rot13.rb

class Rot13
  attr_accessor :text

  def initialize(text)
    @text = text
  end

  def encrypt
    @text = @text.tr('a-zA-Z', 'n-za-mN-ZA-M')
  end

  def decrypt
    @text = @text.tr('n-za-mN-ZA-M', 'a-zA-Z')
  end
end

Example usage:

Encrypt: Rot13.new("This is my string").encrypt #returns 'Guvf vf zl fgevat'

Decrypt: Rot13.new("Guvf vf zl fgevat").decrypt #returns 'This is my string'