Quantum Computing Decoded: The Existential Threat to Bitcoin

What exactly is quantum computing, and how does it differ from traditional computing? More importantly, what does this mean for Bitcoin? New entrants to the Bitcoin space frequently encounter these questions, confronting a potential existential threat to the network if a functional quantum computer is ever developed.

Ownership of bitcoin relies on a foundational assumption: without directly leaking a copy, only the holder of a private key can sign transactions to move the secured coins. Quantum computing challenges this fundamental premise.

Quantum computers are not merely faster versions of classical computers; they operate on a fundamentally different principle, making them vastly more efficient at specific types of calculations. While I will not detail the intricate mechanics of quantum computing here, I will provide the core intuition behind their fundamental differences from classical machines.

Let us examine how both types of computers interact with large cryptographic keys. In a classical computer, all data is stored as a series of 1s and 0s. Each bit is definitively either a 1 or a 0, with no ambiguity. Data is stored and manipulated bit by bit, step by step. A classical computer processes discrete, unambiguous pieces of data linearly, one step at a time, without skipping or taking shortcuts in its computational steps. When generating a private key, a computer acquires a random value and stores it as 1s and 0s. It then multiplies this value by the elliptic curve’s generator point using an algorithm—a series of instructions on which bits to modify, what circuits to push, and how to store the resulting modified value back into memory. Other steps exist to generate a valid address, but they follow the same linear, bit-by-bit logic. If someone wanted to guess another person’s private key, they would face 2^256 possible combinations—a staggering 115,792,089,237,316,195,423,570,985,008,687,907,853,269,984,665,640,564,039,457,584,007,913,129,639,936 different keys. A computer would have to check every single possible key sequentially or in parallel. Checking more keys simultaneously requires exponentially more computing power, with no way to find mathematical shortcuts. The less computing power you use, the longer it takes; the faster you need it, the more power you require. This makes the task effectively impossible. The computational cost exceeds the capacity of every computer on Earth, and the time required surpasses the lifespan of every star in the universe. To succeed, a different approach beyond linear or parallel checking is necessary, which is where quantum computing enters the picture.

Quantum Computers

Quantum computers do not operate with discrete states. The most basic unit of information is a qubit, the quantum equivalent of a bit. Unlike a classical bit, a qubit exists in a superposition, simultaneously representing both a 1 and a 0 until it is observed, at which point it collapses into a single discrete state. Another key building block is entanglement. Qubits are not stored in isolation; the physical atoms representing them are entangled together. When entangled atoms are observed and collapse to a single state, the other entangled atoms collapse to the same state, regardless of the distance separating them. Quantum algorithms do not provide step-by-step instructions to transition between discrete states. Instead, they are instructions on how to operate on entangled qubits to alter the probabilities of various outcomes. Through constructive interference, the probability of a correct outcome is increased, while destructive interference decreases the probability of incorrect outcomes (distinct from the physical noise that hinders quantum computers). Consequently, while a classical computer must check individual private keys one by one to find a match for a public key, a quantum computer can run the appropriate algorithm a few times to arrive at the correct answer. It does not “check all possibilities at once”; rather, it simply modifies the probabilities of what a superposition will collapse into. This is why a quantum computer could break the assumptions underlying elliptic curve cryptography, whereas a classical computer cannot, and why quantum computers are only useful for specific computations with a massive space of potential answers.

Don’t Panic

This fundamental difference means that if a viable, functioning quantum computer is ever produced, the assumption securing all Bitcoiners’ funds is broken, rendering those funds insecure. While this represents a serious risk, we are not entirely unprepared. We understand the problem, we recognize the exposure, and a number of potential solutions addressing various facets of the issue are being developed. There is no need to panic. Over the remainder of this issue, we will walk you through the entire problem.

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