Cryptology ePrint Archive: BibTeX Citation of Report 2004/294

@misc{cryptoeprint:2004:294,
    author = {Souradyuti Paul and Bart Preneel},
    title = {Solving Systems of Differential Equations of Addition and Cryptanalysis of the Helix Cipher},
    howpublished = {Cryptology ePrint Archive, Report 2004/294},
    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.