What is Quantum Cryptography?

In modern society, the Internet of Things (IoT) is advancing in all areas, and various data is exchanged over information and communication networks. This includes sensitive information such as confidential administrative and national security data, financial information, genomic data, and medical information like electronic health records. Cryptographic technology is indispensable for securely sending and receiving such data. Of course, cryptography is also essential in our daily lives. We can safely use familiar online services like shopping site payments, online banking, sending and receiving messages via email and messaging apps, and cryptocurrency transactions because important information is protected by cryptographic technology.

Let's look at a simple flow of information encryption, using payment on a shopping site as an example. When a user attempts to pay for an item online, the user's device (client), such as a computer or smartphone, and the server establish a secure connection. The client encrypts the user's personal information, credit card details, and other data, then sends it to the server. The server decrypts the encrypted data, processes payment authorization based on its content, and returns the result of the transaction to the web browser.

Cryptographic technology enables important information to be delivered only to specific recipients without being deciphered by third parties. Current encryption algorithms are considered computationally secure, making decryption extremely difficult within a realistic timeframe. However, it is said that once quantum computers, which excel at high-speed processing of complex calculations, become practical, current encryption methods could be easily broken. Amid concerns about the vulnerability of current encryption methods, quantum cryptography, a next-generation encryption technology, is attracting attention.

Quantum cryptography is an encryption technology that utilizes the physical laws of quantum mechanics to completely prevent eavesdropping by third parties during data communication. It is said to enable encryption that is information-theoretically impossible to decipher, even by quantum computers with extremely high computational power.

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Current Cryptographic Communication Mechanisms and Their Weaknesses

When encrypting and exchanging information, the sender and receiver must pre-share a common key (a random sequence of 0s and 1s) for the sender to encrypt the data and the receiver to decrypt it. Since the ciphertext could be deciphered if the key falls into the wrong hands, it must be shared securely.

Current cryptographic communication generally employs a "hybrid encryption scheme," which combines "symmetric-key encryption," using a single key called a "symmetric key" for both encryption and decryption, with "public-key encryption," which uses a "public key" for encryption and a "private key" for decryption. On the internet, SSL/TLS is a mechanism (protocol) that enables encrypted communication between clients and servers, and it uses a hybrid encryption scheme for encryption. The flow of the "hybrid encryption scheme" is shown below, using "payment on a shopping site," mentioned earlier, as an example again.

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

2. The server sends an SSL certificate and its "public key" to the client.

3. The client encrypts payment data, such as personal information and credit card details, with a "symmetric key," then further encrypts this "symmetric key" with the "public key," and sends it to the server.

4. The server decrypts the encrypted "symmetric key" using its own "private key," then uses this symmetric key to decrypt the encrypted payment data and process the payment.

When encrypting data with a "public key," it is common to use an algorithm called RSA, which applies prime factorization. Factoring large numbers into their prime factors takes an enormous amount of time. When using RSA, a key length of 2048 bits (617 digits) or more, up to a maximum of 4096 bits (approximately 1200 digits), is recommended for security. With current computer capabilities, it is said to take over a hundred million years to decrypt a 2048-bit encryption key.

Thus, RSA is an algorithm that leverages the impossibility of factoring large numbers within a realistic timeframe. However, this also means that RSA is not "theoretically impossible to decrypt," but rather "difficult to decrypt with current computer processing times."

However, quantum computers are said to have the potential to easily solve this complex prime factorization problem. This means that once quantum computers become practical, the security of RSA will no longer be guaranteed. In such a scenario, our personal information, currently protected when using online payments or internet banking, would be at risk of being stolen by third parties. In an era where quantum computers are widespread, implementing new measures to prevent decryption could become essential.

When encrypting data with a "public key," it is common to use an algorithm called RSA, which applies prime factorization. Factoring large numbers into their prime factors takes an enormous amount of time. When using RSA, a key length of 2048 bits (617 digits) or more, up to a maximum of 4096 bits (approximately 1200 digits), is recommended for security. With current computer capabilities, it is said to take over a hundred million years to decrypt a 2048-bit encryption key.

