Clever:
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.
[...]
The cunning part is that while blinded, Bob’s detector cannot function as a ‘quantum detector’ that distinguishes between different quantum states of incoming light. However, it does still work as a ‘classical detector’ recording a bit value of 1 if it is hit by an additional bright light pulse, regardless of the quantum properties of that pulse.
That means that every time Eve intercepts a bit value of 1 from Alice, she can send a bright pulse to Bob, so that he also receives the correct signal, and is entirely unaware that his detector has been sabotaged. There is no mismatch between Eve and Bob’s readings because Eve sends Bob a classical signal, not a quantum one. As quantum cryptographic rules no longer apply, no alarm bells are triggered, says Makarov.
“We have exploited a purely technological loophole that turns a quantum cryptographic system into a classical system, without anyone noticing,” says Makarov.
Makarov and his team have demonstrated that the hack works on two commercially available systems: one sold by ID Quantique (IDQ), based in Geneva, Switzerland, and one by MagiQ Technologies, based in Boston, Massachusetts. “Once I had the systems in the lab, it took only about two months to develop a working hack,” says Makarov.
Just because something is secure in theory doesn’t mean it’s secure in practice. Or, to put it more cleverly: in theory, theory and practice are the same; but in practice, they’re very different.
The paper is here.
