@misc{cryptoeprint:2009:390,
author = {Jens Groth},
title = {Sub-linear Size Pairing-based Non-interactive Zero-Knowledge Arguments},
howpublished = {Cryptology ePrint Archive, Report 2009/390},
year = {2009},
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.