@misc{cryptoeprint:2003:117,
author = {Chih-Yin Lin and Tzong-Chen Wu},
title = {An identity-based ring signature scheme from bilinear pairings},
howpublished = {Cryptology ePrint Archive, Report 2003/117},
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.