@misc{cryptoeprint:2015:235,
author = {Mashael AlSabah and Ian Goldberg},
title = {Performance and Security Improvements for Tor: A Survey},
howpublished = {Cryptology ePrint Archive, Report 2015/235},
year = {2015},
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.