Jeremy Van der Haegen is a Belgian freelance journalist covering the business and politics of Asia-Pacific, cryptocurrencies and blockchain technology.
One of the most overlooked problems of blockchain systems is their ability to resist fast-evolving machines known as quantum computers.
These powerful computers use quantum physics to solve complex problems beyond the reach of conventional devices using qubits, which are an evolution of the classical binary bit. Qubits can represent a value of 1 or 0 simultaneously, promising to provide an exponential computing power boost.
The world’s greatest superpowers spend billions of dollars developing this technology, and for good reason. The first country or company to use quantum computing will be ready to crack the encryption that protects its competitors’ sensitive documents.
In the case of blockchain systems, the cryptography that protects their tamper-proof ledger may be at risk. Researchers at the University of Sussex estimated in February that a 1.9 billion qubit quantum computer could crack the encryption protecting Bitcoin in just 10 minutes. Only 13 million qubits can do this work in about a day.
Fortunately, the ability to deploy quantum computers with so many qubits still seems many years away. IBM introduced its 127-qubit processor only last year, while a 1,000-qubit unit is scheduled to be completed by the end of 2023.
“We’re not there yet,” said Jens Groth, a Danish professor of cryptology and cryptography researcher at Dfinity. “No one knows what the exact time frame looks like, but blockchain could be at risk in just 10 to 20 years.”
Groth highlights that there is an important distinction between the two types of qubits (physical and logical ones). The second describes a qubit that achieves a superposition between 1 and 0 via a quantum gate. A logical qubit consists of nine physical qubits. “Company announcements about a new qubit milestone are usually about physical qubits, not logical ones,” he explains.
Defenders have the upper hand
Although researchers like Groth do not classify quantum computers as an immediate threat to blockchain technology, experiments with solutions still continue. “Cryptographers are thinking about what an appropriate countermeasure would look like,” Groth says.
Blockchain developers have a clear advantage in the defense race against increasing computing power. Specifically, they can increase the number of digits in the cryptographic keys protecting the chain; this is a process that scales faster than attackers can catch. “The defenders win this war in the long run,” Groth says.
This is evident in the field of symmetric key encryption when reviewing the popular Advanced Encryption Standard (AES). The most common variation of 128 keys can be cracked by quantum computers or even classical attackers. However, the AES 256 variant, which includes twice as many keys, looks powerful enough to fend off brute-force attacks by quantum machines in the foreseeable future.
However, some cryptographers are wary of seeing encryption as an automatic winner in a post-quantum world. “It is very difficult to predict whether we will be able to consistently grow key sizes against powerful quantum computers,” says Angshuman Karmakar, a research fellow in KU Leuven’s Computer Security and Industrial Cryptography group (COSIC).
“When you’re on the defensive, you always have to take a pessimistic approach. A shiny new algorithm could emerge and suddenly give attackers an advantage. The probability of this happening is extremely low, but it can never be ruled out,” says Karmakar.
Meanwhile, lattice-based encryption offers another potential solution to quantum attacks. This type of encryption adds mathematical noise that could confuse even a futuristic supercomputer. “Quantum computers can find a needle in a haystack by constantly doubling the probability of finding them. You have to design structures that these computers can’t exploit,” says Groth.
According to Karmakar, lattice-based solutions are currently in the process of standardization and should soon be available for public use. “A lot will depend on how quickly the industry can implement the new encryption. “On the other hand, it’s still a long time before quantum computers reach a level where they can break a blockchain.”
Migration to a new private key
Implementing a cryptographic upgrade for a blockchain system seems like the biggest headache for cryptographers. In a typical blockchain like Bitcoin, each node will need to be persuaded to switch to a new encryption method. Management protocols such as Internet Computer can automatically update their systems through user voting. Collective determination will be essential in any case.
However, upgrading existing private keys can create new vulnerabilities. This is because, according to Groth, new keys will be generated by the system after successfully implementing post-quantum encryption. To enable migration to the new key, users must sign for approval with their old key.
However, inactive users may never upgrade their private keys, which can cause serious problems. Quite large, dormant wallets, such as those containing around 1 million Bitcoins allegedly owned by Satoshi Nakamoto, will likely never see a crypto upgrade. This could leave some legacy parts of the crypto ecosystem open to quantum-based attacks, even if the blockchain they rely on is securely upgraded.
As a result, while blockchains seem safe from quantum computing for now, developers will need to stay vigilant and be prepared to take new steps to ensure this stays true.
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