Frankering Gecontroleerd is about hacking the postage stamp. Or actually more about hacking the machine that scans and checks the stamps.
I found out that postage stamps have an L-shape of phosphoric ink on them. Which I suspected might be used in the process of checking the authenticity of the stamps.
By silkscreen printing with glow in the dark ink, which is also phosphoric, I could create almost blank stamps that get accepted by the scanning machine in the postal system. Humans can see that these stamps are totally fake, though. So the inauthenticity of the stamps here becomes a secret for the machine kept by us humans.
In addition to the silkscreen printed stamps I also made a toolkit in the form of a plastic stencil that can be washed clean after use, send in an envelope and be used again to spread the secret.
The system was tested by sending mail from and to different locations within the Netherlands and even abroad to Spain. The false stamps always got through the system.