# File lib/em-websocket/masking04.rb, line 5
      def read_mask
        if respond_to?(:encoding) && encoding.name != "ASCII-8BIT"
          raise "MaskedString only operates on BINARY strings"
        end
        raise "Too short" if bytesize < 4 # TODO - change
        @masking_key = String.new(self[0..3])
      end