@misc{cryptoeprint:2013:136,
author = {Dieter Schmidt},
title = {2048XKS-F & 4096XKS-F - Two Software Oriented High Security Block Ciphers},
howpublished = {Cryptology ePrint Archive, Report 2013/136},
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.