Quantum physics could make secure, single-use computer memories possible

Computer security systems may one day get a boost from quantum physics, as a result of recent research from the National Institute of Standards and Technology (NIST). Computer scientist Yi-Kai Liu has devised a way to make a security device that has proved notoriously difficult to build -- a "one-shot" memory unit, whose contents can be read only a single time.

The research, which Liu is presenting at this week's Innovations in Theoretical Computer Science conference, shows in theory how the laws of quantum physics could allow for the construction of such memory devices. One-shot memories would have a wide range of possible applications such as protecting the transfer of large sums of money electronically. A one-shot memory might contain two authorization codes: one that credits the recipient's bank account and one that credits the sender's bank account, in case the transfer is canceled. Crucially, the memory could only be read once, so only one of the codes can be retrieved, and hence, only one of the two actions can be performed -- not both.

"When an adversary has physical control of a device -- such as a stolen cell phone -- software defenses alone aren't enough; we need to use tamper-resistant hardware to provide security," Liu says. "Moreover, to protect critical systems, we don't want to rely too much on complex defenses that might still get hacked. It's better if we can rely on fundamental laws of nature, which are unassailable."

Unfortunately, there is no fundamental solution to the problem of building tamper-resistant chips, at least not using classical physics alone. So scientists have tried involving quantum mechanics as well, because information that is encoded into a quantum system behaves differently from a classical system.

Liu is exploring one approach, which stores data using quantum bits, or "qubits," which use quantum properties such as magnetic spin to represent digital information. Using a technique called "conjugate coding, "two secret messages -- such as separate authorization codes -- can be encoded into the same string of qubits, so that a user can retrieve either one of the two messages. But as the qubits can only be read once, the user cannot retrieve both.

The risk in this approach stems from a more subtle quantum phenomenon: "entanglement," where two particles can affect each other even when separated by great distances. If an adversary is able to use entanglement, he can retrieve both messages at once, breaking the security of the scheme.

However, Liu has observed that in certain kinds of physical systems, it is very difficult to create and use entanglement, and shows in his paper that this obstacle turns out to be an advantage: Liu presents a mathematical proof that if an adversary is unable to use entanglement in his attack, that adversary will never be able to retrieve both messages from the qubits. Hence, if the right physical systems are used, the conjugate coding method is secure after all.

"It's fascinating how entanglement -- and the lack thereof -- is the key to making this work," Liu says. "From a practical point of view, these quantum devices would be more expensive to fabricate, but they would provide a higher level of security. Right now, this is still basic research. But there's been a lot of progress in this area, so I'm optimistic that this will lead to useful technologies in the real world."

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More secure communications thanks to quantum physics

One of the recent revelations by Edward Snowden is that the U.S. National Security Agency is currently developing a quantum computer. Physicists aren't surprised by this news; such a computer could crack the encryption that is commonly used today in no time and would therefore be highly attractive for the NSA.

Professor Thomas Walther of the Institute of Applied Physics at the Technical University of Darmstadt is convinced that "Sooner or later, the quantum computer will arrive." Yet the quantum physicist is not worried. After all, he knows of an antidote: so-called quantum cryptography. This also uses the bizarre rules of quantum physics, but not to decrypt messages at a record pace. Quite the opposite -- to encrypt it in a way that can not be cracked by a quantum computer. To do this, a "key" that depends on the laws of quantum mechanics has to be exchanged between the communication partners; this then serves to encrypt the message. Physicists throughout the world are perfecting quantum cryptography to make it suitable for particularly security-sensitive applications, such as for banking transactions or tap-proof communications. Walther's Ph.D. student Sabine Euler is one of them.

As early as the 1980s, physicists Charles Bennett and Gilles Brassard thought about how quantum physics could help transfer keys while avoiding eavesdropping. Something similar to Morse code is used, consisting of a sequence of light signals from individual light particles (photons). The information is in the different polarizations of successive photons. Eavesdropping is impossible due to the quantum nature of photons. Any eavesdropper will inevitably be discovered because the eavesdropper needs to do measurements on the photons, and these measurements will always be noticed.

