code: func ValidateKey(key []byte) error {
  	if l := len(key); l != 16 && l != 24 && l != 32 {
  		return fmt.Errorf("key size must be 16, 24 or 32 bytes")
  	}
  	return nil
  }
docs: // ValidateKey will check to see if the key is valid and returns an error if not.
  //
  // key should be either 16, 24, or 32 bytes to select AES-128,
  // AES-192, or AES-256.