@misc{cryptoeprint:2013:630,
author = {Joop van de Pol and Nigel P. Smart},
title = {Estimating Key Sizes For High Dimensional Lattice-Based Systems},
howpublished = {Cryptology ePrint Archive, Report 2013/630},
year = {2013},
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.