@misc{cryptoeprint:2004:057,
author = {Alex Biryukov and Christophe De Canni\`ere and Michael Quisquater},
title = {On Multiple Linear Approximations},
howpublished = {Cryptology ePrint Archive, Report 2004/057},
year = {2004},
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.