@misc{cryptoeprint:2009:038,
author = {Bhupendra Singh and Lexy Alexander and Sanjay Burman},
title = {On Algebraic Relations of Serpent S-Boxes},
howpublished = {Cryptology ePrint Archive, Report 2009/038},
year = {2009},
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.