@misc{cryptoeprint:2014:860,
author = {Cihangir Tezcan and Ferruh Özbudak},
title = {Differential Factors: Improved Attacks on SERPENT},
howpublished = {Cryptology ePrint Archive, Report 2014/860},
year = {2014},
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.