@misc{cryptoeprint:2009:526,
author = {Jiazhe Chen and Keting Jia},
title = {Improved Related-Key Boomerang Attacks on Round-Reduced Threefish-512},
howpublished = {Cryptology ePrint Archive, Report 2009/526},
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.