@misc{cryptoeprint:2012:250,
author = {Xu An Wang and Xiaoyuan Yang},
title = {New Identity Based Encryption And Its Proxy Re-encryption},
howpublished = {Cryptology ePrint Archive, Report 2012/250},
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.