"Radio frequency identification" (RFID) systems are ubiquitous in today's world. In an RFID system, it is a desirable to attain mutual authentication between a reader and a tag before commencing application-level communications. This is because tags should not share secret information with unknown parties and readers need to defend against tag impersonation. Authentication protocols designed for communication between computers, however, are not appropriate for RFID systems because tags are extremely resource constrained (low energy, small memory, etc.). Thus, there have been many attempts to design secure and practical authentication protocols for RFID systems over the years since RFID systems became prevalent. This survey summarizes and compares these protocols.