  Almost all existing RFID authentication schemes (tag/reader) are vulnerable
to relay attacks, because of their inability to estimate the distance to the
tag. These attacks are very serious since it can be mounted without the notice
of neither the reader nor the tag and cannot be prevented by cryptographic
protocols that operate at the application layer. Distance bounding protocols
represent a promising way to thwart relay attacks, by measuring the round trip
time of short authenticated messages. All the existing distance bounding
protocols use random number generator and hash functions at the tag side which
make them inapplicable at low cost RFID tags. This paper proposes a lightweight
distance bound protocol for low cost RFID tags. The proposed protocol based on
modified version of Gossamer mutual authentication protocol. The implementation
of the proposed protocol meets the limited abilities of low-cost RFID tags.
