Vajda and Buttyan proposed several lightweight authentication protocols for authenticating RFID tags to readers, and left open the quantifiable cryptographic strength. Our cryptanalysis answers this open question by implementing and measuring attacks against their XOR and SUBSET protocols. A passive eavesdropper can impersonate a tag in the XOR protocol after observing only 70 challenge-response transactions between the tag and reader. In contrast, the theoretical maximum strength of the XOR protocol could have required 16! * 2 observed transactions to break the key. Our experiments also show that a passive eavesdropper can recover the shared secret used in the XOR protocol by observing an expected 1,092 transactions. Additionally, a nearly optimal active attack against the SUBSET protocol extracts almost one bit of information for each bit emitted by the tag