AES (Advanced Encryption Standard)

AES (Advanced Encryption Standard) is the block cipher standardized by NIST in FIPS 197, the workhorse of modern symmetric cryptography. It encrypts 128-bit blocks under a shared key of 128, 192, or 256 bits and underlies TLS record encryption, disk encryption, and countless other protocols.

#Quantum impact

AES has no known exponential quantum attack. Grover's algorithm (Grover's algorithm) searches a k-bit key space in roughly 2^(k/2) evaluations, a quadratic speedup that on paper halves the effective key length: AES-128 drops toward 64-bit security and AES-256 toward 128-bit. The attack requires an extremely deep serial quantum circuit that resists parallelization, so its practical cost is far higher than the raw exponent suggests, and NIST assesses AES-128 as secure against foreseeable quantum attacks.

#Guidance

Conservative practice for long-lived data is AES-256, which retains a 128-bit margin even against an idealized Grover attacker. AES key sizes also anchor the NIST NIST security levels used to grade post-quantum algorithms.