Erik Weijers, a month ago

Can quantum computers crack Bitcoin?

Since the birth of Bitcoin people have been worried about improving decryption technology one day becoming capable of breaking the encryption. A recent paper by Chinese researchers claiming a quantum computation breakthrough - which most consider a hoax - adds fuel to the fire. How likely a candidate is quantum computing to ever break Bitcoin's encryption? 

The first question is: what is there to crack? The Bitcoin protocol makes use of encryption in many places. But the juiciest target are the private keys. Can't a hacker guess your private keys just by trying lots of times? It is... difficult. A private key is a binary number between 1 and 2 to the power of 256 (2^256). That's quite the number. In our decimal system, that's about 4 billion x 4 billion and that eight times. To get a sense of the vastness of this number, see this video: How secure is 256 bit security? 

If a quantum computer would one day have enough power to brute-force this number, that still wouldn't mean it would take down Bitcoin. Why not? Because your digital signatures aren't published, ideally, until you empty a Bitcoin address. Whereas when you receive Bitcoin, you receive it to an address which is a so-called double hash. The public key and digital signature aren't shown until you spend. So, unless you're reusing Bitcoin addresses, cracking the private key will only find an empty balance (most wallets are so called HD wallets, which are like accounts that generate a new address every time you want to receive Bitcoin. Read more about it in the article about self-custody).

But even still. Suppose a quantum computer would be powerful enough to break the algorithm that is responsible for Bitcoin's digital signatures. That wouldn't mean the end of Bitcoin. Why? Because the encryption algorithm can simply be changed and upgraded. In fact, this already happened in the Taproot upgrade of 2021. Not for reasons of quantum resistance by the way, but for reasons of storage efficiency of multi signature transactions. 

Not just Bitcoin would be cracked, but state secrets 

Let's for a moment assume that someone currently has the quantum computational power to crack Bitcoin. In that case, they would also be able to crack every system of encryption on the planet. Bank communications, embassy communications, communications with the nuclear submarines: they all use equally vulnerable cryptographic algorithms. 

The thought experiment then becomes: how would Dr Evil keep it a secret that he has this power? Emptying Bitcoin addresses would be the dumbest way to proceed, as it would immediately show his hand. Compare it to what the British intelligence agencies did, after cracking the Germen Enigma code. They would be super careful to only benefit from the decrypted intelligence if they had a plausible cover story for how they could have obtained the information. Had they not treaded so carefully, the Germans would have smelled a rat and no doubt changed their cryptography settings as soon as possible. 

To conclude, whenever quantum computing becomes powerful enough to decrypt Bitcoin, Bitcoin balances would probably not be the first things in line to be exploited.

Also read: How safe is Bitcoin?

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