@misc{cryptoeprint:2011:645,
author = {Gu Chunsheng},
title = {Fully Homomorphic Encryption Based on Approximate Matrix GCD},
howpublished = {Cryptology ePrint Archive, Report 2011/645},
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.