Password-protected documents can be created by using one of four mechanisms:
40-bit RC4 encryption.
ECMA-376 document encryption [ECMA-376], which can include one of three approaches:
Agile encryption: This approach uses an XML EncryptionInfo structure. The encryption and hashing algorithms are specified in the structure and can be for any encryption supported on the host computer.
Extensible encryption: This approach uses an extensible mechanism to allow arbitrary cryptographic modules to be used.