@misc{cryptoeprint:2013:853,
author = {Alex Biryukov and Vesselin Velichkov},
title = {Automatic Search for Differential Trails in ARX Ciphers (Extended Version)},
howpublished = {Cryptology ePrint Archive, Report 2013/853},
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.