Workbook.PasswordEncryptionAlgorithm (Excel)

Returns a String indicating the algorithm that Microsoft Excel uses to encrypt passwords for the specified workbook.

Use the SetPasswordEncryptionOptions method to specify whether Excel encrypts file properties for password-protected workbooks.

ActiveWorkbook.SetPasswordEncryptionOptions PasswordEncryptionProvider:="Microsoft RSA SChannel Cryptographic Provider", 
 PasswordEncryptionAlgorithm:="RC4", PasswordEncryptionKeyLength:=56, PasswordEncryptionFileProperties:=True