@misc{cryptoeprint:2010:257,
author = {Alex Biryukov and Dmitry Khovratovich},
title = {Feasible Attack on the 13-round AES-256},
howpublished = {Cryptology ePrint Archive, Report 2010/257},
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.