@misc{cryptoeprint:2005:220,
author = {Michael Backes and Birgit Pfitzmann},
title = {Limits of the Cryptographic Realization of Dolev-Yao-style XOR},
howpublished = {Cryptology ePrint Archive, Report 2005/220},
year = {2005},
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.