@misc{cryptoeprint:2012:031,
author = {Da-Zhi Sun},
title = {An error in "On a new formal proof model for RFID location privacy"},
howpublished = {Cryptology ePrint Archive, Report 2012/031},
year = {2012},
note = {\url{http://eprint.iacr.org/}},
}
You will need the url.sty package (comes with most LaTeX installations) for the last line. Otherwise, remove the \url command.