@misc{cryptoeprint:2012:052,
author = {Alexandra Boldyreva and Vipul Goyal and Virendra Kumar},
title = {Identity-based Encryption with Efficient Revocation},
howpublished = {Cryptology ePrint Archive, Report 2012/052},
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.