@misc{cryptoeprint:2015:946,
author = {Alex Biryukov and Dmitry Khovratovich},
title = {Asymmetric proof-of-work based on the Generalized Birthday problem},
howpublished = {Cryptology ePrint Archive, Report 2015/946},
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.