Enhancements to A Lightweight RFID Authentication Protocol
  Vajda and Buttyan (VB) proposed a set of five lightweight RFID authentication protocols. Defend, Fu, and Juels (DFJ) did cryptanalysis on two of them - XOR and SUBSET. To the XOR protocol, DFJ proposed repeated keys attack and nibble attack. In this paper, we identify the vulnerability existed in the original VB's successive session key permutation algorithm. We propose three enhancements to prevent DFJ's attacks and make XOR protocol stronger without introducing extra resource cost. 
Xiaowen Zhang, Zhanyang Zhang, Xinzhou Wei