If Oracle is not running, login as sysdba and run the startup.

su - oracle 
sqlplus / as sysdba 
sql> startup;

If the database is running in NOARCHIVELOG mode, change it to ARCHIVELOG mode.

sql> shutdown immediate;
sql> startup mount;
sql> alter database archivelog;
sql> alter database open;

Exit the sqlplus session and connect to the rman session.

rman target /

List any existing backup files.

rman> list backup;

If there are any backup files, optionally delete them.

rman> delete backup;

Display the default configuration for rman.

rman> show all;

Set the backup and configuration file location and the file name format of the backup files. If the path doesn't exist, create it using the following:  mkdir -p /u02/backup/ORCL/

rman> CONFIGURE CHANNEL DEVICE TYPE DISK FORMAT '/u02/backup/ORCL/ORCL_BKP_%U';
rman> CONFIGURE controlfile autobackup on;

Create the backup with archive logs.

rman> BACKUP DATABASE PLUS ARCHIVELOG;

Create the init file using the pfile.

cd $ORACLE_HOME/dbs/
sqlplus / as sysdba

sql> create pfile from spfile;

After the backup is completed, identify the encryption key using the following query as sysdba on the source machine.

sqlplus / as sysdba
startup;
select KEY_ID,KEY_USE from V$ENCRYPTION_KEYS; 

Search for the section called "Current database master encryption key/s in use".