@misc{cryptoeprint:2010:041,
author = {Orr Dunkelman and Nathan Keller},
title = {The Effects of the Omission of Last Round's MixColumns on AES},
howpublished = {Cryptology ePrint Archive, Report 2010/041},
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.