FEATUREWhite Paper on Quantum Network

Quantum Cryptography and Quantum Key Distribution Network
Toward social deployment of computationally unbreakable cryptosystems

he conventional cryptography that
underpins today’s information society
is facing the risk of decryption as quantum
computers become more powerful. Meanwhile, quantum cryptography is currently
the only cryptographic system that has beenmathematically proved to be impossible todecrypt with any computer, including future
large-scale quantum computers. We take alook at the efforts and future prospects for research and development, and the implementation of a quantum key distribution network,
which is the core technology of quantum
cryptography.

TTAKEOKA Masahiro
Research Executive Director,
Advanced ICT Research Institute
He joined CRL (currently NICT), Japan,in 2001. His research interests include
quantum optics, quantum information
theory, and quantum cryptography.
Since April 2021, he has been a professorof at Keio University, and also a research
executive director at Advanced ICT Research Institute of NICT.

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■ Cryptographic technology which
underpins today’s networked
society, and its issues
The security of the cryptography that iswidely used today is guaranteed by “computational security,” which means that extremely massive computing power is needed fordecryption. This allows us to safely exchange
data every day. However, today’s cryptography is facing the potential threat of becoming
easily decrypted due to the advent of largescale quantum computers and entirely newcomputational technology/mathematical algorithms in future. Especially, critical information that requires secrecy for decades is at
risk of “harvest now, decrypt later” attacks,
in which encrypted data is eavesdropped oracquired, and then decrypted in future when
new computational technology is developed.
This is why an urgent response is required.
Two new types of technology are being developed to address this issue. One is“post-quantum public-key cryptography,”which has the same computational security but has a mathematical structure thatis thought to be difficult to decrypt withcurrently known quantum calculation algorithms, and it is now being implemented
and standardized. The other type is quantum

cryptography, as explained below. Quantum
cryptography is currently the only cryptographic system that has been proved to besafe and impossible in theory to decrypt withany computational technology or computersin future (“information-theoretic security”).

■Quantum cryptography andquantum key distribution (QKD)
network
As shown in Figure 1, quantum cryptography consists of two steps: quantum key
distribution (QKD) and one time pad (OTP)
encryption. QKD is a technology for thesender and receiver to share a secret key thatis kept secure against third parties, whichuses quantum mechanical properties of lightsignals, such as photons. OTP encryption encrypts data using an encryption key having
the same size as the data and, once a key isused, it will not be reused. Using the encryption key provided by QKD achieves information-theoretic security. OTP encryption andthe sending and receiving of encrypted data
are all performed with normal computers andcommunication lines; only the QKD part requires quantum technology.
The technology for managing and distributing keys safely and efficiently by connecting QKD transceivers to the network is called
a quantum key distribution (QKD) network(Figure 2). By sharing the encryption key at
any point on the network with the QKD network, and providing the key to the conventional network, it is possible to provide new
security services using an information-theoretic secure cryptographic key. In addition toQKD networks connected with optical fibers,
QKD using satellite communication is also
under development, which is expected to be
integrated into a global QKD network in future.

Figure 1

Mechanism of Quantum Cryptography
Figure 2

Key delivery from Quantum Key Distribution (QKD) Network■Research and development, andefforts toward implementation
Since the 2000s, in collaboration withuniversities and companies, NICT has beendeveloping the technology for QKD systems
and their networking technologies, such as
QKD network control and management technologies, and has also conducted field demonstrations. In 2010, Tokyo QKD Network, atest bed constructed in collaboration amongindustry, academia, and government, recorded the longest operation time in the world,and various demonstrations of principle andpractice are being conducted. Regarding implementation, companies in Japan, Europe,and China have commercialized QKD devices. Moreover, telecom carriers and startup companies around the world have beenworking toward the commercial provision ofservices using the QKD network.
For the QKD network technology tospread globally, international standardizationis crucial. In collaboration with the government, companies, and universities, NICT
is actively working toward international
standardization at the International Telecommunication Union Telecommunication
Standardization Sector (ITU-T), International Organization for Standardization (ISO)/International Electrotechnical Commission(IEC SC1), European Telecommunications
Standards Institute (ETSI), etc. Especially,at ITU-T, Japan is leading the development
and publication of many recommendations,
including for international standardization of
QKD networks.

Figure 3

Quantum Secure Cloud Technology■Exploitation of QKD networks andfuture prospects: Quantum secure
cloud technology
To apply the high secrecy of quantum
cryptography in society, it is necessary todevelop application technologies by taking
a broad view of communication systems andsecurity technology as a whole. One of thosetechnologies is quantum secure cloud technology, which is being developed by NICT
in collaboration with industry and academia.
This enables the storage and computational
processing of data backups that cannot be decrypted or tampered with by any computers,by merging authentication infrastructure andblind computing with quantum cryptography,
secret sharing, and post-quantum public-key
cryptography (Figure 3). By storing data inseveral servers in a distributed, encrypted
form, this technology realizes both information-theoretic security, in which even if theinformation in some servers is leaked, theoriginal data cannot be restored, and availability, in which if the information in someservers is lost, the original data can be restored from the remaining information at thesame time. This proprietary technology wasdeveloped in Japan, and NICT and companies are conducting demonstrations of storing
important data in various fields including themedical industry. It is expected that a newsecurity infrastructure for the networked society will be developed by properly merging
the QKD network with various contemporary
security technologies. The NICT Quantum
Network White Paper describes the principleof the technology, requirements, and prospects for social implementation in detail.

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