@misc{cryptoeprint:2006:251,
author = {Gregory V. Bard},
title = {Accelerating Cryptanalysis with the Method of Four Russians},
howpublished = {Cryptology ePrint Archive, Report 2006/251},
year = {2006},
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.