"That's the theory" says Walther. However, there are ways to listen without being noticed in practice. This has been demonstrated by hackers who specialize in quantum cryptography based on systems already available on the market. "Commercial systems have always relinquished a little bit of security in the past," says Walther. In order to make the protocol of Bennett and Brassard reality, you need, for example, light sources that are can be controlled so finely that they emit single photons in succession. Usually, a laser that is weakened so much that it emits single photons serves as the light source. "But sometimes two photons can come out simultaneously, which might help a potential eavesdropper to remain unnoticed" says Walther. The eavesdropper could intercept the second photon and transmit the first one.

Therefore, the team led by Sabine Euler uses a light source that transmits a signal when it sends a single photon; this signal can be used to select only the individually transmitted photons for communication. Nevertheless, there are still vulnerabilities. If the system changes the polarization of the light particles during coding, for example, the power consumption varies or the time interval of the pulses changes slightly. "An eavesdropper could tap this information and read the message without the sender and receiver noticing" explains Walther. Sabine Euler and her colleagues at the Institute of Applied Physics are trying to eliminate these vulnerabilities. "They are demonstrating a lot of creativity here" says Walther approvingly. Thanks to such research, it will be harder and harder for hackers to take advantage of vulnerabilities in quantum cryptography systems.

The TU Darmstadt quantum physicists want to make quantum cryptography not only more secure, but more manageable at the same time. "In a network in which many users wish to communicate securely with each other, the technology must be affordable," he says. Therefore, his team develops its systems in such a manner that they are as simple as possible and can be miniaturized.

The research team is part of the Center for Advanced Security Research Darmstadt (CASED), in which the TU Darmstadt, the Fraunhofer Institute for Secure Information Technology and the University of Darmstadt combine their expertise in current and future IT security issues. Over 200 scientists conduct research in CASED, funded by the State Initiative for Economic and Academic Excellence (LOEWE) of the Hessian Ministry for Science and the Arts. "We also exchange information with computer scientists, which is very exciting," says Walther.

After all, the computer science experts deal with many of the same issues as Walther's quantum physicists. For example, Johannes Buchmann of the department of Computer Science at the TU Darmstadt is also working on encryption methods that theoretically can not be cracked by a quantum computer. However, these are not based on quantum physics phenomena, but rather on an unsolvable math problem.

Therefore, it may well be that the answer to the first code-cracking quantum computer comes from Darmstadt.

Bizarre quantum physics and encryption

A quantum computer could quickly crack current encryptions because it can test very many possibilities simultaneously, in the same way as if you could try all possible variations for a password at once. After all, according to the quantum physics principle of superposition, atoms, electrons or photons can have several states simultaneously; for example, they can rotate clockwise and counterclockwise at the same time.

However, if you were to measure a property of a particle, such as the direction of rotation, the superposition is lost. This phenomenon is useful for quantum cryptography. Eavesdroppers inevitably betray themselves because their measurements of the photon change the photon's characteristics. Moreover, quantum physics forbids them to copy the photon with all its properties. Therefore, they can not siphon off any information to retransmit the uninfluenced photons on to the sender of the message.

Story Source:

The above story is based on materials provided by Technische Universit?t Darmstadt. The original article was written by Christian J. Meier. Note: Materials may be edited for content and length.

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Quantum physics secures new cryptography scheme

The way we secure digital transactions could soon change. An international team has demonstrated a form of quantum cryptography that can protect people doing business with others they may not know or trust -- a situation encountered often on the internet and in everyday life, for example at a bank's ATM.

"Having quantum cryptography to hand is a realistic prospect, I think. I expect that quantum technologies will gradually become integrated with existing devices such as smartphones, allowing us to do things like identify ourselves securely or generate encryption keys," says Stephanie Wehner, a Principal Investigator at the Centre for Quantum Technologies (CQT) at the National University of Singapore, and co-author on the paper.

