@misc{cryptoeprint:2013:500,
author = {Ran Canetti and Vinod Vaikuntanathan},
title = {Obfuscating Branching Programs Using Black-Box Pseudo-Free Groups},
howpublished = {Cryptology ePrint Archive, Report 2013/500},
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.