@misc{cryptoeprint:2009:313,
author = {Nigel P. Smart and Bogdan Warinschi},
title = {Identity Based Group Signatures from Hierarchical Identity-Based Encryption},
howpublished = {Cryptology ePrint Archive, Report 2009/313},
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.