With the continuous development of the information age, the Internet has become one of the most important information transmission channels in our lives. However, how to protect important information such as confidential documents, encrypted data, and personal privacy in this vast online world has become a matter of great concern. The key to solving this problem lies in the popularization and application of confidential communications.
In fact, confidential communication is no longer a sophisticated technology in some specific fields, it has penetrated into all aspects of our production and life. In the banking system, how can the financial information of the various accounts be transmitted securely?How do you protect encrypted data within your organization?These are examples of the use of secure communication and demonstrate the power of this technology.
Keep your personal information secure with your account and password (**veer gallery).
We introduced the way classical cryptography builds ciphers in a previous article, and we will continue to look at the methods by which modern secure communication keeps our information secure.
Before talking about "asymmetric encryption", let's first familiarize ourselves with the simpler "symmetric encryption".
Schematic diagram of the "symmetric encryption" method (**wikipedia).
Suppose Xiao Li wants to send an encrypted message to Xiao Wang, but does not want to be eavesdropped by criminals. Then, Xiao Wang and Xiao Li can prepare a safe in advance, and each keep the two keys to open the safe. In this way, Xiao Li can use the key to put the information that needs to be encrypted in the safe every time, and Xiao Wang only needs to use the same key to open the safe to complete the confidential communication between the two of them.
Here, the information that originally needs to be encrypted is called "plaintext", the encrypted information is called "ciphertext", and the key used to encrypt is called "key". At this time, Xiao Li and Xiao Wang used the same key in the process of encryption and decryption, that is, they both used the same key, so this encryption method is also called "symmetric encryption".
In fact, the round wooden stick used to encrypt in ancient Greece and the Caesar cipher used to adjust the alphabetical order mentioned in the previous article are all symmetric encryption methods. While it is true that the encryption and decryption processes use the same key, it is very convenient to use, but it also carries a significant risk of leakage. This is because once the criminals know the encryption key, they can quietly eavesdrop on the encrypted information between Xiao Wang and Xiao Li.
In order to make up for the above-mentioned encryption loopholes, people have invented an "asymmetric encryption" method, that is, the key that Xiao Li used to lock the safe is no longer the same key as the key that Xiao Wang used to unlock the safe.
Schematic diagram of the "asymmetric encryption" method (**Author's own drawing).
For example, if Xiao Li wants to send an encrypted message to Xiao Wang, then Xiao Wang will prepare two different keys, namely the lock key and the unlock key. In this case, Xiao Wang will send the lock key and safe to Xiao Li first, and then Xiao Li will use the lock key to store the information in the safe, and finally Xiao Wang will use the unlock key he left to open the safe.
In this process, only Xiao Wang has the unlock key, so even if the criminals get the safe and the lock key, they will not be able to read the encrypted information. In this process of confidential communication, the key used to lock is also called the "public key", and the key used to unlock is called the "private key", and this asymmetric encryption method is also one of the mainstream means of confidential communication today.
It can be said that the above-mentioned "asymmetric encryption" method is very cleverly designed, and both parties can confidently and boldly disclose the ciphertext and public key to the public without worrying about criminals deciphering the encrypted information.
Concept diagram based on public key cryptography (**veer gallery).
The two keys prepared by Xiao Wang, namelyPublic and private keys are always generated by complex numeracy rules。Xiao Wang and Xiao Li will always update this kind of arithmetic rules regularly, so as to ensure that criminals cannot calculate the internal relationship between the public key and the private key in a limited time. Therefore, Xiao Wang and Xiao Li have the confidence to guarantee that even if the criminals have the public key, they will be helpless to do anything about the confidential communication between them.
However, this encryption is not indecipherable.
If the eavesdropper has super computing power, it is possible to calculate the internal relationship between the public key and the private key in a very short time, thus completely subverting the mainstream encrypted communication method between Xiao Wang and Xiao Li.
The advent of quantum computers is expected to give criminals this kind of super-computing power, thereby breaking the "encryption shield" that guards our information. The fundamental reason why quantum computers have potentially super computing power is that they are based on the basic principles of quantum mechanics, which is fundamentally different from the computing methods used by traditional classical computers.
Concept diagram of quantum computing (**veer gallery).
