@misc{cryptoeprint:2013:456,
author = {Purushothama B R and B B Amberker},
title = {On the Security of Group-based Proxy Re-encryption Scheme},
howpublished = {Cryptology ePrint Archive, Report 2013/456},
year = {2013},
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.