SYSTEM Tablespace ENCRYPTION

Hi,

I want  encrypt  system tablespace . Below statement  is working fine and system tablespace be encrypted with  default encryption algoritm  AES128

ALTER TABLESPACE SYSTEM ENCRYPTION  ENCRYPT;

But  I want  to use  encryption algoritm AES256.  I can use it  for SYSAUX tablespace  and other  tablespaces .

ALTER TABLESPACE SYSTEM ENCRYPTION ONLINE USING 'AES256' ENCRYPT;  --> this command is getting error like below

ORA-28340: A different encryption algorithm has been chosen for the table or

tablespace.

I m wondering  that   We can not use  undefault  encryption algorithm  for  SYSTEM tablespace ?       I did not met a document saying  like that.

Regards.