@misc{cryptoeprint:2003:206,
author = {Benoît Libert and Jean-Jacques Quisquater},
title = {Identity Based Undeniable Signatures},
howpublished = {Cryptology ePrint Archive, Report 2003/206},
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.