@misc{cryptoeprint:2011:237,
author = {Alice Nguyenova-Stepanikova and Tran Ngoc Duong},
title = {The block cipher NSABC (public domain)},
howpublished = {Cryptology ePrint Archive, Report 2011/237},
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.