@misc{cryptoeprint:2008:028,
author = {Henri Gilbert and Matthew J.B. Robshaw and Yannick Seurin},
title = {HB#: Increasing the Security and Efficiency of HB+},
howpublished = {Cryptology ePrint Archive, Report 2008/028},
year = {2008},
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.