When Quantum Cryptography Isn’t Cryptography

Practical Quantum Cryptoby Hal F.

A  research company recently built a secure enterprise network using quantum key distribution. This “quantum encrypted” network is presented as entirely new, but it’s not.  DARPA has funded a functioning quantum encrypted research network and demonstration systems have been running in Europe for some years. There is also intense research in places like Japan and China toward solving the technical problems inherent in the current quantum encryption paradigms.

If you poke deeper at basic quantum cryptography you will find that the user data is really encrypted with a familiar conventional high speed block cipher like AES. The reason for this is that quantum key distribution channels run too slow to support the speeds of a practical modern fiber optical data communications channel. The original idea for quantum cryptography was to create something like a one-time pad — each bit is xor’ed with a random key stream bit transmitted through the quantum channel but in parallel with the data channel.   Chemical impurities in real world fiber optic cable limit the speed at which an individual photon or a weak clump of photons can be sent with reasonable error through the quantum channel, preventing sending data at high speeds.

Researchers originally overcame this speed limitation by devising a very neat trick called “fast re-keying.” With this method the message stream is sent over its own channel parallel with a quantum channel. The quantum channel is used to send a stream of key bits up to the receiving end where it re-keys a conventional cipher. With modern FPGA or ASIC  implementations, common ciphers can run comfortably at the speeds of a modern fiber optic channel. The idea is to change conventional keys as fast as the quantum channel can deliver them. This means short spans of message data will be encrypted with different conventional keys. With fast re-keying the desire is to make something that resembles a one-time pad.

So, what is quantum mechanical aspect of this really doing? The answer is a bit astonishing.  It turns out that the heart of a quantum key distribution system: the quantum channel,  functions as a channel with a built-in alarm system. If the bits flowing through the quantum channel are copied or altered in any way the receiver quickly knows about it because the bit error rate goes sky high. Modern quantum key distribution systems must also use complex protocols to prevent sophisticated attacks that can exploit the quantum channel. So, is quantum cryptography really cryptography? Some specialists think it’s really a sophisticated type of alarm system and not cryptography at all. Then again, almost everyone else thinks it is.

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Today’s post pic is from Imgur.com.

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