@misc{cryptoeprint:2011:151,
author = {Murtaza, G. and Ikram, N.},
title = {Direct Exponent and Scalar Multiplication Classes of an MDS Matrix},
howpublished = {Cryptology ePrint Archive, Report 2011/151},
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.