@misc{cryptoeprint:2015:614,
author = {Fangguo Zhang},
title = {Bit Security of the Hyperelliptic Curves Diffie-Hellman Problem},
howpublished = {Cryptology ePrint Archive, Report 2015/614},
year = {2015},
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.