What is Quantum Cryptography?

In today’s society, IoT (Internet of Things) is advancing across all fields, leading to the exchange of various types of data over information and communication networks. Among this data are sensitive pieces of information, including government and security-related confidential information, financial data, genomic information, and medical records such as electronic health records. Cryptographic technology is essential for securely transmitting and receiving this data. Naturally, cryptographic technology is also indispensable in our daily lives. It is thanks to cryptographic technology that we can safely use familiar online services, such as making payments on shopping sites, online banking, sending and receiving messages via email and messaging apps, and conducting cryptocurrency transactions, as it protects important information.

Let’s take a brief look at the process of data encryption using the example of making a payment on a shopping website. When a user attempts to make a payment online, their device, whether it be a computer or smartphone (the client), establishes a secure connection with the server. The client encrypts the user’s personal information, such as credit card details, and sends it to the server. The server decrypts the encrypted data, uses the information to authorize the payment, and then returns the result of the transaction to the web browser.

Cryptographic technology enables the secure transmission of important information to a specific recipient without being deciphered by a third party by encrypting the data. The encryption algorithms currently in use are considered computationally secure, meaning that decryption within a realistic timeframe is extremely difficult. However, it is said that when quantum computers, known for their ability to perform complex calculations at high speeds, become practical, they will easily break the current encryption methods. As concerns grow about the potential obsolescence of current encryption methods, attention is increasingly focused on quantum cryptography, the next generation of cryptographic technology.

Quantum cryptography leverages the principles of quantum mechanics to create an encryption method that is theoretically impossible to intercept by a third party during data transmission. It is believed that, even with the high computational power of quantum computers, this form of encryption remains absolutely unbreakable according to information theory.

The Mechanism and Weaknesses of Current Encrypted Communication

When exchanging encrypted information, the sender encrypts the data, and the recipient decrypts the encrypted data using a common key (a random sequence of 0s and 1s) that must be shared between the two parties in advance. Since there is a risk that the encrypted message could be deciphered if the key is leaked to a third party, it must be shared using a secure method.

In current encrypted communication, it is common to use a ‘hybrid encryption method,’ which combines two approaches: the ‘symmetric key encryption method,’ where the same key, known as the ‘symmetric key,’ is used for both encryption and decryption, and the ‘asymmetric key encryption method,’ where a ‘public key’ is used for encryption and a ‘private key’ is used for decryption. On the internet, the protocol SSL/TLS is used to enable encrypted communication between clients and servers, and it employs the hybrid encryption method. Using the example of ‘making a payment on a shopping website,’ which was mentioned earlier, the following explains the process of the hybrid encryption method.

1. When a user attempts to make a payment on an online shopping site, the client sends a request for SSL/TLS communication to the server.

2. The server sends the SSL certificate and the “public key” to the client.

3. The client encrypts the payment data, such as personal information and credit card details, using a “symmetric key,” then encrypts the symmetric key itself using the “public key,” and sends it to the server.

4. The server decrypts the encrypted symmetric key using its “private key,” then decrypts the payment data using this symmetric key and processes the payment.

When encrypting data with a ‘public key,’ it is common to use an algorithm called RSA, which is based on the factorization of large prime numbers. Factoring a large number into its prime factors requires an enormous amount of time. When using RSA, it is recommended to use a key length of 2048 bits (617 digits) or more, with a maximum of 4096 bits (approximately 1200 digits), for security reasons. It is said that with the current computational power, it would take over a hundred million years to decrypt a 2048-bit encryption key.

In this way, RSA is an algorithm that takes advantage of the fact that factoring large numbers into their prime factors cannot be done within a realistic timeframe. However, this also means that RSA is not ‘theoretically impossible to decrypt,’ but rather that ‘it is difficult to decrypt with the processing power of current computers.

However, it is said that quantum computers could potentially solve this complex factorization problem easily. In other words, once quantum computers become practical, the security of RSA will no longer be guaranteed. This would introduce the risk that our personal information, which is currently protected when using online payments or internet banking, could be stolen by third parties. In an era where quantum computers are widespread, it may become essential to implement new measures to prevent decryption.

