@misc{cryptoeprint:2011:226,
author = {Eric Miles and Emanuele Viola},
title = {Substitution-permutation networks, pseudorandom functions, and Natural Proofs},
howpublished = {Cryptology ePrint Archive, Report 2011/226},
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.