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

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SQL> ALTER TABLE ENC_TEST3 ADD (ssn VARCHAR2(11) ENCRYPT USING 'AES192');
ALTER TABLE ENC_TEST3 ADD (ssn VARCHAR2(11) ENCRYPT USING 'AES192')
                           *
ERROR at line 1:
ORA-28340: a different encryption algorithm has been chosen for the table

Cause:
There are already some encrypted column in the table using different encryption algorithm. A table must use the same encryption algorithm to encrypt all its column. Identify the existing Algorithm and use it for encryption

Solution:
Remove the using clause and rerun the query 
SQL> ALTER TABLE ENC_TEST3 ADD (ssn VARCHAR2(11) ENCRYPT );

OR

Identify the Existing Algorithm used and use this algorithm to encrypt the column.
SQL>  select OWNER,TABLE_NAME,COLUMN_NAME,SALT,INTEGRITY_ALG,ENCRYPTION_ALG from dba_encrypted_columns where OWNER='TEST' and TABLE_NAME='ENC_TEST3';
OWNER      TABLE_NAME           COLUMN_NAME     SAL INTEGRITY_AL ENCRYPTION_ALG
---------- -------------------- --------------- --- ------------ -----------------------------
TEST       ENC_TEST3            EMPID           YES NOMAC        3 Key Triple DES 168 bits key
SQL> ALTER TABLE ENC_TEST3 ADD (ssn VARCHAR2(11) ENCRYPT USING '3DES168');