@misc{cryptoeprint:2013:782,
author = {Dorit Ron and Adi Shamir},
title = {How Did Dread Pirate Roberts Acquire and Protect His Bitcoin Wealth?},
howpublished = {Cryptology ePrint Archive, Report 2013/782},
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.