Community Submission - Author: John Ma
社区提交 - 作者：约翰·马
Introduction 介绍
Quantum computers are powerful machines that can solve complex equations much more quickly than regular computers. Some experts estimate that they could crack encryption that would take the fastest computers of today thousands of years in mere minutes. As a result, most of today’s digital security infrastructure could be at risk — including the cryptography underlying Bitcoin and cryptocurrencies.
量子计算机是强大的机器，可以比普通计算机更快地解决复杂方程。一些专家估计，它们可以在几分钟内破解需要今天最快的计算机花费数千年才能完成的加密。因此，今天大多数数字安全基础设施可能面临风险——包括支撑比特币和加密货币的密码学。
This article will give an introduction to how quantum computers are different from regular computers and what risks they pose to cryptocurrencies and digital infrastructure.
本文将介绍量子计算机与常规计算机的不同之处，以及它们对加密货币和数字基础设施构成的风险。
Asymmetric cryptography and Internet security
非对称加密与互联网安全
Asymmetric cryptography (also known as public-key cryptography) is a critical component of the cryptocurrency ecosystem and most Internet infrastructure. It relies on a key pair to encrypt and decrypt information - namely, a public key to encrypt and a private key to decrypt. In contrast, symmetric key cryptography only uses one key to encrypt and decrypt data.
非对称加密（也称为公钥加密）是加密货币生态系统和大多数互联网基础设施的关键组成部分。它依赖于一对密钥来加密和解密信息——即使用公钥进行加密，使用私钥进行解密。相比之下，对称密钥加密仅使用一个密钥来加密和解密数据。
A public key can be freely shared and used to encrypt information, which can then only be decrypted by the corresponding private key. This ensures that only the intended recipient can access the encrypted information.
公钥可以自由分享并用于加密信息，而这些信息只能通过相应的私钥解密。这确保了只有预期的接收者可以访问加密的信息。
One of the main advantages of asymmetric cryptography is the ability to exchange information without needing to share a common key across an untrusted channel. Without this crucial ability, basic information security would have been impossible on the Internet. It is difficult to imagine online banking, for example, without the ability to safely encrypt information between otherwise untrusted parties.
非对称加密的主要优势之一是能够在不需要通过不可信通道共享共同密钥的情况下交换信息。如果没有这一关键能力，基本的信息安全在互联网上将是不可能的。例如，想象一下没有能够安全加密不可信方之间信息的在线银行业务是很困难的。
If you’d like to read more on the subject, check out Symmetric vs. Asymmetric Encryption.
如果您想了解更多相关内容，请查看对称加密与非对称加密。
Some of the security of asymmetric cryptography relies on the assumption that the algorithm generating the key pair makes it incredibly difficult to calculate the private key from the public key, while it is simple to calculate the public key from the private key. In mathematics, this is called a trapdoor function, because it is easy to calculate in one direction but difficult in the other.
非对称加密的安全性部分依赖于这样一个假设：生成密钥对的算法使得从公钥计算私钥变得极其困难，而从私钥计算公钥则很简单。在数学中，这被称为陷门函数，因为在一个方向上计算很简单，而在另一个方向上则很困难。
Currently, most modern algorithms used to generate the key pair are based on known mathematical trapdoor functions. These trapdoor functions are not known to be solvable in a timeframe that would be feasible for any existing computer. It would take immense amounts of time for even the most powerful of machines to perform these computations.
目前，大多数用于生成密钥对的现代算法基于已知的数学陷门函数。这些陷门函数在现有计算机的可行时间内尚未被证明是可解的。即使是最强大的机器，执行这些计算也需要巨大的时间。
However, this might soon change with the development of new computing systems called quantum computers. To understand why quantum computers are so powerful, let’s examine how regular computers work first.
然而，随着新计算系统——量子计算机的发展，这种情况可能很快会改变。为了理解量子计算机为何如此强大，我们先来看看常规计算机是如何工作的。
Classical computers 经典计算机
Computers that we know today can be called classical computers. This means that computations are done in a sequential order - a computational task is executed, and then another one can be started. This is due to the fact that the memory in a classical computer must obey the laws of physics and can only have a state of either 0 or 1 (off or on).
我们今天所知的计算机可以称为经典计算机。这意味着计算是按顺序进行的——一个计算任务执行完毕后，才能开始另一个。这是因为经典计算机中的内存必须遵循物理法则，只能有 0 或 1（关或开）两种状态。
Various hardware and software methods exist that allow computers to break up complex computations into smaller chunks to gain some efficiency. However, the basis remains the same. A computational task must be completed before another one can be started.
Let’s consider the following example, where a computer must guess a 4-bit key. Each of the 4 bits can either be a 0 or a 1. There are 16 possible combinations, as shown in the table:
A classical computer needs to guess each combination separately, one at a time. Imagine having a lock and 16 keys on a keychain. Each of the 16 keys has to be tried separately. If the first one does not open the lock, the next one can be tried, then the next one, and so on until the right key opens the lock.
