@misc{cryptoeprint:2004:347,
author = {Marco Macchetti},
title = {Addendum to ``On the Generalized Linear Equivalence of Functions over Finite Fields''},
howpublished = {Cryptology ePrint Archive, Report 2004/347},
year = {2004},
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.