@misc{cryptoeprint:2015:687,
author = {Sean Hallgren and Adam Smith and Fang Song},
title = {Classical Cryptographic Protocols in a Quantum World},
howpublished = {Cryptology ePrint Archive, Report 2015/687},
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.