@misc{cryptoeprint:2013:829,
author = {Arthur Gervais and Ghassan Karame and Srdjan Capkun and Vedran Capkun},
title = {Is Bitcoin a Decentralized Currency?},
howpublished = {Cryptology ePrint Archive, Report 2013/829},
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.