@misc{cryptoeprint:2013:188,
author = {Daniel Loebenberger and Michael Nüsken},
title = {A family of 6-to-4-bit S-boxes with large linear branch number},
howpublished = {Cryptology ePrint Archive, Report 2013/188},
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.