The Quantum Era and Blockchain

Quantum computers threaten several of the cryptographic algorithms most widely relied upon today because they can, in principle, run fundamentally different classes of algorithms than classical computers. Shor's algorithm, devised by mathematician Peter Shor in 1994, can factor large numbers and solve the discrete logarithm problem in polynomial time on a sufficiently powerful quantum computer, which would break RSA, classic Diffie-Hellman, and elliptic curve cryptography, all of which depend on precisely those problems being computationally hard for classical machines. Grover's algorithm, another quantum algorithm, offers a more modest quadratic speedup for searching an unsorted space, which effectively halves the practical security strength of symmetric ciphers and hash functions, meaning AES-256 and SHA-256 remain considered adequately secure against quantum attack, unlike current public-key algorithms, because doubling their key or output length restores the lost security margin. As of the mid-2020s, no publicly known quantum computer has come close to the scale and error-correction capability needed to run Shor's algorithm against real-world key sizes, but the concern is not purely theoretical for data that must remain confidential for a long time: adversaries can record encrypted traffic today and decrypt it once a capable quantum computer becomes available, a scenario known as "harvest now, decrypt later," which is the primary driver behind current efforts to migrate to quantum-resistant algorithms well ahead of that day.