@misc{cryptoeprint:2012:138,
author = {Nicolas T. Courtois},
title = {An Improved Differential Attack on Full GOST},
howpublished = {Cryptology ePrint Archive, Report 2012/138},
year = {2012},
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.