@misc{cryptoeprint:2009:269,
author = {Thomas Roche and Cédric Tavernier},
title = {Side-channel attacks based on linear approximations},
howpublished = {Cryptology ePrint Archive, Report 2009/269},
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.