@misc{cryptoeprint:2014:613,
author = {Gordon Procter},
title = {A Security Analysis of the Composition of ChaCha20 and Poly1305},
howpublished = {Cryptology ePrint Archive, Report 2014/613},
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.