gistfile1.rb

class Foo
  class << self
    protected
    def foo
      :foo
    end
  end

  class Bar
    class << self
      public
      def bar
        Foo.foo
      end
    end
  end
end

Foo::Bar.bar # => :foo
Foo.foo # => 
# ~> -:20: protected method `foo' called for Foo:Class (NoMethodError)