@misc{cryptoeprint:2004:094,
author = {Alex Biryukov},
title = {Block Ciphers and Stream Ciphers: The State of the Art },
howpublished = {Cryptology ePrint Archive, Report 2004/094},
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.