That's the right idea John, but it would be bothersome to have to keep entering passwords as the user moved back and forth from sheet to sheet while working, they already entered the password once, so that should "stick" for their session.
I would recommend trying out putting all the passwords into the Workbook_Open macro to reset them all when the book is first opened, any sheets that are legitimately unlocked will stay unlocked for that session.
In ThisWorkbook
This is less cumbersome, but this is still far from secure. If a person opens your workbook with macros disabled, this wouldn't trigger. It's a whole 'nother topic, so Google "Excel VBA force macros" for some great suggestions on making your workbook unusable if macros are disabled.
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