@misc{cryptoeprint:2009:489,
author = {Jiali Choy and Huihui Yap},
title = {Impossible Boomerang Attack for Block Cipher Structures},
howpublished = {Cryptology ePrint Archive, Report 2009/489},
year = {2009},
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.