugboard.new()
   machine
end

Instance Method Details

#encipher(text) ⇒ String

Encipher (or decipher) a string.

Each character of the string is, in turn, passed through the machine. This process is documented in the class comment for the RotorMachine::Machine class.

Because the Enigma machine did not differentiate uppercase and lowercase letters, the source string is upcase’d before processing.

Parameters:

  • text (String) —

    the text to encipher or decipher

Returns:

  • (String) —

    the enciphered or deciphered text

Raises:

  • (ArgumentError) 


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# File 'lib/rotor_machine/machine.rb', line 140

def encipher(text)
  raise ArgumentError, "Cannot encipher; no rotors loaded" if (@rotors.count == 0)
  raise ArgumentError, "Cannot encipher; no reflector loaded" if (@reflector.nil?)
  text.upcase.chars.collect { |c| self.encipher_char(c) }.join("")
end

#encipher_char(c) ⇒ String

Encipher a single character.

Used by #encipher to walk a single character of text through the signal path of all components of the machine.

Parameters:

  • c (String) —

    a single-character string containing the next character to encipher/decipher

Returns:

  • (String) —

    the enciphered/deciphered character. After the character passes through the machine, a call is made to #step_rotors to advance the rotors.



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# File 'lib/rotor_machine/machine.rb', line 202

def encipher_char(c)
  ec = c

  unless @plugboard.nil?
    ec = @plugboard.transpose(ec)
  end

  @rotors.each { |rotor| ec = rotor.forward(ec) }
  ec = @reflector.reflect(ec)
  @rotors.reverse.each { |rotor| ec = rotor.reverse(ec) }

  unless @plugboard.nil?
    ec = @plugboard.transpose(ec)
  end

  self.step_rotors
  ec
end

#set_rotors(init_val) ⇒ Object

Set the initial positions of the set of rotors before begining an enciphering or deciphering operation.

This is a helper method to avoid having to manipulate the rotor positions individually. Starting with the leftmost rotor, each character from this string is used to set the position of one rotor.

If the string is longer than the number of rotors, the extra values (to the right) are ignored. If it’s shorter, the values of the “extra” rotors will be unchanged.

for the rotors.

Parameters:

  • init_val (String) —

    A string containing the initial values



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# File 'lib/rotor_machine/machine.rb', line 171

def set_rotors(init_val)
  init_val.chars.each_with_index do |c, i|
    @rotors[i].position = c if (i < @rotors.length)
  end
end

#step_rotors ⇒ Object

Coordinate the stepping of the set of rotors after a character is enciphered.



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# File 'lib/rotor_machine/machine.rb', line 149

def step_rotors
  @rotors.reverse.each do |rotor|
    rotor.step
    break unless rotor.wrapped?
  end
end

#to_s ⇒ String

Describe the current state of the machine in human-readable form.

state.

Returns:

  • (String) —

    A description of the Rotor Machine’s current internal



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# File 'lib/rotor_machine/machine.rb', line 182

def to_s
  buf = "a RotorMachine::Machine with the following configuration:\n"
  buf += "  Rotors: #{@rotors.count}\n"
  @rotors.each { |r| buf += "    - #{r.to_s}\n" }
  buf += "  Reflector: #{@reflector.nil? ? "none" : @reflector.to_s}\n"
  buf += "  Plugboard: #{@plugboard.nil? ? "none" : @plugboard.to_s}"
  return buf
end