@misc{cryptoeprint:2009:571,
author = {N.P. Smart and F. Vercauteren},
title = {Fully Homomorphic Encryption with Relatively Small Key and Ciphertext Sizes},
howpublished = {Cryptology ePrint Archive, Report 2009/571},
year = {2009},
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.