@misc{cryptoeprint:2009:248,
author = {Simon R. Blackburn, Carlos Cid and Ciaran Mullan},
title = {Cryptanalysis of the MST_3 Public Key Cryptosystem},
howpublished = {Cryptology ePrint Archive, Report 2009/248},
year = {2009},
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.