How is algorithm encryption provided by MariaDB in RHSCL ?

Issue

• MariaDB uses key of 256-bit to encrypt with AES algorithm. In file_key_management_filename system variable , 128-bit, 192-bit or 256-bit keys are supported. But in file_key_management_encryption_algorithm system variable, if the value is set to AES_CBC, the plugin will use AES with 128-bit keys in the CBC mode. 128-bit key is length of block use in CBC mode. So which 128-bit key is used for 256-bit key length ?