How do quantum computers handle data encryption and decryption?

Quantum computers handle data encryption and decryption differently from classical computers by leveraging quantum mechanics principles like superposition and entanglement. Unlike classical bits, which are either 0 or 1, quantum bits (qubits) can exist in multiple states simultaneously. This allows quantum algorithms to perform certain calculations exponentially faster. For encryption, the most significant impact comes from algorithms like Shor’s algorithm, which can factor large numbers efficiently—a task that is computationally infeasible for classical computers. For example, RSA encryption relies on the difficulty of factoring large primes, and Shor’s algorithm could break RSA-2048 in hours on a sufficiently powerful quantum computer. This poses a direct threat to widely used asymmetric encryption methods.

In symmetric encryption, quantum computers are less disruptive but still impactful. Grover’s algorithm, a quantum search method, can theoretically reduce the effective security of symmetric keys by half. For instance, AES-256, which is considered secure against classical attacks, would have its security reduced to 128 bits under Grover’s algorithm. However, doubling the key length (e.g., moving to AES-512) would mitigate this. Decryption in quantum systems also differs: quantum key distribution (QKD) protocols like BB84 use quantum properties to securely exchange keys, ensuring eavesdropping attempts disrupt the quantum state and are detectable. This provides a theoretically unbreakable way to share encryption keys, though it requires specialized hardware and isn’t yet widely deployed.

To address these challenges, the field of post-quantum cryptography focuses on developing algorithms resistant to quantum attacks. Lattice-based cryptography, for example, relies on the hardness of problems like learning with errors (LWE), which even quantum algorithms struggle to solve. The National Institute of Standards and Technology (NIST) is standardizing post-quantum algorithms, with CRYSTALS-Kyber (for encryption) and CRYSTALS-Dilithium (for signatures) being leading candidates. Developers today can start integrating these algorithms into systems to future-proof encryption. While practical, large-scale quantum computers capable of breaking RSA or ECC don’t yet exist, preparing for their eventual arrival is critical, as transitioning cryptographic infrastructure takes time.