hackers blind quantum cryptographers

Nature | Quantum hackers have performed the first 'invisible' attack on two commercial quantum cryptographic systems. By using lasers on the systems — which use quantum states of light to encrypt information for transmission — they have fully cracked their encryption keys, yet left no trace of the hack.

Quantum cryptography is often touted as being perfectly secure. It is based on the principle that you cannot make measurements of a quantum system without disturbing it. So, in theory, it is impossible for an eavesdropper to intercept a quantum encryption key without disrupting it in a noticeable way, triggering alarm bells.

Vadim Makarov at the Norwegian University of Science and Technology in Trondheim and his colleagues have now cracked it. "Our hack gave 100% knowledge of the key, with zero disturbance to the system," he says.

In standard quantum cryptographic techniques, the sender — called 'Alice' for convenience — generates a secret key by encoding classical bit values of 0 and 1 using two different quantum states of photons, or particles of light. The receiver, 'Bob', reads off these bit values using a detector that measures the quantum state of incoming photons. In theory, an eavesdropper, 'Eve', will disturb the properties of these photons before they reach Bob, so that if Alice and Bob compare parts of their key, they will notice a mismatch.

In Makarov and colleagues' hack, Eve gets round this constraint by 'blinding' Bob's detector — shining a continuous, 1-milliwatt laser at it. While Bob's detector is thus disabled, Eve can then intercept Alice's signal. The research is published online in Nature Photonics today.