@misc{cryptoeprint:2013:268,
author = {Majid Rahimi, Mostafa Barmshory, Mohammad Hadi Mansouri, Mohammad Reza Aref},
title = {Dynamic Cube Attack on Grain-v1},
howpublished = {Cryptology ePrint Archive, Report 2013/268},
year = {2013},
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.