Build 3.5.0.82 of xlsgen makes it possible that some auto-encrypted XLSX files can be auto-opened with xlsgen without the hassles of passing a password.

Auto-encryption occurs in Excel depending on the version of Excel being used, and obviously the action triggering this encryption.

The action in question is the "Protect workbook" feature that you can find in the Review ribbon tab. When this button is clicked, Excel will provide the ability to check/uncheck "Structures" and "Windows", thereby giving you the ability to ask Excel to lock the ability to add/move/delete sheet tabs and the ability to move windows hosting the sheet tabs, respectively. Usually this is enforced by passing a password.

As it turns out, if you are using Excel 2003 (or past versions) or Excel 2007, the file will be encrypted after you used this "Protect workbook" feature. It won't if you are using Excel 2010 or Excel 2013 however, but it does not matter to you.

And, as it turns out too, this encryption uses a hardcoded password, not the password mentioned above. This hardcoded password can be found over on the internet by just using the correct keywords in Google.

Without knowledge of this password, xlsgen could not in the past open such files. Beginning with this build xlsgen automatically tries to open a file (or an input buffer in memory) as an encrypted file with said password. If the file isn't encrypted, xlsgen knows it soon in the code execution so there is no performance loss compared to what we did before. The benefits is that xlsgen can now automatically open those files.

This behavior does not prevent the regular password file encryption/decryption from working correctly.