Cryptology ePrint Archive: BibTeX Citation of Report 2001/021

@misc{cryptoeprint:2001:021,
    author = {Biham Eli, Orr Dunkelman, Nathan Keller},
    title = {The Rectangle Attack - Rectangling the Serpent},
    howpublished = {Cryptology ePrint Archive, Report 2001/021},
    year = {2001},
    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.