Qubits and Blockchain

Every time news comes out about Google, IBM, or some new superconducting qubit, experts start crawling out of every corner claiming:

“Bitcoin is doomed.”
“Blockchain will be hacked.”
“Crypto will die.”

Nonsense.

And nonsense on multiple levels simultaneously.

First, most people do not understand what in a blockchain can actually be attacked by quantum computing.

Hash functions like SHA-256 and Keccak are not “broken” by Shor’s algorithm. For them, only Grover’s algorithm applies, which gives a quadratic speedup of brute-force search.

That means instead of $2^{256}$ operations, you would need roughly $2^{128}$.

Is $2^{128}$ small? To brute-force $2^{128}$ keys, even with a quantum computer, you would need to consume more energy than is required to boil all oceans on Earth. SHA-256 remains completely safe for the foreseeable future.

### Public Keys and ECDSA — The Real Target

Where quantum computers actually pose a threat is asymmetric cryptography (ECDSA, RSA).

Shor’s algorithm allows calculating a private key from a known public key in polynomial time. If your public key is exposed on the network (for example, when sending a transaction), a quantum computer could theoretically calculate your private key before the transaction is mined into a block.

### Post-Quantum Cryptography (PQC)

How is the blockchain community solving this?

1. **Address Hashing:** In Bitcoin, your address is a hash of the public key. The public key is not revealed until you spend funds from that address. As long as you don’t reuse addresses, your public key remains hidden from Shor’s algorithm.
2. **Post-Quantum Signatures:** Cryptographers are already testing quantum-resistant signature algorithms (Dilithium, Falcon, SPHINCS+). Upgrading a blockchain to post-quantum signatures requires a soft fork, similar to previous upgrades like SegWit or Taproot.

Quantum computing is a natural evolutionary challenge for cryptography, not an apocalyptic catastrophe.