@misc{cryptoeprint:2008:107,
author = {Helger Lipmaa},
title = {Private Branching Programs: On Communication-Efficient Cryptocomputing},
howpublished = {Cryptology ePrint Archive, Report 2008/107},
year = {2008},
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.