# File lib/gdata/auth/authsub.rb, line 62
      def private_key=(key)
        begin
          if key.nil? or key.class == OpenSSL::PKey::RSA
            @private_key = key
          elsif File.exists?(key)
            key_from_file = File.read(key)
            @private_key = OpenSSL::PKey::RSA.new(key_from_file)
          else
            @private_key = OpenSSL::PKey::RSA.new(key)
          end
        rescue
          raise ArgumentError, "Not a valid private key."
        end
      end