Like many of the classes in the %SYSTEM package, you can easily access its methods via $SYSTEM, e.g.
USER>d $system.Encryption.Help()
'Do $system.Encryption.Help(method)' will display a full description of an individual method.
Methods of the class: %SYSTEM.Encryption
AESCBCDecrypt(ciphertext,key,IV)
This method performs AES decryption in Cipher Block Chained (CBC) mode.
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Please do not use DES as it's currently a broken cypher with bruteforce time of about a day.
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.