Add AES key length validation

[alan-null]

Add a validation to ensure the input key length is either 16, 24, or 32 bytes (AES-128, AES-192, AES-256).

These key lengths come directly from the AES specification (FIPS PUB 197), set by the National Institute of Standards and Technology (NIST). No other key lengths are valid — the AES algorithm won’t work if the key length is anything other than these three sizes.