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Office 2013 shows tamper warning when loading encrypted file

Feb 9 at 9:45 PM
When I encrypt a file using this library, and then load it into Excel 2013, Excels displays a warning suggesting that the file may have been tampered with. If you click through the warning, Excel continues to load the file properly. I'm trying to determine what causes Excel to display that warning when loading encrypted files generated by this library.

I suspect it may be a problem in the AddIntegrityCheck section of Hmac.cs (since the actual contents of the file seem to be correct) - but I cannot quite figure out where (or if) the bug is.

Any pointers would be appreciated.