@misc{cryptoeprint:2015:477,
author = {John Mattsson},
title = {Authentication Key Recovery in Galois/Counter Mode (GCM)},
howpublished = {Cryptology ePrint Archive, Report 2015/477},
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.