@misc{cryptoeprint:2013:258,
author = {Sanjam Garg and Craig Gentry and Amit Sahai and Brent Waters},
title = {Witness Encryption and its Applications},
howpublished = {Cryptology ePrint Archive, Report 2013/258},
year = {2013},
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.