@misc{cryptoeprint:2010:282,
author = {Kerry A. McKay and Poorvi L. Vora},
title = {Pseudo-Linear Approximations for ARX Ciphers: With Application to Threefish},
howpublished = {Cryptology ePrint Archive, Report 2010/282},
year = {2010},
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.