@misc{cryptoeprint:2009:623,
author = {Masaaki Shirase},
title = {Universally Constructing 12-th Degree Extension Field for Ate Pairing},
howpublished = {Cryptology ePrint Archive, Report 2009/623},
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.