How does Excel set a password?

There are several forms of encrypting Excel:

1, open the password in the tool--option-security-This workbook file encryption settings open the password to enter your password, OK, repeat input can be. This type of password is open permission password, forget the software can only be cracked.

2, modify the password in the tool--option-security-This workbook file sharing settings to modify the permissions password to enter the password you set, OK, repeat input can be. This password is to modify the permissions password, forgot the password this file is read-only form, can not be modified.

3, another is to protect the part of the structure of the worksheet in the tool-protection-Protection Worksheet/Protection Workbook, check the appropriate protection and password. This kind of password forgets can use the macro code to crack