@misc{cryptoeprint:2012:099,
author = {Craig Gentry and Shai Halevi and Nigel P. Smart},
title = {Homomorphic Evaluation of the AES Circuit},
howpublished = {Cryptology ePrint Archive, Report 2012/099},
year = {2012},
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.