Most security applications, for instance, access to buildings or digital signatures, use cryptographic keys that must at all costs be kept secret. That also is the weak link: Who will guarantee that the key doesn't get stolen or hacked? Using a physical unclonable key (PUK), which can be a stroke of white paint on a surface, and the quantum properties of light, researchers of the University of Twente and Eindhoven University of Technology have presented a new type of data security that does away with secret keys. They present their method in the journal Quantum Science and Technology.