Quick Tip
From v.3.0.1, the statement CREATE OR ALTER USER SYSDBA PASSWORD <password> can be used to initialize an empty securityN.fdb security database.

./isql -user sysdba employee
Database: employee, User: SYSDBA
SQL> CREATE OR ALTER USER SYSDBA PASSWORD 'MyMasterkeY';
Statement failed, SQLSTATE = 08001
I/O error during "open" operation for file "/opt/firebird/security3.fdb"
-Error while trying to open file
-No such file or directory
SQL>