@misc{cryptoeprint:2015:366,
author = {Arjen K. Lenstra and Benjamin Wesolowski},
title = {A random zoo: sloth, unicorn, and trx},
howpublished = {Cryptology ePrint Archive, Report 2015/366},
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.