@misc{cryptoeprint:2002:157,
author = {Elad Barkan and Eli Biham},
title = {In How Many Ways Can You Write Rijndael?},
howpublished = {Cryptology ePrint Archive, Report 2002/157},
year = {2002},
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.