@misc{cryptoeprint:2013:868,
author = {Lear Bahack},
title = {Theoretical Bitcoin Attacks with less than Half of the Computational Power (draft)},
howpublished = {Cryptology ePrint Archive, Report 2013/868},
year = {2013},
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.