Cryptology ePrint Archive: BibTeX Citation of Report 2015/261

@misc{cryptoeprint:2015:261,
    author = {Joseph Bonneau, Andrew Miler, Jeremy Clark, Arvind Narayanan, Joshua A. Kroll, Edward W. Felten},
    title = {Research Perspectives and Challenges for Bitcoin and Cryptocurrencies},
    howpublished = {Cryptology ePrint Archive, Report 2015/261},
    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.