@misc{cryptoeprint:2010:521,
author = {Xiutao Feng and Chunfang Zhou and Chuankun Wu},
title = {Linear Approximations of Addition Modulo $2^n$-1},
howpublished = {Cryptology ePrint Archive, Report 2010/521},
year = {2010},
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.