@misc{cryptoeprint:2013:835,
author = {Thomas Shrimpton and R. Seth Terashima},
title = {A Modular Framework for Building Variable-Input Length Tweakable Ciphers},
howpublished = {Cryptology ePrint Archive, Report 2013/835},
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.