Physically Unclonable Functions are a promising tool to protect against counterfeiting attacks. Yet, as with any security system, it is important to embed them in a sound protocol, ensuring that no unexpected weakness is present in the “mortar” binding the components together. This paper proposes an anti-counterfeiting protocol that provably reduces to natural properties of its underlying components, namely an image-based Physical Function System bearing physical unclonability and an existentially unforgeable signature scheme. Experiments confirm the practical feasibility of our construction.