@misc{cryptoeprint:2010:227,
author = {Neal Koblitz},
title = {A Security Weakness in Composite-Order Pairing-Based Protocols with Imbedding Degree $k>2$},
howpublished = {Cryptology ePrint Archive, Report 2010/227},
year = {2010},
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.