@misc{cryptoeprint:2015:649,
author = {David Bernhard and Marc Fischlin and Bogdan Warinschi},
title = {On the Hardness of Proving CCA-security of Signed ElGamal},
howpublished = {Cryptology ePrint Archive, Report 2015/649},
year = {2015},
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.