24 November 2017

Jonathan Jogenfors’ doctoral thesis discusses security in what many people consider to be the most secure system of all – quantum cryptography. And it explains how to fix the holes he found. He is to continue developing both computer security and education in the field.

Jonathan Jogenfors and Niklas Johansson Jonathan Jogenfors and Niklas Johansson Photo credit: Thor BalkhedJonathan Jogenfors presented his doctoral thesis at the Division for Information Coding recently, and has already been interviewed many times in the media. The most recent interview was in the Swedish national news TV programme “Aktuellt” and dealt with combatting viruses, but he is most often asked about his other speciality – Bitcoin. The digital payment system Bitcoin, however, is dealt with only superficially in the thesis and is only included as an idea in an article in which he combines blockchain technology with quantum computers to obtain a form of quantum Bitcoin.

“To be honest – it was really just a wacky idea.”

Security loopholes

The thesis otherwise deals with quantum cryptography and the security loopholes he and his colleagues have found.

“It is possible to show scientifically that quantum cryptography is completely secure, but this is true only under ideal conditions. At our division, we like to try unconventional thinking, and we want to be a bit more practical and work in the world as it really is. We have concentrated on a three-stage process: we show that a problem exists; we find out why the problem exists; and we come up with a solution,” he says.

This is not always popular among his colleagues in the field. The end of the thesis presents a small errata sheet – a correction that one of the prestigious journals was compelled to publish after the LiU researchers had pointed out that one important assumption made in a published article, written by colleagues, was no longer valid.

Energy-time entanglement

In the autumn of 2015, Jonathan Jogenfors and his supervisor Jan-Åke Larsson, professor of information coding, showed that a method on which several of the current systems for quantum cryptography are built, energy-time entanglement, is vulnerable to attack. They also demonstrated several possible solutions. The article was published in Science Advances, and is included in Jonathan’s thesis.

“The method has been considered to be effective, and the test has been easy, but the consequence of our research is that the system will be more difficult to build than previously thought.”

Are the requirements for optimal conditions so stringent that it is impossible to use the technology?

Jonathan Jogenfors doesn’t think so.

“In ten years or so the technology for secure quantum cryptography could exist, but this assumes that there is a need for better methods for encrypting information than those currently available. As soon as quantum computers, which can test billions of different combinations extremely rapidly and thus defeat standard cryptography techniques, are a reality, then quantum cryptography may be commercially viable. One possible example of use is the transfer of election results,” he replies.

Research manager

The security company Sectra, which was founded by LiU researchers, has recruited Jonathan Jogenfors as research manager Sectra Communications. He will be moving to this job in a few weeks. But he will not be giving up his teaching role at LiU.

“We want to strengthen once again the connection between LiU and Sectra, and continue the development of computer security education at LiU. The Division of Information Coding is not the only place at which education in computer security is carried out: expertise is available at other places in the university.”

Jonathan Jogenfors defended his doctoral thesis on 17 November 2017. His supervisor has been Professor Jan-Åke Larsson.
Breaking the Unbreakable, Exploiting Loopholes in Bell’s Theorem to Hack Quantum Cryptography, Jonathan Jogenfors, Division of Information Coding, Department of Electrical Engineering, Linköping University 2017.


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