@misc{cryptoeprint:2014:273,
author = {Craig Gentry and Allison Bishop Lewko and Brent Waters},
title = {Witness Encryption from Instance Independent Assumptions},
howpublished = {Cryptology ePrint Archive, Report 2014/273},
year = {2014},
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.