@misc{cryptoeprint:2014:578,
author = {Peter Gaži and Krzysztof Pietrzak and Michal Rybár},
title = {The Exact PRF-Security of NMAC and HMAC},
howpublished = {Cryptology ePrint Archive, Report 2014/578},
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.