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November 28, 2023


The Quantum Frontier: NTT's Quest for Unbreakable Cryptography

Quantum computers leverage the principles of quantum mechanics to perform calculations at speeds unattainable by classical computers for specific problems. They have the potential to revolutionize a number of fields, including material science and medicine. For example, quantum computers are able to efficiently simulate molecular structures for drug discovery and optimize complex systems in logistics and finance.

The power of quantum computers also brings potential problems, however. Although they are not inherently dangerous, their potential computational capabilities pose significant challenges to current cryptographic systems. A key way in which quantum computers could affect security is by breaking public-key cryptography. The RSA algorithm, which is widely used for securing online transactions, emails, and more, is a public-key cryptographic algorithm introduced in 1978, which has become one of the most widely used encryption and digital signature algorithms. RSA relies on the mathematical properties of prime numbers and their difficulty to factor when it comes to large composite numbers. The security of RSA is primarily based on the computational difficulty of factoring large composite numbers into their prime components, a problem for which no efficient solution currently exists for classical computers.

However, Shor's factorization algorithm, developed in 1994, predicted that the RSA cipher could be broken by quantum computers, which can solve problems exponentially faster than the best-known algorithms running on classical computers. For this reason, research has taken place in recent years on whether it might be possible to develop computer cryptography that cannot be deciphered—even by quantum computers.

Thanks to NTT, a solution may be in sight.

NTT has succeeded in creating a world-first commitment that balances high quantum-resistant security and communication efficiency solely through one-way functions—foundational technology for constructing safe and efficient cryptographic protocols against attackers with quantum computers. A commitment scheme in computer security is a cryptographic element that allows one party (the sender) to "commit" to a certain value while keeping it hidden from another party (the receiver), and reveal the committed value later. Once the sender has committed to a value, they cannot change it without the receiver knowing. This ensures that the sender cannot cheat by changing their mind after the fact. Meanwhile, the receiver cannot determine the committed value until the sender chooses to reveal it. This ensures that the sender's choice remains confidential until they are ready to disclose it. Think of a sealed bid in an auction: you put your bid in an envelope and seal it (commit); once all bids are collected, all envelopes are then opened (reveal). If the scheme is secure, you cannot change your bid once the envelope is sealed (binding), and no one can see your bid until all envelopes are opened (hiding).

NTT's Distinguished Researcher Takashi Yamakawa, in collaboration with Dr. Xiao Liang of NTT Research Cryptography & Information Security Lab and Associate Professor Omkant Pandey of Stony Brook University, has designed a commitment solution that simultaneously achieves non-malleability against quantum computers and communication efficiency—"constant-roundness"—that is independent of the desired security level, using only one-way functions as the minimal assumption. Non-malleability refers to the property whereby once a user commits to a particular message, another user cannot tamper with it to commit to a related message. Constant-roundness is the property where the number of communication rounds between the sender and the receiver remains constant, irrespective of the desired security level.

When it comes to designing safety for classical computers, commitment schemes have been known since 2011; however, achieving similar solutions for quantum computers has been an impossibility until now, because quantum computers operate on different principles from classical computers and traditional safety proofs for attackers using classical computers cannot be applied to attackers using quantum computers.

Professor Yamakawa and his colleagues have redesigned the commitment scheme in a completely different way and have achieved secure commitment with non-malleability, in which multiple users perform computations cooperatively while keeping their own data secret.

NTT's research is expected to lead to the development of a more secure and efficient secure computation protocol for quantum computers in the future. Designing commitment schemes is only the beginning—in the future, NTT intends to apply its method to other cryptographic protocols such as secure computing protocols and aim to improve quantum security resistance. Eventually, NTT aims to develop quantum-resistant security through the application of its research to other cryptographic protocols such as secret computation protocols and zero-knowledge proofs.

NTT—Innovating the Future of Online Security

Picture: Daniel O'Connor

Daniel O'Connor joined the NTT Group in 1999 when he began work as the Public Relations Manager of NTT Europe. While in London, he liaised with the local press, created the company's intranet site, wrote technical copy for industry magazines and managed exhibition stands from initial design to finished displays.

Later seconded to the headquarters of NTT Communications in Tokyo, he contributed to the company's first-ever winning of global telecoms awards and the digitalisation of internal company information exchange.

Since 2015 Daniel has created content for the Group's Global Leadership Institute, the One NTT Network and is currently working with NTT R&D teams to grow public understanding of the cutting-edge research undertaken by the NTT Group.