Hackers blind quantum cryptographers

via Nature via Liquidmatrix Security Digest:

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.

Extra credit: Browse the resources from this comment, debating the pragmatic viability of quantum crypto, which relates directly to (among other things) the way these researchers were able to authenticate the message, but not validate the source.