During the historic nuclear arms negotiations between the United States and the Soviet Union in the 1980s, "trust, but verify" became a common phrase. Today, with control of the spread of nuclear weapons as urgent a problem as ever, MIT researchers are working on a system similar to computer encryption that allows arms inspectors to keep track of nuclear warheads without their owners worrying about giving away vital military secrets.

The paradox of nuclear arms control is that monitoring strategic weapons is easiest when it's needed least, and hardest when it's needed most. The United States and Britain, for example, don't worry much about one another's arsenals, but US and Russia worry very much about one another's capabilities. Almost since the invention of the first atomic bombs, how to control their possession and use has vexed the imagination of statesmen, diplomats, and generals.

Part of the problem is that nuclear weapons are unique. They are so destructive that even without a fully-fledged delivery system a stockpile of H-bombs still poses a formidable threat. If you eliminate all of a country's artillery or bomber planes, then its armories filled with shells and bombs don't stand for much, but a one-megaton nuke is in a completely different class that can act as a threat even if its in storage.

Nuclear weapons have another problem. The details of their construction are a closely guarded secret that no power wants to divulge, but unless nuclear warheads are directly inspected, how can the inspectors be sure that the bomb allegedly in storage is really the bomb or a replica? It's for this reason that, for decades, most arms control agreements have focused more on the delivery systems, including missiles, guns, submarines, and bombers rather than the bombs themselves. It's unsatisfactory, but it was at least something.

At top, a diagram shows the configuration that could be used to verify that a nuclear warhead is real. At left, the key component of a nuclear weapon, called the pit, which consists of a hollow sphere of plutonium, is lined up with a specially made second component, called the reciprocal, which has the opposite characteristics(Credit: Areg Danagoulian/MIT)

In an effort to facilitate better monitoring, MIT has taken a page from the book of digital security. Most people who spend any time worrying about keeping their data safe on the internet have heard of encryption. The most common type uses a pair of very large numerical keys derived from prime numbers that allow people to encrypt data, send it or receive it, and have it decrypted without having to share information like the decryption key with the other person.

The thinking of the MIT team led by assistant professor of nuclear science and engineering, Areg Danagoulian, is that by using a similar technique, only one from physical properties instead of digital keys, it should be possible for inspectors to scan a suspect warhead and confirm it's the real one on the inventory without learning anything about how it's actually put together.

In the case of digital encryption, its possible to compare two sets of encrypted data and show that they are identical because they were encrypted using the same encryption key, which anyone can openly know, and the sequences should be the same. You don't know what the original message is, but if the two strings don't match, they aren't the same message.

The MIT team's idea is to take information from the atomic isotopes used in the construction of a warhead that contain all the information needed to confirm the identity of a warhead, yet doesn't reveal anything about its design or components.

The way in which MIT describes this process as being similar to making a full-color image of an object, then using this to create a second image of the first, only in the complementary colors. If these two are lined up, the final image is black and featureless. By using a neutron scan of the isotopes in a warhead, a similar featureless record can be created.

The clever bit is, in the photographic example, if you try to put a fake image in against the complementary one, it won't be black any longer, but will show colors where the two fail to match. The colors don't tell you much, but they do show that something isn't right.

In the case of arms control, warheads can be kept sealed in a black box and scanned with a neutron beam to create the cryptographic reciprocal or a cryptographic foil. If it's re-scanned and the two reciprocals don't match, then it's a fake. In other words it's a Zero Knowledge Proof where the only thing that is revealed is whether something is true without explaining what that truth it.

In fact, the MIT researchers point out that it's a bit better that. They argue that it actually deters cheating because if the bomb is what it claims it is, then nothing about its secrets are revealed, but a fake will provide some clues because the encryption breaks down in spots.

So far, MIT has tested the neutron concept against extensive simulations and now want to go on to trying it on actual fissile materials. In addition, a similar concept using photons instead of neutrons is under investigation. But, ultimately, the goal is a safer world.

"[E]veryone will be better off," says Danagoulian. "There will be less of this waiting around, waiting to be stolen, accidentally dropped or smuggled somewhere. We hope this will make a dent in the problem."

Source: MIT

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