@misc{cryptoeprint:2001:001,
author = {Helger Lipmaa, Shiho Moriai},
title = {Efficient Algorithms for Computing Differential Properties of Addition},
howpublished = {Cryptology ePrint Archive, Report 2001/001},
year = {2001},
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.