@misc{cryptoeprint:2012:238,
author = {Hongbo Yu and Jiazhe Chen and XIaoyun Wang},
title = {The Boomerang Attacks on the Round-Reduced Skein-512},
howpublished = {Cryptology ePrint Archive, Report 2012/238},
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.