The proliferation of RFID tags means that an RFID reader will often interact with large groups of RFID tags. However, the majority of RFID security research is still focused on securing single reader-single tag interaction, which is unlikely to be scalable to larger groups of tags. In this paper, we propose a protocol that allows a large group of tags to authenticate a reader. Our technique relies on using a more powerful RFID tag, known as a computational RFID tag (CRFID), which is included into the large group of tags. Theoretical analysis and simulation experiments show that our new protocol is secure and efficient.