The problem:

I have modified some app which works as a EXCEL add-in and is mostly contained in a .xll file. When I try to load it, EXCEL complains: "The macros in this file do not match the digital signature . . .blah blah "
and refuses to load it.
There are ways around it, of course. Like lowering the security level for macros, or loading the add-in after EXCEL is running, but I would like to re-sign the updated file without perturbing the EXCEL security level, and without modifying the EXCEL code.

Microsoft provides info about signing macro libraries that you make, but it is not trivial when the macros are in an already compiled module .xll .
My question to the public is: does anybody know how the MSdigital signature operates? or knows of a source about the issue?
At first value looks like a CRC check of some sort, and I have some ideas about how to locate it, but pehaps some one here already invented the wheel before I do.

Thank you

A wild guess would be that the xll file is simply signed with a normal authenticode signature, in which case you can quite easily remove the signature data (just an appended binary chunk) and then use the Microsoft authenticode tools to apply your own, or am I wrong?