@misc{cryptoeprint:2011:165,
author = {XinJie Zhao and Tao Wang and ShiZe Guo},
title = {Improved Side Channel Cube Attacks on PRESENT},
howpublished = {Cryptology ePrint Archive, Report 2011/165},
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.