@misc{cryptoeprint:2007:274,
author = {Chunbo Ma, Jun Ao, and Jianhua Li},
title = {Group-based Proxy Re-encryption scheme},
howpublished = {Cryptology ePrint Archive, Report 2007/274},
year = {2007},
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.