@misc{cryptoeprint:2008:226,
author = {Chunbo Ma and Jun Ao},
title = {Proxy Key Re-encapsulation Mechanism for Group Communications},
howpublished = {Cryptology ePrint Archive, Report 2008/226},
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.