Why is Quantum Cryptography Considered Secure?

Quantum cryptography is expected to overcome the weaknesses of traditional encryption methods and ensure security even in a society where quantum computers are widespread. Communication using quantum cryptography (quantum cryptography communication) involves sharing a common key between the sender and receiver using ‘Quantum Key Distribution’ (QKD), and encrypting the data with a ‘One-Time Pad’ (OTP). Here, we will introduce an overview of a representative method called ‘BB84.

In the BB84 protocol, the sender converts a random sequence, which forms the basis of the encryption key, into specific states of photons, the smallest units of light, and transmits them to the receiver. During this process, each photon carries 1 bit of key information, which is transmitted to the receiver through a dedicated optical fiber. The receiver measures the state of each received photon using a photon detector, reads the bit information, and obtains the random sequence. Finally, the sender and receiver compare parts of the random sequence to ensure that no third party has intercepted the transmission, and then generate the encryption key. This process is known as ‘Quantum Key Distribution’ (QKD).

Why is it possible to detect eavesdropping? The reason lies in a peculiar property of quantum mechanics: once a photon is measured, its state changes and cannot be restored to its original state. Because of this, even if a third party intercepts the communication, measures the photons sent by the sender, and then returns them to the communication path, the photons that reach the receiver will be in a different state than those originally sent by the sender. As a result, a high probability of mismatched bits will appear between the sender and receiver. Moreover, since photons cannot be split or copied before they are observed, if a third party intercepts the photons, the number of photons reaching the receiver decreases, again leading to mismatches. By applying the principles of quantum mechanics, it becomes possible to reliably detect eavesdropping and securely share keys between the sender and receiver.

In the One-Time Pad (OTP) encryption method, the sender encrypts the data they wish to transmit using the key shared through Quantum Key Distribution (QKD) and sends it to the receiver via a conventional communication channel. When encrypting the data, a key of the same length as the data is used. Additionally, the key is disposable, meaning that a different key must be used for each new transmission. One-Time Pad encryption is impossible to decrypt without knowing the key, and it has been theoretically proven to be absolutely unbreakable.

The First Step Toward the Practical Implementation of Quantum Cryptography

With the advent of the quantum computing era, research and development, as well as efforts toward standardization, are being actively pursued worldwide to achieve the social implementation of quantum cryptography, which will be essential. In Japan, based on the government’s Quantum Technology Innovation Strategy, the National Institute of Information and Communications Technology (NICT) was designated as a ‘Quantum Security Hub’ in 2021. The NICT is comprehensively advancing research and development related to quantum security technology, promoting social implementation through the development and utilization of testbeds, driving standardization, and fostering human resources, all in collaboration with domestic companies. Moving forward, the examination of use cases and demonstration experiments are being conducted with the aim of achieving social implementation by 2030.

Key factors in realizing quantum cryptographic communication are the acceleration of key generation and the extension of communication distances. Japan possesses world-class technological capabilities in both areas and has already achieved commercialization. Additionally, demonstration experiments are underway to establish a global communication network between space and the ground using satellites, and international standardization efforts are also being actively pursued.

In Japan, there is the ‘Tokyo QKD Network,’ a testbed with a long track record of operation that aims to verify the principles of quantum cryptographic communication. This network is used for various demonstration experiments and developments aimed at the social implementation of quantum cryptography. The basic specifications for quantum cryptographic communication devices, formulated based on the long-term operation of the Tokyo QKD Network, were adopted as an international standard in 2020.

Quantum cryptographic communication is expected to be used not only by government agencies handling sensitive information but also in the financial and medical sectors. Already, demonstration experiments on the Tokyo QKD Network have been conducted regarding the quantum cryptographic communication of biometric authentication information (such as facial recognition data), medical information (such as electronic health records and genomic data), and financial information, with technical evaluations also progressing. The financial sector, in particular, is said to have high compatibility with quantum cryptography due to the frequent use of dedicated communication lines. Currently, the expansion of the Tokyo QKD Network is underway, and collaboration between the Quantum Security Hub and government and financial users is expected to clarify issues related to social implementation and promote the adoption of the technology by early adopters.