@misc{cryptoeprint:2012:233,
author = {Qi Chai and Guang Gong},
title = {A Cryptanalysis of HummingBird-2: The Differential Sequence Analysis},
howpublished = {Cryptology ePrint Archive, Report 2012/233},
year = {2012},
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.