@misc{cryptoeprint:2008:076,
author = {Kashif Ali and Howard M. Heys},
title = {Results from a Search for the Best Linear Approximation of a Block Cipher},
howpublished = {Cryptology ePrint Archive, Report 2008/076},
year = {2008},
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.