@misc{cryptoeprint:2008:463,
author = {Giuseppe Ateniese and Karyn Benson and Susan Hohenberger},
title = {Key-Private Proxy Re-Encryption},
howpublished = {Cryptology ePrint Archive, Report 2008/463},
year = {2008},
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.