Bitcoin security relies on elliptic curve cryptography (ECDSA), a digital signature algorithm vulnerable to Shor’s algorithm, which is why Quantum Computing FUD is back in focus. The key question is timing, and Q-Day is when this threat becomes real at scale. Estimates vary, with Nick Szabo viewing it as inevitable and Adam Back arguing the timeline could extend over decades, while Vitalik Buterin has pointed to “about a 20% chance” that quantum computers could break modern cryptography