Hopper-Blum (HB) Theory

Theory describes two protocols that provide symmetric authentication for low cost RFID tags: the HB protocol and the HB+ protocol. It is claimed that HB is secure against passive attacks, while HB+ is secure against both passive and active attacks. Recently, Gilbert- Robshaw-Sibert have shown that HB+ is vulnerable to active attacks with linear computational and communication complexity, Juels-Weis and Katz-Shin have shown that HB+ is provably secure in a weak security model in which the adversary is not allowed to attack an ongoing authentication session in order to learn the key.

Additonal Reading

Secure Anonymous RFID Authentication Protocols 

Security Parallels Between People and Pervasive Devices