@misc{cryptoeprint:2002:173,
author = {Giuseppe Ateniese and Breno de Medeiros},
title = {Efficient Group Signatures without Trapdoors},
howpublished = {Cryptology ePrint Archive, Report 2002/173},
year = {2002},
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.