@misc{cryptoeprint:2013:467,
author = {Jian Guo and Pierre Karpman and Ivica Nikolic and Lei Wang and Shuang Wu},
title = {Analysis of BLAKE2},
howpublished = {Cryptology ePrint Archive, Report 2013/467},
year = {2013},
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.