@misc{cryptoeprint:2011:405,
author = {Kristin Lauter and Michael Naehrig and Vinod Vaikuntanathan},
title = {Can Homomorphic Encryption be Practical?},
howpublished = {Cryptology ePrint Archive, Report 2011/405},
year = {2011},
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.