@misc{cryptoeprint:2015:053,
author = {Peter Gaži and Krzysztof Pietrzak and Stefano Tessaro},
title = {Tight Bounds for Keyed Sponges and Truncated CBC},
howpublished = {Cryptology ePrint Archive, Report 2015/053},
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.