@misc{cryptoeprint:2011:417,
author = {Eli Biham and Orr Dunkelman and Nathan Keller and Adi Shamir},
title = {New Data-Efficient Attacks on Reduced-Round IDEA},
howpublished = {Cryptology ePrint Archive, Report 2011/417},
year = {2011},
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.