Classical computers use classical bits, which are like two sides of a coin, either 0 or 1. Quantum computers, on the other hand, use qubits, which can be not only 0 states, but also 1 states, and can magically be superposition states of 0 and 1 at the same timeIt's like having both heads and tails of a coin. This feature allows quantum computers to process multiple possibilities in parallel at astonishing speeds in certain situations, eliminating the need to queue them one by one, thus greatly speeding up the computational process.
Therefore, with the emergence of practical quantum computers in the future, the criminals Xiao Wang will make it impossible for Xiao Wang and Xiao Li to guarantee the absolute security of the above-mentioned "asymmetric encryption" method. Therefore, in order to resist the "quantum spear" computing power attack, we have to start to change our thinking and start looking for more effective encryption methods to ensure communication security.
In fact, no matter how complex and changeable the encryption method is, there are always two vulnerabilities that need to be filled to ensure its absolute security.
Web Security Firewall (**veer gallery).
First, once the eavesdropper has super computing power, they can decipher the keys of both sides of the communication in a very short timeSecond, neither Xiao Wang nor Xiao Li knew whether the eavesdropper had stolen the encrypted information. Comparatively, the second flaw described above tends to be more lethal because the eavesdropper will pretend that they have not deciphered the encrypted message and steal the communication between the two for a long time.
Fortunately, quantum mechanics is not a "quantum spear" that is biased towards deciphering keys, and scientists have also designed a more powerful "quantum shield" based on the basic principles of quantum mechanics to protect the absolute security of encrypted information.
The first loophole is relatively easy to fix, if Xiao Wang and Xiao Li will randomly change the key every time they communicate, then even if the eavesdropper has super computing power, he can only decipher a single confidential message. This method in which both parties need to change the encryption key every time they send a message is called "one secret at a time".
To close the second loophole, it is necessary to take advantage of a special property in quantum mechanics, namely quantum entangled states.
To understand quantum entangled states more vividly, we can give an interesting example.
Let's say that a pair of twin sisters are studying in Beijing and Shanghai, and a classmate in Beijing asks one of the twins, "Are you a sister or a sister?"Then this classmate can instantly deduce the situation of the other twin in Shanghai based on her answer. This is because before the two twins' sisterhood was questioned, in the eyes of the outside world, they were always in a "state of entanglement" of "sister or sister", and their respective states would be determined at the moment of answering.
Similarly, if we can make a pair of identical qubits, they will be in an entangled state of 0 and 1 before they are measured. No matter how far apart they are, if the state of one of the qubits changes, the other related qubit will also change instantaneously, a phenomenon known as "quantum entanglement".
Therefore, Xiao Wang and Xiao Li can send and receive a series of qubits as the key to encrypt information, so as to complete the confidential communication between each other. In addition, if the eavesdropper starts stealing the key, the quantum entanglement state between Xiao Wang and Xiao Li will change due to interference.
Schematic diagram of the quantum key distribution scheme (**self-drawn by the author).
In other words, if Xiao Wang and Xiao Li find that their quantum entangled states are not disturbed, they can be sure that the communication is not stolen, that is, it is safe. Therefore, Xiao Wang and Xiao Li can use these quantum entangled states to generate a shared encryption key that can be used to encrypt and decrypt their communication.
And this way of using the basic principles of quantum mechanics for confidential communication is also calledQuantum key distribution schemes（quantum key distribution, qkd）。
It can be said that the quantum key distribution scheme not only has the characteristics of "one secret at a time", but also makes full use of the wonderful properties of "quantum entangled states" to monitor whether confidential communications are eavesdropped.
In 1984, the quantum key distribution scheme was proposed, which attracted widespread attention from scientists. The quantum key distribution scheme provides a new way to ensure the security of communication, which can be used as a "quantum shield" to protect the absolute security of our communication.
Quantum Key Concept Diagram (**veer Gallery).
After nearly 40 years of development, quantum secure communication technology is also maturing and is entering our real life step by step. I guess you can't get enough of it here, right?So next, let's tell you the wonderful story of "going to the sky" and "entering the ground" in quantum secure communication!
References: 1] Haitjema, M .a survey of the prominent quantum key distribution protocols. cse.wustl.edu.
2]buttler w t, hughes r j, kwiat p g, et al. free-space quantum-key distribution[j]. physical review a, 1998, 57(4): 2379.
Producer: Popular Science China.
Author: Luan Chunyang (Department of Physics, Tsinghua University).
Producer: China Science Expo.