@misc{cryptoeprint:2005:439,
author = {T. C. Lam},
title = {A note on the n-spendable extension of Ferguson's single-term off-line coins},
howpublished = {Cryptology ePrint Archive, Report 2005/439},
year = {2005},
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.