Finally, i was successful with the interbase encryption with interbase XE. Here are the main parts of the script to encrypt a DB, make an encypted backup and restore it also encrypted. The database must be created with the embedded user authentication (EUA) enabled.
// connected with SYSDSO: prepare keys and grants
ALTER DATABASE SET SYSTEM ENCRYPTION sepPass;
CREATE ENCRYPTION dbKey FOR DES with LENGTH 56 BITS INIT_VECTOR RANDOM;
CREATE ENCRYPTION backupKey FOR DES with LENGTH 56 BITS password backupKeyPass;
GRANT ENCRYPT ON ENCRYPTION dbKey to SYSDBA;
GRANT ENCRYPT ON ENCRYPTION backupKey to SYSDBA;
// connected with SYSDBA: encrypt database
ALTER DATABASE ENCRYPT WITH dbKey;
// To backup and restore db with gbak
Backup:
gbak -b -v EMPLOYEE.IB EMPLOYEE.IBAK -user SYSDBA -pas masterkey -sep sepPass -encrypt backupKey -se service_mgr
Restore:
gbak -r -v EMPLOYEE.IBAK EMPLOYEE.IB -user SYSDBA -pas masterkey -sep sepPass -decrypt backupKeyPass -se service_mgr