@misc{cryptoeprint:2010:307,
author = {Ruilin Li and Bing Sun and Chao Li},
title = {Impossible Differential Cryptanalysis of SPN Ciphers},
howpublished = {Cryptology ePrint Archive, Report 2010/307},
year = {2010},
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.