@misc{cryptoeprint:2006:168,
author = {Nicolas T. Courtois},
title = {How Fast can be Algebraic Attacks on Block Ciphers ?},
howpublished = {Cryptology ePrint Archive, Report 2006/168},
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.