@misc{cryptoeprint:2012:641,
author = {Bin Wang},
title = {A unidirectional conditional proxy re-encryption scheme based on non-monotonic access structure},
howpublished = {Cryptology ePrint Archive, Report 2012/641},
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.