@misc{cryptoeprint:2011:093,
author = {Miia Hermelin and Kaisa Nyberg},
title = {Linear Cryptanalysis Using Multiple Linear Approximations},
howpublished = {Cryptology ePrint Archive, Report 2011/093},
year = {2011},
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.