Symmetric Encryption (Quantum Era)

Symmetric encryption is the cryptography that survives quantum computing relatively intact. In symmetric systems, the same key encrypts and decrypts — both parties share a secret key. AES, ChaCha20, 3DES are symmetric algorithms. The quantum threat to symmetric crypto comes from Grover's algorithm, which provides quadratic speedup for brute-force search. This weakens security but doesn't break it completely. AES-128 provides 64-bit quantum security, AES-256 provides 128-bit quantum security.  

The solution is simple: use AES-256 or equivalent 256-bit keys. Unlike public-key crypto which needs complete replacement, symmetric crypto just needs key length doubling. The practical challenge is key distribution — how do both parties get the same key securely? Classical solutions (Diffie-Hellman, RSA key transport) are quantum-vulnerable. Quantum solutions include QKD from systems like QNu Labs' Armos, or post-quantum key exchange using CRYSTALS-Kyber. Either way, symmetric encryption remains your workhorse for bulk data encryption. It's fast, proven, hardware-accelerated in modern CPUs, and quantum-resistant with appropriate key lengths.

The fact that AES survives quantum computing is enormously important — it means we don't have to replace everything. Keep using AES for data encryption, TLS record protocol, VPN tunnel encryption, database encryption.

Just upgrade the key exchange mechanism to post-quantum or QKD, and use 256-bit keys. This significantly reduces the scope and cost of quantum migration.

For organizations planning quantum transitions, focusing on public-key replacement while keeping symmetric encryption provides a practical migration path. You can upgrade TLS ciphersuites to use Kyber for key exchange while keeping AES-256-GCM for record encryption.  

VPNs can adopt post-quantum key exchange while keeping their current tunnel encryption. This incremental approach reduces risk and cost.