@misc{cryptoeprint:2013:222,
author = {Shan Chen, John Steinberger},
title = {Tight security bounds for key-alternating ciphers},
howpublished = {Cryptology ePrint Archive, Report 2013/222},
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.