Quantum Computing: Could it Break Blockchain as we Know it?

Every day, we inch closer to a reality that humanity may not be prepared for: the quantum computing revolution. Indeed, this technology, which remains shrouded in mystery for most due to its extreme complexities, has some researchers convinced it will fundamentally change the way we compute and even help us travel through space at a speed faster than light and find extraterrestrial life. On Earth, though, it may have the unintended consequence of breaking our strongest encryption mechanisms and by extension, the blockchains and cryptocurrencies currently touted as “uncrackable.” Let’s gain a simple understanding of what quantum computing is, how it could break blockchains, and what’s being done to prevent this from happening.

To understand why quantum computing can be powerful and dangerous, it is first important to comprehend what the term means. Until now, we have been using classical computing methods that align with our current understanding of physics. We use bits, the smallest unit of data in computing that can be set to two states: 0 or 1. By manipulating a large number of bits via electricity, we can create modern computers and processors that are capable of incredible feats. 

Quantum computing, on the other hand, relies on the counter-intuitive and not yet fully understood world of quantum physics. It uses quantum bits, or qubits, as its unit of computation. Qubits are capable of a phenomenon known as superposition - instead of being either 0 or 1, they can be in both states simultaneously. However, qubits are subject to information decay if subject to environmental factors like high temperature and electromagnetic radiation, so massive centers are required to supercool quantum processors and keep them isolated; hence, we don’t see any home-friendly quantum computers yet. 

Because of the property of superposition, quantum computers can accomplish some computing feats significantly faster than their classical counterparts. Some examples include traffic optimization, probabilistic financial modeling, and AI modeling. Most importantly for cryptography, quantum computing can solve the problem of taking a large number and factoring it into its two prime numbers. Classical computers are very slow at this task, which is why it is the basis for many of our modern cryptographic schemes, including many of those used by blockchains like Ethereum and the privacy-preserving Monero, which could lead to individual wallets having coins stolen and private transactions revealed to the world. Additionally, quantum computing could be used to mine cryptocurrencies like Bitcoin exponentially faster than our current ASIC-based miners, which could cause network destabilization, centralization, and the potential for a 51% attack.

Fortunately, we are at least a decade out from quantum computing evolving to the point of becoming a threat. A computer strong enough to complete these tasks would require hundreds of millions of qubits, while current computers only have hundreds. Given this time advantage, most, but not all, blockchains and cryptocurrencies will be able to adapt and prevent any real damage from occurring. To mitigate the risk of 51% mining attacks and wallet hacks, blockchains could upgrade to a quantum-resistant cryptographic scheme, some of which have already been created. 

Monero, the best private blockchain that completely hides user transactions, could have their privacy broken with quantum computing. Since encrypted data was published to the blockchain, it cannot be retroactively re-encrypted, and quantum computers could be able to reveal the transaction information. As a countermeasure, users could send funds from an exploited transaction to a new account after quantum-resistance measures have been implemented, which could bring back their privacy. However, if they ever sent funds to an exchange or an address associated with them, their entire transaction history pre-quantum could be pieced together. There are similar implications for any encrypted on-chain data, and anything that must be secure for a long time should not be put on-chain.

There may be a few exceptions to this rule, one being ZK-STARKS. This type of zero-knowledge cryptographic proof is theoretically quantum resistant. StarkWare, the company behind the StarkNet zero-knowledge rollup meant to scale Ethereum, created this technology and utilizes these proofs to guarantee off-chain transaction execution.

While quantum computing can seem scary and confusing, the reality is that its dangers can be mitigated through preparation and planning. While some encrypted data will be subject to exposure, blockchains can take precautions to prevent themselves from becoming vulnerable to hacks or exploits. Hopefully, quantum will cause the same information revolution we had thanks to classical computers and change the world for the better.

By Lincoln Murr

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