@misc{cryptoeprint:2005:202,
author = {Nicolas Courtois},
title = {The Best Differential Characteristics and Subtleties of the Biham-Shamir Attacks on DES},
howpublished = {Cryptology ePrint Archive, Report 2005/202},
year = {2005},
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.