@misc{cryptoeprint:2004:281,
author = {Patrick P. Tsang and Victor K. Wei},
title = {Short Linkable Ring Signatures for E-Voting, E-Cash and Attestation},
howpublished = {Cryptology ePrint Archive, Report 2004/281},
year = {2004},
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.