@misc{cryptoeprint:2003:261,
author = {Javier Herranz and Germ\'an S\'aez},
title = {A provably secure ID-based ring signature scheme},
howpublished = {Cryptology ePrint Archive, Report 2003/261},
year = {2003},
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.