Module: Crypt::Noise

Included in:
String
Defined in:
lib/crypt/noise.rb

Instance Method Summary collapse

Instance Method Details

#add_noise(new_length) ⇒ Object 



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# File 'lib/crypt/noise.rb', line 7

def add_noise(new_length)
  message = self
  usable_noisy_message_length = new_length / 9 * 8
  bitmap_size = new_length / 9
  remaining_bytes = new_length - usable_noisy_message_length - bitmap_size
  if (message.length > usable_noisy_message_length)
    minimumnew_length = (message.length / 8.0).ceil * 9
    puts "For a clear text of #{message.length} bytes, the minimum obscured length"
    puts "is #{minimumnew_length} bytes which allows for no noise in the message."
    puts "You should choose an obscured length of at least double the clear text"
    puts "length, such as #{message.length / 8 * 32} bytes"
    raise "Insufficient length for noisy message" 
  end
  bitmap = []
  usable_noisy_message_length.times { bitmap << false }
  srand(Time.now.to_i)
  positions_selected = 0
  while (positions_selected < message.length)
    position_taken = rand(usable_noisy_message_length)
    if bitmap[position_taken]
      next
    else
      bitmap[position_taken] = true
      positions_selected = positions_selected.next
    end
  end

  noisy_message = ""
  0.upto(bitmap_size-1) { |byte|
    c = 0
    0.upto(7) { |bit|
      c = c + (1<<bit) if bitmap[byte * 8 + bit]
    }
    noisy_message << c.chr
  }
  pos_in_message = 0
  0.upto(usable_noisy_message_length-1) { |pos|
    if bitmap[pos]
      meaningful_byte = message[pos_in_message]
      noisy_message << meaningful_byte
      pos_in_message = pos_in_message.next
    else
      noise_byte = rand(256).chr
      noisy_message << noise_byte
    end
  }
  remaining_bytes.times {
      noise_byte = rand(256).chr
      noisy_message << noise_byte
  }
  return(noisy_message)
end

#remove_noise ⇒ Object 



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# File 'lib/crypt/noise.rb', line 61

def remove_noise
  noisy_message = self
  bitmap_size = noisy_message.length / 9
  actual_message_length =  bitmap_size * 8

  actual_message_start = bitmap_size
  actual_message_finish = bitmap_size + actual_message_length - 1
  actual_message = noisy_message[actual_message_start..actual_message_finish]

  bitmap = []
  0.upto(bitmap_size - 1) { |byte|
    c = noisy_message[byte]
    0.upto(7) { |bit|
      bitmap[byte * 8 + bit] = (c[bit] == 1)
    }
  }
  clear_message = ""
  0.upto(actual_message_length) { |pos|
    meaningful = bitmap[pos]
    if meaningful
      clear_message << actual_message[pos]
    end
  }
  return(clear_message)
end