@misc{cryptoeprint:2005:233,
author = {Luke O'Connor},
title = {On the Entropy of Arcfour Keys},
howpublished = {Cryptology ePrint Archive, Report 2005/233},
year = {2005},
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.