@misc{cryptoeprint:2009:463,
author = {Benoit Gérard and Jean-Pierre Tillich},
title = {On Linear Cryptanalysis with Many Linear Approximations},
howpublished = {Cryptology ePrint Archive, Report 2009/463},
year = {2009},
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.