@misc{cryptoeprint:2006:420,
author = {Shenghui Su and Shuwang Lv},
title = {The REESSE1+ Public Key Cryptosystem v 2.21},
howpublished = {Cryptology ePrint Archive, Report 2006/420},
year = {2006},
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.