How to programmatically add a digital signature to a workbook
 

I want to write a macro that a user could run to digitally sign their
own workbook (the user will already have a digital certificate with
which to sign). I have found code to add a digital signature to a Word
document (or, more accurately, to display the Select Certificate dialog
box and the commit the certificate the user selects). I would like to
do the same thing in Excel, but it doesn't appear that Excel 2003 makes
the Signatures collection available the way it is in Word. Am I
missing something or is this "by design"? Any workarounds?