@misc{cryptoeprint:2012:481,
author = {John Steinberger},
title = {Improved Security Bounds for Key-Alternating Ciphers via Hellinger Distance},
howpublished = {Cryptology ePrint Archive, Report 2012/481},
year = {2012},
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.