@misc{cryptoeprint:2007:208,
author = {Goutam Paul and Subhamoy Maitra},
title = {RC4 State Information at Any Stage Reveals the Secret Key},
howpublished = {Cryptology ePrint Archive, Report 2007/208},
year = {2007},
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.