@misc{cryptoeprint:2013:255,
author = {Rodolphe Lampe and Yannick Seurin},
title = {How to Construct an Ideal Cipher from a Small Set of Public Permutations},
howpublished = {Cryptology ePrint Archive, Report 2013/255},
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.