Abstract: Re-encryption is an approach for RFID security，which periodically renames the tags in order to avoid tag tracking.Re-encryption demands that the name of tag is rewriteable，so the adversary can swap two valid names of tags，forming swapping attack.Defending swapping attack is still an un-solved problem in re-encryption with the difficulty of keeping anonymity and anti-swapping simultaneously.The paper promotes a swapping defending algorithm in re-encryption under the “attack-invalid” model，and realizes the ID anti-swapping and public-key anti-swapping，and gives proof.The tag requires a hardware multiplier，and it’s applicable with the current RFID chip technology.