However, as the key length grows, the number of possible combinations grows exponentially. In the example above, adding an extra bit to increase the key length to 5 bits would result in 32 possible combinations. Increasing it to 6 bits would result in 64 possible combinations. At 256 bits, the number of possible combinations is close to the estimated number of atoms in the observable universe.
In contrast, computational processing speed only grows linearly. Doubling the processing speed of a computer results in only a doubling of the number of guesses that can be made in a given time. Exponential growth far outstrips any linear progress on the guessing side.
It is estimated that it would take millennia for a classical computing system to guess a 55-bit key. For reference, the minimum recommended size for a seed used in Bitcoin is 128 bits, with many wallet implementations using 256 bits.
It would appear that classical computing is not a threat to the asymmetric encryption used by cryptocurrencies and Internet infrastructure.
Quantum computers
There is a class of computers currently in their very early stages of development for which these classes of problems would be trivial to solve - quantum computers. Quantum computers are based on fundamental principles described in the theory of quantum mechanics, which is concerned with how subatomic particles behave.
In classical computers, a bit is used to represent information, and a bit can have a state of either 0 or 1. Quantum computers work with quantum bits or qubits. A qubit is the basic unit of information in a quantum computer. Just like a bit, a qubit can have a state of 0 or 1. However, thanks to the peculiarity of quantum mechanical phenomena, the state of a qubit can also be both 0 and 1 at the same time.
This has spurred research and development into the field of quantum computing, with both universities and private companies investing time and money into exploring this exciting new field. Tackling the abstract theory and practical engineering problems that this field presents is on the cutting edge of human technological achievement.
Unfortunately, a side effect of these quantum computers would be that the algorithms that form the basis of asymmetric cryptography would become trivial to solve, fundamentally breaking the systems that rely on them.
Let’s consider the example of cracking the 4-bit key again. A 4-qubit computer would theoretically be able to take all 16 states (combinations) at once, in a single computational task. The probability of finding the correct key would be 100% in the time that it would take for it to perform this computation.
Quantum-resistant cryptography
The emergence of quantum computing technology could undermine the cryptography that underlies most of our modern digital infrastructure, including cryptocurrencies.
This would put the security, operations, and communications of the entire world at risk, from governments and multinational corporations to the individual user. It is no surprise that a considerable amount of research is being directed at investigating and developing countermeasures to the technology. Cryptographic algorithms that are assumed to be secure against the threat of quantum computers are known as quantum-resistant algorithms.
On a basic level, it appears that the risk associated with quantum computers could be mitigated with symmetric key cryptography through a simple increase in key length. This field of cryptography was sidelined by asymmetric key cryptography due to the issues arising from sharing a common secret key across an open channel. However, it may reemerge as quantum computing develops.
The problem of securely sharing a common key across an open channel might also find its solution itself in quantum cryptography. Advances are being made to develop countermeasures against eavesdropping. Eavesdroppers on a shared channel could be detected using the same principles that are required for the development of quantum computers. This would make it possible to know if a shared symmetric key had been previously read or tampered with by a third party.
There are other avenues of research being investigated to defeat possible quantum-based attacks. These can involve basic techniques such as hashing to create large message sizes or other methods such as lattice-based cryptography. All of this research aims to create types of encryption that quantum computers would find difficult to crack.
Quantum computers and Bitcoin mining
Bitcoin mining also uses cryptography. The miners are competing to solve a cryptographic puzzle in exchange for the block reward. If a single miner would have access to a quantum computer, it may gain dominance over the network. This would reduce the decentralization of the network and potentially expose it to a 51% attack.
However, according to some experts, this isn’t an immediate threat. Application-Specific Integrated Circuits (ASICs) can reduce the effectiveness of such an attack — at least for the foreseeable future. Also, if multiple miners have access to a quantum computer, the risk of such an attack is significantly reduced.
Closing thoughts
The development of quantum computing and the resulting threat to current implementations of asymmetric encryption seems to be only a matter of time. However, it isn’t a problem of immediate concern - there are gigantic theoretical and engineering hurdles to overcome before it is fully realized.
Due to the immense stakes involved in information security, it is reasonable to start laying the groundwork against a future attack vector. Thankfully, there is a great deal of research being conducted into potential solutions that could be deployed to existing systems. These solutions, in theory, would future-proof our critical infrastructure against the threat of quantum computers.
Quantum-resistant standards could be distributed to the wider public in the same way that end-to-end encryption was rolled out through well-known browsers and messaging applications. Once these standards are finalized, the cryptocurrency ecosystem could integrate the strongest possible defense against these attack vectors with relative ease.