In cryptography, the problem of providing a secure way for two mutually distrustful parties to interact is known as 'two-party secure computation'. The new work, published in Nature Communications, describes the implementation using quantum technology of an important building block for such schemes.

CQT theorists Wehner and Nelly Ng teamed up with researchers at the Institute for Quantum Computing (IQC) at the University of Waterloo, Canada, for the demonstration.

"Research partnerships such as this one between IQC and CQT are critical in moving the field forward," says Raymond Laflamme, Executive Director at the Institute for Quantum Computing. "The infrastructure that we've built here at IQC is enabling exciting progress on quantum technologies."

"CQT and IQC are two of the world's largest, leading research centres in quantum technologies. Great things can happen when we combine our powers," says Artur Ekert, Director of CQT.

The experiments performed at IQC deployed quantum-entangled photons in such a way that one party, dubbed Alice, could share information with a second party, dubbed Bob, while meeting stringent restrictions. Specifically, Alice has two sets of information. Bob requests access to one or the other, and Alice must be able to send it to him without knowing which set he's asked for. Bob must also learn nothing about the unrequested set. This is a protocol known as 1-2 random oblivious transfer (ROT).

ROT is a starting point for more complicated schemes that have applications, for example, in secure identification. "Oblivious transfer is a basic building block that you can stack together, like lego, to make something more fantastic," says Wehner.

Today, taking money out of an ATM requires that you put in a card and type in your PIN. You trust the bank's machine with your personal data. But what if you don't trust the machine? You might instead type your PIN into your trusted phone, then let your phone do secure quantum identification with the ATM (see artist's impression). Ultimately, the aim is to implement a scheme that can check if your account number and PIN matches the bank's records without either you or the bank having to disclose the login details to each other.

Unlike protocols for ROT that use only classical physics, the security of the quantum protocol cannot be broken by computational power. Even if the attacker had a quantum computer, the protocol would remain secure.

Its security depends only on Alice and Bob not being able to store much quantum information for long. This is reasonable physical assumption, given today's best quantum memories are able to store information for minutes at most. Moreover, any improvements in memory can be matched by changes in the protocol: a bigger storage device simply means more signals have to be sent in order to achieve security. (The idea of 'noisy storage' securing quantum cryptography was developed by Wehner in earlier papers.)

To start the ROT protocol, Alice creates pairs of entangled photons. She measures one of each pair and sends the other to Bob to measure. Bob chooses which photons he wants to learn about, dividing his data accordingly without revealing his picks to Alice. Both then wait for a length of time chosen such that any attempt to store quantum information about the photons is likely to fail. To complete the oblivious transfer, Alice then tells Bob which measurements she made, and they both process their data in set ways that ensure the result is correct and secure within a pre-agreed margin of error.

In the demonstration performed at IQC, Alice and Bob achieved a random oblivious transfer of 1,366 bits. The whole process took about three minutes.

The experiment adapted devices built to do a more standard form of quantum cryptography known as quantum key distribution (QKD), a scheme that generates random numbers for scrambling communication. Devices for QKD are already commercially available, and miniaturised versions of this experiment are in principle possible using integrated optics. In the future, people might carry hand-held quantum devices that can perform this kind of feat.

"We did the experiment with big and bulky optics taking metres of space, but you can well imagine this technology being shrunk down to sit happily next to classical processing circuits on a small little microchip. The field of integrated quantum optics has been progressing in leaps and bounds, and most of the key pieces required to implement ROT have already been successfully demonstrated in integrated setups a few millimetres in size," says Chris Erven, who performed the experiments at IQC as a PhD student under the supervision of Raymond Laflamme and Gregor Weihs. Weihs is now at the University of Innsbruck, Austria. Erven is now a postdoctoral fellow at the University of Bristol, UK.

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