@misc{cryptoeprint:2012:155,
author = {Laila El Aimani and Marc Joye
},
title = {Toward Practical Group Encryption
},
howpublished = {Cryptology ePrint Archive, Report 2012/155},
year = {2012},
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.