The Quantum Gap

"Animals will be bred und slaughtered." This is a famous line from the Kubrick film Dr. Strangelove, where the titular character postulates about the realities of life in a mine shaft after World War Three.

Immediately, George C. Scott's character, General Turgidson, insists on including weapons and warheads, so that when the radiation levels drop, America won't be caught helpless against sinister Russians who no doubt will have stockpiled their own weapons in their own mineshafts.

Turgidson was worried about planning for a war that wouldn't occur for generations after the apocalypse. He was adamant that the United States could not allow a mineshaft gap.

Encryption is a lot like that. You define an encryption technique with 56 bits of randomness. Your enemy develops a way to try all 2^56 permutations in a reasonable length of time. You bump up the complexity: 64 bits. Then 128, then 256, and so on.

These days, the truly paranoid use 2048-bit RSA keys. I'd say that there will be 4096-bit keys in my lifetime, but sometime in the next 20 years someone will develop a quantum computer and keysizes won't matter any more, at least not in the conventional sense.

Quantum computing is a kind of holy grail. It can, at least in theory, crack an RSA key by circumventing a common assumption in conventional cryptology: that factoring is hard. Quantum computers make factoring a simple operation, and so codes built upon a reliance that factoring is hard immediately disintegrate.

At the same time, quantum encryption, which already exists in laboratory environments, is unbreakable. Unbreakable, but apparently not untappable.

Don't worry. Scientists saw this coming years ago. If you eavesdrop on a quantum conversation in a conventional sense, you interfere with the particles and that's detectable by the recipient. If you replace the particles with new particles, you'll only guess right about half the time, and that's detectable, too.

Now, if I read the article correctly, they've developed a way to clone a quantum exchange of particles so that you don't tamper with them. Great. Quantum encryption is still safe. Why? Because quantum encryption only handles one part of a secure communication, the "getting there" part. So you can entangle your messages into particles and push them at the intended recipient. And, at some point between hither and yon, some ne'er-do-well goes and duplicates your particle stream.

That bastard. But what the duplicate stream can't provide is which particles are important. So you can pick up the phone, call your friend, and you can go through the data together. You can error-check, of course, and when you're happy that the stream has been correctly received you can go and decide which particles were the important ones. Maybe the only qubits that mattered were the first, sixth, and eighteenth. Your eavesdropper wouldn't be able to determine that without also eavesdropping on your phone call.

So go ahead and wait for quantum cryptography to become commonplace. It's gonna be great.