I would like to be be able to mark an excel file with a key which will come from the system so that when an excel file is imported it would try to find that key to determine if the file came from the system or not. I am currently doing it by specifying the key in the file name but that is very easy to fail. My google-fu is not working, my search results are returning links to adding keys/identification to the cells.

In the VB.Net section it was recommended that I use a custom property of the excel file. I am still looking for other more foolproof ways if possible. Would it be possible to binary edit the file and include a secret data?

TIA