Teaching my 10yr old daughter about ROT-13

rot13.rb

require 'test/unit'

$alphabet = 'abcdefghijklmnopqrstuvwxyz'
$rot13 =    'nopqrstuvwxyzabcdefghijklm'

class Rot13 < Test::Unit::TestCase

  @@message = "the quick brown fox jumps over the lazy dog"
  @@encoded = "gur dhvpx oebja sbk whzcf bire gur ynml qbt"

  def test_encode
    assert_equal @@encoded, @@message.tr($alphabet, $rot13)
  end

  def test_decode
    assert_equal @@message, @@encoded.tr($rot13, $alphabet)
  end

end