@misc{cryptoeprint:2014:922,
author = {Alex Biryukov and Arnab Roy and Vesselin Velichkov},
title = {Differential Analysis of Block Ciphers SIMON and SPECK},
howpublished = {Cryptology ePrint Archive, Report 2014/922},
year = {2014},
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.