@misc{cryptoeprint:2011:023,
author = {Ming Duan and Xuajia Lai},
title = {Improved zero-sum distinguisher for full round Keccak-f permutation},
howpublished = {Cryptology ePrint Archive, Report 2011/023},
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.