Definition
AES is a symmetric-key block cipher selected by NIST in 2001 to replace the aging DES algorithm. It was designed by Belgian cryptographers Joan Daemen and Vincent Rijmen and was originally called Rijndael. AES encrypts data in fixed 128-bit blocks using keys of 128, 192, or 256 bits. In the VPN industry, AES-256 is the de-facto standard, often marketed as 'military-grade encryption.'
How It Works
AES is a substitution-permutation network. Plaintext is divided into 128-bit blocks and run through multiple rounds: SubBytes (byte substitution via an S-box), ShiftRows, MixColumns, and AddRoundKey (XOR with a round key). AES-128 uses 10 rounds, AES-192 uses 12, and AES-256 uses 14. The same key encrypts and decrypts — this is why it is symmetric — so VPNs first establish a shared secret over an asymmetric handshake (RSA or Diffie-Hellman), then switch to AES for the bulk traffic.
Modes of Operation
Raw AES only encrypts a single block, so it must be combined with a mode of operation. The common modes in VPNs are AES-CBC (Cipher Block Chaining), which is older and requires a separate authentication step (HMAC), and AES-GCM (Galois/Counter Mode), which is faster, parallelizable, and provides built-in authentication. Modern VPNs overwhelmingly favor AES-GCM. WireGuard, by contrast, uses ChaCha20-Poly1305 because it performs better on devices without AES hardware acceleration.
Why It Matters
AES has withstood more than two decades of cryptanalysis. There is no known practical attack against the full algorithm, and brute-forcing AES-256 is far beyond the reach of any conventional computer. When a VPN says it uses AES-256, it means the data inside the tunnel is, in practical terms, unreadable to an outside observer. The strength of a session, however, depends on the entire stack — key exchange, authentication, implementation quality, and the random number generator — not the cipher alone.