only. This will not work for encrypting a workbook. This method is based on the algorithm provided by
Daniel Rentz of OpenOffice and the PEAR package
Spreadsheet_Excel_Writer by Xavier Noguer <xnoguer@rezebra.com>.
See also http://blogs.msdn.com/b/ericwhite/archive/2008/02/23/the-legacy-hashing-algorithm-in-open-xml.aspx