Bitcoin Magazine
The Quantum Issue: WTF Is Quantum Computing?
What is quantum computing? How does a quantum computer differ from a standard computer? And why does any of this matter for Bitcoin?
New Bitcoiners inevitably encounter these questions, forcing them to confront the existential threat that a viable quantum computer poses to Bitcoin’s security.
The ability to own bitcoin relies on the core assumption that—barring a direct leak of a copy—only the individual possessing a private key can sign transactions for coins secured by that key. Quantum computing directly challenges that foundational premise.
Quantum computers are not simply “faster computers.” They operate on entirely different principles, making them vastly more efficient than classical computers at very specific types of computations. While explaining the exact mechanics of quantum computers in detail is impossible here, we can outline the core intuition behind how they fundamentally differ from classical machines.
Let’s examine how both computer types interact with large cryptographic keys.
Classical Computers
Everything stored within a classical computer (referred to simply as a computer hereafter) is represented as a sequence of 1s and 0s. Every bit is definitively either a 1 or a 0 with no ambiguity. Data storage consists of 1s and 0s, and data modification occurs bit by bit, step by step, across those values.
This is how conventional computers operate. They process discrete, unambiguous pieces of data linearly, one step after another. They cannot skip ahead or take shortcuts regarding their processing steps (putting aside more efficient mathematical methods); they must execute computation steps sequentially.
When a computer generates a private key, it captures a random value—sourced from dice rolls, general user input, or device hardware randomness—and stores it in memory as 1s and 0s. It then multiplies this value by the elliptic curve’s generator point to yield a public key. At its most fundamental level, the executing algorithm consists of explicit instructions detailing which bits to select, how to modify them, which physical circuits to push them down, and how to return the newly modified value bit-by-bit back into memory.
Additional steps are required to generate a valid address, but they follow the exact same pattern: step-by-step instructions for altering 1s and 0s in memory.
So, what if someone attempted to use a computer to guess another person’s private key?
There are 2256 possible private keys. That equals 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 potential keys.
A classical computer would need to test every single possible private key sequentially—or however many it can process simultaneously in parallel—following the exact key-generation instructions outlined above. Checking more keys in parallel requires proportionally more computing power, with no way to bypass that cost.
Using less computing power increases the time required; conversely, demanding shorter completion times requires astronomical amounts of computing power.
This task is impossible for a classical computer. The computational cost exceeds the resources of every computer on Earth, and the time required is so long that every star in the universe would burn out before all keys could be checked.
Achieving this goal requires an alternative to linear or parallel one-by-one checking. That is where quantum computing enters the picture.
Quantum Computers
Quantum computers do not rely on discrete states where everything is strictly a 1 or a 0. The fundamental unit of information in a quantum computer is a qubit (the quantum counterpart to a bit). Unlike a standard bit, a qubit exists in a superposition, acting as both a 1 and a 0 simultaneously. It collapses into a single discrete state only when it is observed.
This superposition is one foundational building block of quantum computation. The other is entanglement. Qubits are not isolated; the physical atoms representing them—which collapse to discrete states upon observation—are entangled. Consequently, when entangled atoms are observed and collapse, they collapse into the identical state regardless of the physical distance between them.
While this sounds strange and requires a slightly generalized explanation, the intuitive takeaway is that quantum computers operate fundamentally differently from classical computers. A classical algorithm provides step-by-step instructions to manipulate a specific set of bits until arriving at a final set of modified bits, transforming one discrete state into another.
Qubits do not hold discrete states until observed; instead, they store probabilities. When a set of entangled qubits of any given size—such as 2256 in our hypothetical scenario—is used, every potential outcome state carries a distinct probability of collapse.
Rather than executing step-by-step instructions on discrete states, quantum algorithms instruct the system on how to manipulate entangled qubits to alter outcome probabilities. Constructive interference increases the likelihood of a correct result, while destructive interference decreases the likelihood of incorrect outcomes (note that this differs from the physical noise and interference that hinder physical quantum computers).
Therefore, whereas a classical computer must evaluate private keys individually to match a public key, a quantum computer running the correct algorithm can arrive at the right answer in just a few runs. It achieves this without “checking all possibilities simultaneously”; it simply alters the probabilities governing how the superposition collapses.
This capability explains why a quantum computer could compromise elliptic curve cryptography while a classical computer cannot—and why quantum computers are only useful for specific types of computations featuring a massive space of potential answer candidates.
Don’t Panic
This fundamental distinction between classical and quantum computing means that if a functional, viable quantum computer is successfully built, the foundational assumptions securing individual bitcoin holdings will indeed be broken, leaving those funds insecure.
While this presents a serious risk should such a device ever be manufactured and operated successfully, the community is not entirely unprepared. We understand the problem, recognize our exposure, and are actively developing a range of potential solutions for various facets of the issue.
Breathe and relax. The remainder of this issue will guide you through the entirety of the problem.
This piece is featured in the latest Print edition of Bitcoin Magazine, The Quantum Issue. We’re sharing it here as an early look at the ideas explored throughout the full issue.
What is quantum computing?
Quantum computing uses qubits that exist in a superposition of both 1 and 0 simultaneously, utilizing entanglement and probability to perform specific computations much more efficiently than classical computers.
Why is quantum computing a threat to Bitcoin?
Quantum computers could theoretically bypass the elliptic curve cryptography that secures private keys, allowing malicious actors to derive private keys from public keys and access funds.
What is the difference between a bit and a qubit?
A classical bit is strictly a 1 or a 0, whereas a qubit can exist as both a 1 and a 0 at the same time until it is observed and collapses into a single discrete state.
Are we prepared for quantum computers in Bitcoin?
Yes, the Bitcoin community understands the exposure and is actively researching and developing various solutions to mitigate the threat before viable quantum computers emerge.
