@misc{cryptoeprint:2014:059,
author = {John Tromp},
title = {Cuckoo Cycle: a memory bound graph-theoretic proof-of-work},
howpublished = {Cryptology ePrint Archive, Report 2014/059},
year = {2014},
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.