Take a look at methods in the %SYSTEM.Encryption class.
Like many of the classes in the %SYSTEM package, you can easily access its methods via $SYSTEM, e.g.
'Do $system.Encryption.Help(method)' will display a full description of an individual method.
Methods of the class: %SYSTEM.Encryption
This method performs AES decryption in Cipher Block Chained (CBC) mode.