@misc{cryptoeprint:2010:543,
author = {Dan Boneh and Amit Sahai and Brent Waters},
title = {Functional Encryption: Definitions and Challenges},
howpublished = {Cryptology ePrint Archive, Report 2010/543},
year = {2010},
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.