@misc{cryptoeprint:2012:653,
author = {Periklis A. Papakonstantinou and Charles W. Rackoff and Yevgeniy Vahlis},
title = {How powerful are the DDH hard groups?},
howpublished = {Cryptology ePrint Archive, Report 2012/653},
year = {2012},
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.