I had tried that before using Mathematica instead of a website and I was also surprised to see the data in the barcode is not really signed in any way. IIRC it was a European low cost like Ryan air, and I thought it was scary that I could have generated the same exact bar code just by knowing my name and the flight.

Of course there is an extra step of validation, because the airline has the passenger list, so you can't just add yourself to a flight.