31.2. Assign a Password to a Workbook

Assigning a password to a workbook is one technique you can use to secure the workbook. You can actually assign two passwords to a workbook — one password to open the workbook and another password to modify the workbook — and you can use the same or different passwords.

If you assign a password to open the workbook, Excel encrypts the workbook using AES 128-bit advanced encryption and prompts users who try to open the workbook to supply the password. If you assign a password to modify a workbook, Excel prompts users when they open the workbook for a password to save changes they subsequently make to the workbook. The password to modify a workbook is intended to help you control who makes changes to the workbook; it does not provide any other kind of security.