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Why is Quantum Cryptography Considered Secure?

Quantum cryptography is expected to overcome the weaknesses of the aforementioned cryptographic technologies and ensure security even in a society where quantum computers are widespread. Communication using quantum cryptography (quantum cryptographic communication) is a system where a common key used for encryption and decryption is shared between sender and receiver via "quantum key distribution," and data is encrypted using a "one-time pad: OTP." Here, we will provide an overview of a representative method called "BB84."

In BB84, the sender converts a random number sequence, which will become the basis of the encryption key, into specific states of photons (the smallest unit of light) and transmits them to the receiver. In this process, 1 bit of key information is attached to each photon and transmitted to the receiver via a dedicated optical fiber. The receiver measures the state of each received photon with a photon detector, reads out the bit information, and obtains the random number sequence. Finally, the sender and receiver compare a portion of the random number sequence to confirm that no third party has eavesdropped during transmission, and then generate the encryption key. This mechanism is called "Quantum Key Distribution" (QKD).

Why is it possible to detect eavesdropping? This is because photons possess a peculiar quantum mechanical property: "once measured, their state changes and cannot be restored to its original form." Therefore, even if a third party eavesdrops on the communication, "steals" and 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 from those originally sent by the sender. In such a case, a high probability of bit discrepancies will appear between the sender and receiver. Furthermore, since photons cannot be split or copied before observation, if a third party extracts photons and does not return them, the number of photons reaching the receiver will decrease, also leading to discrepancies. By applying the principles of quantum mechanics, it is possible to reliably detect eavesdropping by a third party and securely share keys between the sender and receiver.

In "one-time pad encryption," the sender encrypts the data they wish to transmit using the key shared via quantum key distribution and sends it to the receiver using a conventional communication line. When encrypting data, a key of the same length as the transmitted data is used. Additionally, since the key is used only once, a different key must be used if data is to be sent or received again. One-time pad encryption is impossible to decipher without knowing the key, and it has been theoretically proven to be absolutely unbreakable.

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The First Step Towards Practical Quantum Cryptography

Towards the societal implementation of quantum cryptography, which is indispensable in the age of quantum computers, research and development and standardization efforts are currently underway worldwide. 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. NICT is comprehensively promoting research and development in quantum security technology, fostering societal implementation through the construction and utilization of testbeds, advancing standardization, and developing human resources, all in collaboration with domestic companies. Moving forward, studies of use cases and demonstration experiments are progressing with the aim of societal implementation by 2030.

Key to realizing quantum cryptographic communication are faster encryption key generation and longer communication distances. Japan possesses world-leading technological capabilities in both areas and has achieved commercialization. Furthermore, demonstration experiments for building a global space-to-ground communication network using artificial satellites are underway, and international standardization activities are also being promoted.

Japan hosts the "Tokyo QKD Network," a testbed with one of the longest operational track records globally, dedicated to verifying the principles of quantum cryptographic communication. Various demonstration experiments and developments aimed at the societal implementation of quantum cryptographic communication are being conducted there. The basic specifications for quantum cryptographic communication devices, formulated based on the Tokyo QKD Network's long-term operational experience, were adopted as an international standard in 2020.

In addition to its use by government agencies handling sensitive information, quantum cryptographic communication is also expected to see practical application in the financial and medical sectors. Demonstration experiments concerning quantum cryptographic communication for biometric information (feature data for facial recognition), medical information (electronic health records and genomic data), and financial information have already been conducted on the Tokyo QKD Network, and technical evaluations are progressing. The financial sector, in particular, is said to have high compatibility with quantum cryptography due to its frequent use of dedicated lines for communication. Currently, the Tokyo QKD Network is undergoing expansion and development. It is expected that the Quantum Security Hub will collaborate with government and financial users to clarify challenges for societal implementation and promote adoption by early adopters.

Q-STAR Member Companies: Quantum Cryptographic Communication Services

Toshiba
https://www.global.toshiba/jp/products-solutions/security-ict/qkd.html

NEC
https://jpn.nec.com/q-crypt/index.html

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