@misc{cryptoeprint:2010:358,
author = {M.-F. Lee, N.P. Smart and B. Warinschi},
title = {The Fiat--Shamir Transform for Group and Ring Signature Schemes},
howpublished = {Cryptology ePrint Archive, Report 2010/358},
year = {2010},
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.