@misc{cryptoeprint:2011:308,
author = {Michael Backes, Ian Goldberg, Aniket Kate, Esfandiar Mohammadi},
title = {Provably Secure and Practical Onion Routing},
howpublished = {Cryptology ePrint Archive, Report 2011/308},
year = {2011},
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.