@misc{cryptoeprint:2006:193,
author = {Victor K. Wei and Tsz Hon Yuen},
title = {(Hierarchical Identity-Based) Threshold Ring Signatures},
howpublished = {Cryptology ePrint Archive, Report 2006/193},
year = {2006},
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.