In principle, harnessing the laws of quantum theory can drastically boost the security of modern communication networks, from public key encryption to electronic voting and online banking. Real-world implementation, however, opens a whole new spectrum of attacks and loopholes which must be characterized.

We provide an introduction to some very simple yet powerful attacks on photonic demonstrations of quantum-cryptographic tasks, and how to take them into account in security proofs. We then illustrate this with two quantum network tasks: quantum money and quantum weak coin flipping. The first exploits the no-cloning property of quantum physics to generate unforgeable tokens, banknotes, and credit cards, stored in a malicious quantum memory. The second is a fundamental cryptographic primitive, which allows two distant parties to flip a coin when they both desire opposite outcomes. Using quantum entanglement then prevents any party from biasing the outcome of the flip beyond a certain probability.