You need to use an interposing relay. This is simply a mechanical or solid state relay that has a 12Vdc coil and at least one contact (typically a form C contact, aka single pole double throw). Wire 3.3Vdc from your Pi to one side of the contact and the other to a GPIO input. Ensure you use a pull down resistor with the relay contact and a current limiting resistor directly in front of the GPIO point. The 12Vdc will be wired to the relay coil. When the coil is powered on from the security panel, the contacts will change state.
You could use the/a DC to DC converter, but depending on the type can cause issues....but generally these are expensive devices and more complicated than required.
FYI I have implemented this application at home with my ADT panel to send SMS text messages. I used the relay and it works flawlessly.
You can find terminal block style relays that are relatively inexpensive. I like Omron. Take a look at their G2RV-SR series. You can find these at a variety of online dealers or at your local electrical distributor.